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fragasaurus

04:29AM | 07/14/04
Member Since: 12/01/02
93 lifetime posts
Bvbasement
We are adding on to our house by extending out one area which is a 4' high crawlspace and another area which will be our garage.

I specified a 4" slab with wire mesh and vapor barrier in both areas but my concrete contractor is telling me all I need is a 2" rat slab in the crawlspace.

The slab will be poured mostly on ledge so I'm not too worried about it going anywhere but I'd like to hear opinions if I should get 4" instead of 2". In the end, it's only another few hundred dollars for material so I will probably just do it.

For what it is worth, the crawlspace is over ledge and although there is a 4' clearance, we will not be using it for any storage. Also, we have high radon levels so I'm curious as to whether the slab thickness plays a part assuming we put in a vapor barrier (2 layers of 6 mill plastic I think).

Any thoughts would be appreciated.

Thanks.

Glenn Good

11:34AM | 07/14/04
Member Since: 09/10/03
320 lifetime posts
fragasaurus,

If you send me an email with your email address I will send you a good .pdf file concerning radon gas and the various ways to help control it.

A 2" rat slab may be enough but the main problem is that when pouring a 2" slab it is difficult to control the thickness of the concrete and this often results in areas that are thinner than others. This causes weak spots and results in frequent cracking which can allow radon gas to enter the space more readily. Spend the extra money and go to at least 3" thick fiber reinforced concrete to prevent cracking.

You might also want to visit this website for a sealer you can use on the slab as added protection from both moisture and radon:

http://www.radonseal.com/

Glenn

Moderator: Construction Systems, Foundations, and Masonry & Stone

For more information about me and/or my qualifications please visit my website at:

www.consultationdirect.com

fragasaurus

04:38AM | 07/23/04
Member Since: 12/01/02
93 lifetime posts
Glenn,

Thank you again for your reply. I have 3 follow-up questions.

1. When putting down the 6mil vapor barrier, what can be used to 'tape' the seams? Also, is there a standard overlap? I know for roofing I've seen 6" but not sure if it should be more for under the slab. I'm not worried about cost... I just want to do it right.

2. There are different materials for waterproofing/damproofing a foundation. My local building department says there is no requirement and just applying the damp-proofing is enough but again, the material is cheap compared to the problems it may alleviate so I want to do it right. Is applying a 6mil barrier over walls that have a damp-proofing already applied (Henry brand for example) reasonable? I've heard of other rubberized materials but am having trouble locating retailers lcoally. I understand how to apply the material on the walls and over and down the footing, but I don't know if it is typical to use 6mil plastic for this application.

3. Lastly, you gave some great advice about using fiber reinforced concrete instead of wire mesh. Would this apply to a garage application as well? Our addition consists of 1 area of crawlspace, 1 area of slab on grade (3 season sun room) and a garage. The plan was to use 4" of fiber reinforced concrete for the crawlspace and the slab on grade. The local building department said there is no requirement for mesh in the garage but I'd think it may be more important here than the other areas. Any thoughts on whether fiber reinforced is applicable for a garage as well? The garage will be attached to the house but not heated.

Thanks in advance,

Jonathan

Glenn Good

05:29AM | 07/23/04
Member Since: 09/10/03
320 lifetime posts
They make a polyethylene tape for the seams. If you have problems finding it check with a contractor supply company that handles concrete accessories.

For the damp proofing you might want to consider Bituthene® System 4000 (below grade). This is a much better material to use and is easy to install. Go to http://www.na.graceconstruction.com/product.cfm?mode=c&did=11&id=80 for more information about it. We use this product a lot with good results. The problem with 6-mil polyethylene is that it is generally made to be biodegradable and will deteriorate over time below ground.

The fiber reinforced concrete should work fine for you garage as well. I do recommend using a thicker slab for a garage (5”-6”). Many contractors do not do this but it is worth the extra expense to avoid cracking under the vehicle weight that will be placed on it. I think of it as buying insurance. This is true for either wire of fiber reinforced concrete.

Glenn

Moderator: Construction Systems, Foundations, and Masonry & Stone

For more information about me and/or my qualifications please visit my website at:

www.consultationdirect.com
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