Category: Foundation & Framing


Factory-Made Flooring and Roofing Systems

Photo: flickr.com

Most of the wood-frame houses built in the last hundred years have been assembled of dimensional lumber, the standardized two-by-fours, two-by-sixes, two-by-eights, and the rest that you encounter at your lumberyard. You shouldn’t be surprised to learn, given the finite number of trees to be harvested, that new products have been developed that take better advantage of the trees we have. Thus, a number of factory-made wood products have begun to appear at the construction site.

Trusses
Perhaps the most common variety of prefabricated structural members are trusses. Trusses are carefully engineered arrangements of triangles that can carry large loads over broad spans. They’re most often used in home construction to form the triangular gable roof, but other roof shapes and even interior floors are being framed today using trusses.

Trusses use the inherent rigidity of the triangle. Most are assemblages of two-by-fours. At the same time that they conserve materials, they also give the designer the option of creating larger uninterrupted spaces. Trusses are fabricated elsewhere, delivered to the job site oh a flatbed truck, lifted into place by a crew or even a crane, and nailed in place much like traditional solid-wood joists or rafters. The costs are roughly comparable, especially when savings in labor are considered.

Laminated-veneer lumber
Made of thin layers (veneers, roughly Vio inch thick) of wood that have been glued together, LVL is extremely strong and stable. Unlike the veneers in traditional plywood, all the layers in LVL are glued together with the grain in parallel. This produces a very consistent and uniform product suitable for use as beams, joists, and headers.

LVL actually costs a bit more than solid lumber but there can be labor economies in its installation. The price may also come down as these products become more commonplace, but one argument for the use of LVL is the material’s uniformity (there’s less spoilage than with solid lumber, where a loose knot, check, or twist can render a piece unusable). Another is its strength—structural members made of LVL can reach across much tifoader spans. LVL can also be purchased in lengths of 60 or even 80 feet, allowing the builder to span an entire structure without overlapping joists.

LVL comes in two basic configurations. As their name suggests, I-joists are a cross between steel I- beams and traditional wooden joists. When looked at in cross section, they have the shape of the letter I The vertical portion is called the web, the horizontals the flanges. J-joists are made of LVL and can be used both as joists and rafters. Micro=Lams are structural lengths of LVL without flanges that are used for rim joists, headers, ridge beams, and other applications.

The advantage of all these members is they’re strong but light. One man can lift, and two men can position, lengths of 60 feet or more with ease. Trusses and LVLs are all very stable when kept dry in delivery and installation. Shrinkage is minimal, unlike with traditional kiln- dried lumber where shrinkage routinely separates baseboards from floors and cracks plasterboard walls when moisture content is too high. Trusses have the added advantage that their open structure makes the installation of wiring, plumbing, and HVAC systems easier.

Don’t be surprised if your designer specifies trusses or LVL in designing your addition, particularly if you are creating a broad open expanse within the house.


Green Homes—Advanced Framing Techniques

With the increased emphasis on saving the environment and costs, there is renewed interest in “advanced framing” construction techniques, which were proven effective more than a decade ago.

Advanced Framing

Photo: hawkinshouse.ca

With the increased emphasis on saving the environment and costs, there is renewed interest in “advanced framing” construction techniques, which were proven effective more than a decade ago.

Advanced Framing Basics
Advanced framing is the name given to techniques designed to reduce the amount of lumber used and waste generated in a residential construction project and to improve a home’s energy efficiency.  Also known as Optimum Value Engineering, advanced framing includes such practices as building corners with two studs instead of three, which allows more insulation to be included.

The ideas have been known about for years, though the homebuilding industry has been slow to adopt them. A Natural Resources Defense Council handbook from 1998 included advanced framing among the ways to reduce waste of resources.

NRDC Senior Sustainable Building Specialist Kevin Mo says the techniques can be used as a package or separately depending on specific needs. The main objective is to use less lumber without compromising structural integrity so that more insulation can be put on the enclosure.

“The techniques are not rocket science but do take time for builders to adopt,” says Mo. techniques. “Now, more local building codes approve the techniques, and more contractors have gone through the learning curve. Builders are more familiar with the techniques and willing to apply the advanced framing techniques for energy efficiency.”

Proven in the Field
Advanced framing is one of many green methods Ferrier Builders & Ferrier Custom Homes of Fort Worth, TX, employs in both new homes and remodeling projects. “We have always specialized in extremely energy-efficient homes, with the first one back in 1982,” says Don Ferrier, chief executive officer. The company emphasized air sealing as well as reducing, reusing. and recycling long before the idea of “green builder” became popular.

The company works with the Building America research teams of the U.S. Department of Energy to employ cutting-edge and proven energy-efficiency techniques, including the advanced framing methods. Some potential subcontractors still balk at the idea of switching from standardz

techniques to advanced framing. “It’s not a difficult thing but it’s just enough different that we’ve heard people say ‘Never done that and don’t know if I want to,’ ” says Ferrier, who was named Green Building Advocate of the Year in 2007 by the National Association of Home Builders (NAHB).

“Studs 24 inches on center instead of 16, single top plate instead of double top plates, no headers on non-load-bearing walls—the differences are subtle,” but they add up, Ferrier says. For example, instead of two smaller headers, advanced framing would place one larger header that would allow for up to two inches of foam insulation that can increase R-value from 1.5 to 7.5.

Cost and Energy Savings
Kevin Morrow, NAHB program manager for green building standards, says the organization is doing all it can to increase builder education in all levels of green building. He suggests consumers look for a builder with the NAHB designation as certified green professional to ensure they are schooled in innovative techniques such as advanced framing.

According to the U.S. Department of Energy, advanced framing not only means the saving of resources, it also means savings for the homeowner. It estimates materials cost savings of $500 for a 1,200-square-foot house and $1,000 for a 2,400-square-foot house—a labor savings of three to five percent and heating and cooling costs savings up to five percent. The NRDC has estimated that using advanced framing techniques can reduce framing costs as much as $1.20 per square foot and reduce the amount of wood used for framing by 11 to 19 percent.

Here are several concepts to keep in mind when planning to use advanced framing on a project:

  1. Consider designing a home or remodeling project based on 24-inch modules. It makes the most efficient use of such building materials as framing lumber, wood sheathing, drywall, and trim that are typically stocked in two-foot dimensions.
  2. Consider how just one area, an exterior corner, can be changed with advanced framing. With advanced framing, insulation can be added in a commonly uninsulated area, an installed drywall clip can accommodate drywall and one less stud is used.
  3. Check with local codes first. Some advanced framing techniques may not be suitable for areas with high wind or seismic activity.
  4. Familiarize yourself with advanced framing and other green concepts before you start planning your home building or remodeling project. Section 2.1.2 of the NAHB’s Model Green Home Building Guidelines details some advanced framing techniques.

Pre-Cast Foundation System

An alternative to poured concrete, pre-case foundations have many benefits.

Precast Foundation

Photo: Buildipedia.com

In Season Five of “Bob Vila’s Home Again,” the popular “Cabin in the Woods” project showcased a unique array of innovative building textiles and technology that saved time and money.  One product that continues to generate interest is the state-of-the-art precast wall and foundation system developed by Superior Walls of America.

The Superior Walls System consists of pre-cast, studded concrete walls. The ready-to-finish wall panels feature built-in plumbing and electrical access holes, and SWA crews can install an average system in about five hours, in almost any kind of weather.

Pre-insulated with DOW Styrofoam and sealed with Bostik Chem-Caulk, Superior Walls are manufactured to National Standards and recognized Building Codes with 5,000 psi concrete. This eliminates the need for additional waterproofing or tarring.

In the words of SWA literature, “Ten times stronger than a block foundation, the Superior Walls System is guaranteed to prevent water infiltration and moisture build-up. Because of its innovative design, Superior Walls keep homes warmer and drier than conventional foundations while adding valuable living space and increasing resale values.”

Specifics
To enhance strength and durability, Superior Walls panels are manufactured with steel-reinforced concrete studs, rigid insulation, a reinforced top and bottom bond (footer) beam, and a 2-inch-thick concrete facing.

The bond beams and concrete facing are cast in one continuous pour. They connect to the studs by encapsulating vertical rebars and galvanized hooks and pins that protrude from the top, bottom, and back of each stud.

Pressure-treated furring strips are preattached to the inner face of each stud to provide a base to accommodate a variety of wall finishes. In addition 1-inch diameter holes are cast into each stud, allowing for the installation of wiring and plumbing.

The top bond beam is perforated with pre-formed 1/2-inch holes approximately every 24 inches to allow the bolting on of pressure-treated sill plates. The system is delivered to the jobsite with a built-in footer and is installed on crushed stone sub-footer.

The walls are pre-insulated with 1-inch DOW Styrofoam, with an R-5 rating. Additional insulation may be added to the 7-1/2-inch-deep wall cavity between the studs to increase the R-value up to R-26. A triple bead of Bostik Chem-Caulk provides a watertight sealant at the panel seams. Set 12 inches from the precast wall, a 4-inch perforated drain pipe assures a drier basement by collecting and channeling excess water away from the foundation.

Superior Walls are completely custom made and designed to accommodate door and window openings. Panels are normally formed in lengths up to 16 feet and standard heights of 4-foot, 4-foot 8 inches, 8-foot 2-inches, 9-foot and 10-foot, and can be cast to virtually any shape for unlimited design flexibility.

Installation Basics
Factory-trained crews can install an average foundation system in about five hours, regardless of most weather conditions. Backfilling can begin as soon as the floor is poured and the subfloor is properly attached to the top of the wall system. The panels come to the job site already cured, so construction may proceed immediately after installation.

Site Preparation and Installation Process
1. A 35 x 35-foot level area clear of overhead obstructions must be provided for the crane.

2. The basement must have an overdig of 24 inches at the bottom of the excavation.

3. The drainage system must be in place and functional.

4. Corner pins of the foundation must be clearly indicated.

5. Crushed stone must cover the entire floor area and be level to within one inch.

6. The builder must provide bracing materials for the wall system. Various lengths of 2x4s are preferred.

7. The site must be accessible for the delivery truck and crane. Check for mud, sharp turns, hills, bumps, trees, and overhead wires.

Cold-Weather Guidelines
1. Mix calcium with the stone all the way down to the virgin soil in an area of at least 30 inches wide around the footing.

2. Cover the area with plastic sheeting or other waterproof and nonporous material, extending two feet on each side of the center of the footing. Place stones or other heavy material along the edges to prevent air from getting under the plastic blanket.

3. Scatter at least 6 inches of loose straw over the blanket — more in severe freezing conditions.

These wintertime steps will keep the footing base stable and prevent weather-related delays. After the walls are placed, reapply the straw until backfilling is completed.

Radon Ventilation
Superior Walls can easily accommodate a simple and economical ventilation system to remove contaminated air and radon gas from the basement. Supplied and installed by the builder, a small in-line fan and piping system can be very effective. Special standard features of the SWA system add to the efficacy of this air exchange system.

The cast concrete panels offer a very low permeability rate, which is even further enhanced by the factory-installed DOW Styrofoam insulation. The crushed stone foundation allows for the free flow of air from all points of the excavation into the exhaust system beneath the floor.


Strengthen Your Roof with Trusses

Engineered roof truss systems even stand up to hurricanes.

Photo: caudilltrussandmetal.com

After hurricanes ravaged Florida in recent years, building codes were strengthened to keep future damage to a minimum. Officials and builders have learned that keeping the lid on a house means forming a tight bond between the sail-like roof deck and the walls below. That job falls to the engineered roof truss system that holds it all together.

Trusses Surpass Traditional Framing
Carpenters used to use two-by lumber to frame into stringers and rafters. Engineers and architects now design roof trusses built of 2x4s in triangular configurations that are joined together with metal connector plates. The result is a cohesive roof truss that stands up to state, local, and national building codes. Trusses perform to such a high degree because the lumber is uniform in size, density, and quality, and metal connector plates ensure rigidity at joints.

Engineered trusses have been on the building scene for 35 years, a track record that impresses many builders and homeowners. Kirk Grundahl, executive director of the Wood Truss Council of America (WTCA), in Madison, WI, says homeowners can be assured their roof trusses are engineered to exacting design standards nationwide since manufacturers must meet the standards set by WTCA and the Truss Plate Institute (TPI).

Sean O’Connor of Robbins Engineering, in Tampa, FL — designers, plate fabricators, and truss system engineers — explains that roof trusses create a stronger roof structure because they are engineered using CAD (computer-aided design) design techniques and computer analysis for worst-case scenarios.

“Because every one of the roof trusses are engineered, it literally takes into consideration all the forces acting on the truss, from gravity loads to wind loads, seismic loads, and uplift loads,” O’Connor says.

Roof Trusses Allow Open Floor Plans
Trusses have many pluses, including their overall strength, ability to be placed quickly, and span capability. Since they’re built from shorter lengths of lumber, roof truss systems are typically less expensive to build than roofs with conventional framing.

Trusses are engineered to span larger distances than conventionally framed roofs. Since they transmit weight from the roof to the exterior walls, none of the interior walls needs to be load bearing. This opens up interior space and allows for many interior design options.

Wood, Steel, and Engineered Timbers
Roof trusses, historically composed of wood with metal connector plates, now have competition. As steel-framed homes catch on, so have all-steel roof trusses. O’Connor says that to date steel roof trusses are typically reserved for the light commercial and industrial markets, with wooden trusses still dominating home construction.

Engineered wood products such as I-joists have also made a big surge in the market. “They can be used almost like framing lumber, but unlike conventional lumber, they’ll span up to 60 feet in length,” O’Connor says.

Wood roof trusses with metal connectors can also be treated with fire retardant and have an “excellent fire rating,” according to O’Connor, “and from a budgetary and quality standpoint, are the price point winners.” When they are properly engineered and put together, the wood roof truss with low-cost connector plates will perform to engineered lumber parameters. “So they are a nice, low-cost solution to framing problems,” O’Connor says.

Coupled with hurricane straps for fastening the trusses to the walls, the roof system is typically better than any stick-built roof, according to O’Connor.

Roof Trusses Allow Open Floor Plans
Trusses have many pluses, including their overall strength, ability to be placed quickly, and span capability. Since they’re built from shorter lengths of lumber, roof truss systems are typically less expensive to build than roofs with conventional framing.

Trusses are engineered to span larger distances than conventionally framed roofs. Since they transmit weight from the roof to the exterior walls, none of the interior walls needs to be load bearing. This opens up interior space and allows for many interior design options.

Wood, Steel, and Engineered Timbers
Roof trusses, historically composed of wood with metal connector plates, now have competition. As steel-framed homes catch on, so have all-steel roof trusses. O’Connor says that to date steel roof trusses are typically reserved for the light commercial and industrial markets, with wooden trusses still dominating home construction.

Engineered wood products such as I-joists have also made a big surge in the market. “They can be used almost like framing lumber, but unlike conventional lumber, they’ll span up to 60 feet in length,” O’Connor says.

Wood roof trusses with metal connectors can also be treated with fire retardant and have an “excellent fire rating,” according to O’Connor, “and from a budgetary and quality standpoint, are the price point winners.” When they are properly engineered and put together, the wood roof truss with low-cost connector plates will perform to engineered lumber parameters. “So they are a nice, low-cost solution to framing problems,” O’Connor says.

Coupled with hurricane straps for fastening the trusses to the walls, the roof system is typically better than any stick-built roof, according to O’Connor.


Know Your Building Lot

Go over the ground and study site conditions before you plan your house.

Building Lots

Photo: dougfrancis.com

In your mind you’ve got a dream house, but in reality you have a building lot. Before you get locked into a building plan, research your site, because site conditions affect your design and the cost to build it. No designer should draw house plans for you without a detailed site plan, and no builder should estimate the construction costs without knowing what’s under foot.

Gathering Information
It’s best to have complete site information before you build, but you can gather a lot of good data on your own before you hire a civil or geotechnical engineer. Ask neighbors; they’ll probably know if there’s a ledge, a high water table, or problem soils. Get a local soils map from the building department or local library. Take a good look at the site, noticing exposed rock, water plants, or new plant growth that may indicate fill.

Start with Soil
Since you may have layers of different soil types on site, your builder and designer need to know what’s there. The critical layers go from the surface down to about eight feet below the depth of your planned foundation.

Foundation codes are written for sand or gravel soils, which are the best natural soils for construction. Heavier silts and softer clays are not ideal and may require more than the minimum code requirements. Most building departments will want information on soils before they sign off on a permit; they may even require an engineer’s site report or stamp on your foundation design.

An engineering report is based on a site survey and test pit samples. If real problem soils are suspected, the engineer may do “soil borings,” but they are usually reserved for commercial projects.

Watch for Water
Quite often, the excavator discovers water when digging the foundation hole or test pit. This is not necessarily a problem, since water levels fluctuate from season to season in response to rainfall, drought, and melts. Engineers and site planners do, however, need to identify the water table (the depth where water sits year-round) and its high point. They do this by analyzing the color or “mottling” of the soil in the pit.

Foundation footings and basement slabs should sit above the water table so that groundwater will not put pressure on the foundation or cause a dampness problem. On a site with a high water table, you may prefer to build a shallow foundation, or bring in fill to raise the grade.

Drainage Is Essential
Soil drainage varies depending on the type of soil. Sands and gravels drain better than silts and clays, and this affects the project. If the native soil is sand or gravel, you can use soil from your excavation to backfill the foundation, placing it back against the foundation walls. But silts or clays, which don’t drain well, should not be used as fill because they tend to hold water against the foundation. This added pressure creates a structural load in addition to the obvious moisture concern. So if the original soil was a poorly draining silt or clay, it’s best to bring in gravel or sand for backfilling, and dispose of the original soil elsewhere.

Septic Planning
If your house needs a septic system, the water table and soil drainage are issues once again. Septic disposal or “leach” fields are usually four feet above the water table. You may need to build up with fill to meet that requirement, which is complicated and expensive since trucking clean fill is very costly.

Your septic permit will also depend on a “perc test,” which is done by filling a test pit with water and measuring the time it takes to drain. For a septic field to work, the wastewater has to seep through the “treatment zone” fast enough to dissipate easily, but slowly enough to give soil bacteria time to break down the wastes. Make sure your site will pass a perc test; otherwise, you can’t build there.

Building On Bedrock
Rock outcroppings on or near your property are a sign that there’s rock near the surface, commonly known as “ledge.” Blasting rock requires an expert, and costs far more than standard excavation—on the order of $20,000 a day. You may opt to forgo the full basement if your site has ledge, building instead on what you find.

The good news is that rock is strong. As long as the whole house rests on rock, settling is unlikely to be a concern. If you fill, put the whole house on engineered fill (preferably gravel); if you don’t fill, put the whole house right on the rock. At all costs, avoid uneven settling.


Get Down in Your Dirt

Study the soil of your lot before you build.

Testing Soil

Photo: yardcare.com

Foundations rest on soil, soil pushes against their sides, and wet soil pushes water and humidity against them, so it’s hard to plan for a foundation without a basic understanding of soils. The average person thinks of soil as dirt. For engineers, soil is a complex material worthy of a lot of study. In fact, there are thousands of soil varieties, but the main categories are gravel, sand, silt, and clay. What separates them is basically the size of the particles. Gravel is made of big chunks; sand consists of grains as small as the width of a human hair; silt is made of still smaller particles that are nearly microscopic in size; clay has particles too small to see. Most soils are blends of these main types, with names like “clayey sand” or “sandy silt.” Soil also has air and water mixed into it, so compacting the soil with rollers, pounding or vibrating equipment densifies and strengthens it.

Getting Down to the Dirt
To be absolutely sure of your soil, you have to send a sample to a soils lab. If they find more than 12 percent clay, the clay will be analyzed for its behavior when wet. This is because clay can turn to liquid, reduce the soil’s bearing strength, and cause the soil to exert pressure on the foundation. On a large commercial project, soil “borings” are taken vertically in two-foot increments. On a residential project, builders often rely on instinct and rule of thumb, because some building departments don’t insist on a soils report. Unfortunately, it can be hard to identify a soil by eye, or to predict its behavior by guesswork. A soil that seems to have a lot of gravel or sand in it could still contain 20 to 30 percent clay. If it does, it’s going to act like clay, which can give your project poor drainage and plenty of problems.

Testing Basics
So, do some creative detective work on your site. First, walk on the soil. If you leave a boot mark, try driving a stake into the soil. Since it usually takes six or seven whacks to drive a stake into the ground, a stake that goes in with one or two solid drives probably indicates soil that lacks strength and needs to be compacted.

Next, if your site is already under excavation, take a handful of damp soil from the bottom of the excavation and ball it up in your hands. If it crumbles apart when you release it, it is a granular soil (with lots of sand or gravel). If it holds together, it’s a silt. If it stays in a ball when you drop it from two feet, it’s probably a clay. To be sure, you might also try rolling the ball of soil into a noodle or worm shape. If you can roll it into a pencil shape without having it crumble, consider it clay, and make sure your next call is to a soils engineer. If ever you suspect clay in your soil, a full workup will be in order. It’s always worth investing $1,000 or so in engineering work before you invest your life savings in a home site.

The Bottom Line On Soils
For home sites, the bottom line is pretty simple: You want soil that has good bearing capacity, exerts relatively low lateral pressure, and drains well, so that you can have a stable, dry foundation. The best natural soils for these purposes are sands and gravels. Silts and clays are fair, but the softest ones are poor. Then there are soils such as peat, expansive clay, and improperly deposited fill, which are so bad that they must usually be removed and replaced — often at considerable cost to you.


When a Chimney Tilts (A Foundation Problem Lurks Below)

Tilting Chimney

Photo: ashireporter.org

It’s not something you really want to acknowledge. You tell yourself it’s an optical illusion, or clutch to the consoling thought that, hey, the tower in Pisa has leaned for centuries.

But a tilting chimney is a serious home problem that should be dealt with as soon as possible. The bricks could fall on someone’s head or crash on your roof. Water and bugs could get in the gap where the chimney has pulled away from the siding. The chimney liner may even be cracked, leaking combustible gases into the home. And since the chimney is probably connected to the foundation, there could be problems there, too.

The good news is that with today’s foundation repair methods, it may be possible to move your chimney into its original plumb position without incurring the high cost and disruption of demolition and rebuilding.

Symptoms
Look at the joint between the house siding and an exterior chimney. If a gap has opened up, it’s a pretty sure sign the chimney has begun to lean. Previous owners may have filled the gap with mortar, caulk, or foam insulation, but these measures only mask the problem. You may also see metal straps that have been used to fasten the chimney in place.

Tilting Chimney - Siding

Photo: Foundation Star

If your chimney runs through the interior of the house, look in the attic to see if it is centered in its framed opening. If it’s pressing against the opening to one side or another, that means it is leaning.

Leaks due to dislodged flashing are another sign that a chimney has settled. You may also use a long level to check whether the chimney is plumb (vertical in two planes). Alternatively, check the horizontal mortar joints for level.

Be aware, however, that some chimneys are designed to “tilt”. If the fireplace is not centered, the builder may have chosen to offset the brick courses so the chimney could exit at the roof ridge, giving the house a more symmetrical appearance. In some cases, the offset is slight and it looks like tilting, but as long as the horizontal mortar joints are level, you can rest assured that the chimney was built that way.

Causes
Masonry chimneys weigh many tons, and that weight is concentrated on a small area. So a chimney needs to be built on a concrete footing, sometimes called a chimney pad, in order to keep from sinking. (A chimney may be attached to the house for stability as well, but that’s not what’s holding it up.) The footing may be poured at the same time as the foundation or afterwards, as would be the case if the chimney were an add-on.

A number of things can cause a footing to fail and undermine a chimney. They include:

- Undersized footing. To ensure stability, the footing should be at least one-foot-thick and project six inches beyond the chimney on all sides.

- Poor soil. Loose soil and soils that expand and contract with changing water content (called expansive soils) will not bear the load of the chimney. Erosion and placement on backfilled soil may also weaken support.

- Shallow footing. If the ground beneath a footing freezes and expands, the resulting heaving will weaken the footing.

- Deteriorated footing. Concrete may crack due to water infiltration and repeated freeze-thaw cycles. Poor concrete quality, lack of reinforcement (rebar), or improperly installed rebar may also cause footings to crack.

- Missing footing. In such cases, the chimney will need to be stabilized, so a footing can be poured beneath it.

Repair
Foundation repair companies often use steel helical piers to stabilize and sometimes straighten leaning chimneys without dismantling them.

Helical piers look a little like giant screws and can be installed with hydraulic rotary drive equipment. The piers are driven deep under your chimney until they reach a firm, load-bearing soil strata. Brackets are then placed on the piers and slipped under the chimney footing.

Once in place, hydraulic jacks are used to slowly lift the chimney back into its original position. The bracket is then secured to the pier and the jacks are removed. Because helical piers do not require excavation and bring up nothing in the way of spoils (stone and soil), your yard is virtually undisturbed.

For more on fireplaces and chimneys, consider:

Gas Fireplaces 101
Bob Vila Radio: Fireplaces
Quick Tip: Make Your Fireplace More Efficient


Concrete, Block, and Slab Foundations

Climate, including high water tables, frost lines, harsh winters, and vulnerability to storm surge and high winds, will determine whether a slab or below-grade foundation is chosen.

Concrete Foundations, Block Foundations, Slab Foundations

Photo: concreteworkz.com

When building a house, two main types of foundations are used: slab-on-grade or below-grade foundations with a basement slab. Climate, including high water tables, frost lines, harsh winters, and vulnerability to storm surge and high winds, will determine whether a slab or below-grade foundation is chosen.

Poured Footings
Poured and block foundations both sit on concrete footings, or poured pads that serve as a base for the walls. Footings are constructed in trenches dug beneath the level of the basement floor. These trenches are wider and longer than the walls they support and function like feet to distribute the weight of the wall and the structure above it. Footings provide a firm surface to resist sinking or shifting into the ground or substrate. A footing trench ranges from six inches to three feet deep, depending on the building size and soil characteristics.

Poured Concrete Walls
Poured concrete is more popular for basement construction than block because it is seamless and resists water intrusion. When pouring an integral foundation, aluminum or insulated wall forms are placed on the footings, clamped together, and supported to maintain their shape while the concrete is poured.

Once the forms are set, rebar is placed vertically inside the wall channel to support and add additional strength to the concrete wall once the molds are removed. Concrete is then poured into the mold to form the walls.

Concrete walls should be created as a continuous pour to ensure good bonding and avoid seam cracking where a first concrete layer has already set.

Cement can be poured in place with a cement-pumper truck, or offloaded down the chute of a ready-mix truck if it can get close enough to the foundation. Set-up time depends on the slurry used, the time of year, heat, and humidity. Temporary forms are usually taken down after one week, at which time the concrete is cured enough to support itself. The concrete will continue to cure and emit moisture for much longer. When using insulated concrete forms, they remain in place and insulate the home.

Reinforced Block and Concrete Walls
Block foundations use cinder blocks (8 x 8 x 16 inches) that are stacked on each other and cemented in place with mortar. The process starts on the top of the footings with each row forming its own course. The blocks are then reinforced with rebar placed vertically in the holes or cells and filled with concrete.

Block walls can also be used to form stem walls that support a slab above. When building stem walls, block courses on footings are set below grade and reinforced with rebar before concrete is poured in a continuous pour for a seamless, integral slab. Stem-wall slab foundations prevent water intrusion and the separation of the slab from the substrate that can be caused by uplift or hydrostatic pressure.

Both poured and block foundations are reinforced with rebar. With poured walls, a pencil vibrator is inserted into the slurry to vibrate the concrete into place and ensure there are no air pockets or voids left in the wall.

Finishing the Basement Floor
When building slab foundations, the concrete pour comes after the footings have set and before walls are erected. Dirt is compacted and backfilled with four to six inches of gravel. Typically, a six-mil polyethylene sheet provides a vapor barrier between the soil and the slab. A two-inch layer of sand goes on top of the vapor barrier, followed by a 6×6-inch wire-mesh grid that reinforces the concrete. If radiant in-floor heating will be used, the plastic tubing is placed on top of the wire mesh. Once the tubing is pressure-tested, the four-to-six-inch concrete slab is poured.

When building with poured walls, the basement floor is prepared as if it were a slab floor, often with the concrete floor poured after the top floors are in place and the roof, windows, and doors are set.

Basement plumbing for floor drains and piping must be roughed in before the pour. Like a slab floor, the basement floor will be lined with a six-inch aggregate bed followed by a six-mil polyethylene vapor barrier. One to two inches of foam board can go on top of the vapor barrier for insulation and further waterproofing. Wire mesh comes next for structural strength, and flex tubing is set in place if using in-floor radiant heating. Finally, the concrete is poured on top and leveled with a screed.


Tradeoffs in Roof Structures

Two options in roof framing affect space and cost.

Rafters, Trusses

Photo: home-improvement-and-financing.com

There are two common options for framing a home’s roof. A rafter system that connects to a ridge beam is the most flexible, allowing for design elements like vaulted and cathedral ceilings and wide-open attic spaces. A rafter system, however, must be built piece-by-piece on location. A truss roof can span much farther than a one using beams and rafters, allowing the creation of larger open spaces below. Truss systems are faster to erect, as they arrive on site pre-assembled and ready to install. However, the wide-open space created by a rafter system is often lost when using roof trusses, as the lumber webbing that give the truss its strength often crisscross through the space traditionally left open by rafters.

At Home Again’s Vermont Farmhouse project in Quechee Lakes, Vt., architect Hunter Ulf of UK Architects in Norwich, Vt., took advantage of both the speed and cost effectiveness of truss systems, while still making accommodation for a bonus space above the garage. “The truss we used is setup so there are no webs in the center of the truss allowing you to use the space,” says Ulf. Although not as spacious as a roof system formed by rafters, “it gives you an affordable roof system and useable space,” notes the architect. Bob Vila says, “The prefabricated trusses are delivered to the job site ready for installation. Assembling the roof structure with this system is very fast, and a crew of about a half a dozen workers can raise the truss system and sheath the roof with plywood in a single day.”

Plan ahead carefully when working with your architect or designer when choosing a roof system. Truss systems cannot be modified once installed. Cutting individual lengths of wood in the trusses can significantly weaken the entire roof structure (boring holes to run electrical wires is even discouraged). “The trusses used in our farmhouse project constructed from 2x4s. Ordinarily, 2x4s are too small for use as roof rafters, but the engineered construction of each truss allows the use of smaller dimensional lumber in this application,” Bob notes.