In the overall scheme of things there isn't an appreciable difference between the weight of wood vs. asphalt shingles. Wood is usually calculated at 1.5 to 2 PSF (pounds per square foot) while asphalt weighs about 3 PSF. Overshadowing this, the roof structure itself usually weighs 7 to 10 PSF and must be designed to carry an additional 20 to 40 PSF live load for access and snow. These numbers vary depending upon the actual construction and regional location, but as you can see an extra 1.5 PSF isn't of concern.

Often roofers will apply a second layer of shingles over the old shingles to save the cost of removal and disposal. Even this is not usually considered to be a structural problem. However, if you were changing from wood shingles to clay tile (9 to 14 PSF), structural improvements would be called for.

Good Luck

Thank you Bravey for your speedy reply!

You're right about snow load. Looks like snow load factors in our area (Colorado) are about 44 psf. Roof material weight seems to pale by comparision!

Does that asphalt weight (3 psf) follow for the different varieties of composition shingles (such as 40-yr dimensional)? (I guess even if it didn't, it wouldn't add that much--unlike clay tile...)

Alas, I don't think clay tile is in our future, given its cost!

One of the engineering challenges I'd like to do (if we replace the roof) is to put some sort of overhang on. (Currently there are no eaves at all; kind of a bonehead design if you ask me.) I'd like to do this even if it entails just adding a little bit (say a few inches). Assuming, of course, that it "worked" with the overall integrity of the roof, ventilation, etc.

In fact I think ventilation could be an issue with asphalt (we don't have an attic).

Any other follow up is welcome; I do appreciate your feedback!

Regards,

-k2 in CO

Moderator, Miscellaneous Forum

http://www.bobvila.com/BBS/Miscellaneous

Here again shingle weights for 20, 30, or 40 year products do not differ enough to be a structural consideration.

You might want to talk to some local builders or designers about adding overhangs. Here in the south (coastal Texas) overhangs make a lot of sense - we get a lot of sun and no snow. In heavy snow regions, overhangs can be a problem. If your attic has any heat sources it can cause the snow above to melt and the resulting water to run toward the eave. Once at the eave it will freeze to ice just like water on a bridge. After several such cycles the ice can build to dangerous levels. One foot of ice weighs 60 PSF which exceeds the design capacity of many roofs. Liquid water can dam up behind the ice and leak inside. When the ice finally falls its danger is obvious.

Good Luck

Thanks again Bravey!

Very interesting. After reading your post, maybe we're starting to LIKE the house the way it is (no eaves)! Never knew they could cause so much trouble. I guess the 'grass is always greener'.

Some of the neighbors' homes do have eaves. I guess the reason we'd like a small overhang is that you don't have to be so concerned if windows are open (during rain).

You talk about dangerous overloading during snowmelt. I know this is exactly what happened during the 100-year blizzard that occurred here a couple of years ago. Many structures failed during that one. One thing about it was there was very high moisture content in the snow. From what I'd learned, this caused (kind of like you said) the water to run to the edges of houses--where the weight became a problem.

Thanks again for the informative posts, and for sharing information that might otherwise be difficult to find. Is this a great forum or what! :)

Regards,

-k2 in CO

Moderator, Miscellaneous Forum

http://www.bobvila.com/BBS/Miscellaneous