Rigid foam on the roof
user-5644012
| Posted in Green Building Techniques on
We are remodeling the exterior of our 115 year old farm house this year and felt it would be a great time to tighten the envelope and add rigid foam to the exterior. However, our contractor has some concerns about the extra weight of five inches of foam over the roof. Is this a valid concern? It’s an old house. The frame of the roof consists of actual 2x4s placed 24 inches apart. We nailed 2x12s to the 2x4s eight years ago to provide a cavity to put blown in cellulose.
Thanks for the advice, Ryan
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Replies
Are the 2x12s nailed to the 2x4s ? Are the resting on the top plates of the walls?
The heaviest foam you'd likely be adding to the roof is about 2lbs per cubic foot, so even at 6" the additional dead load weighting of the foam itself is only about 1lb per square foot. A 2x12 raftered roof 24" o.c. would barely notice that additional load.
If the cellulose was dense-packed it's adding about 3lbs per square foot of dead load.
Ryan,
I agree with Dana. Hopefully, someone consulted an engineer when the 2x12s were sistered to the 2x4s -- because (as Dana notes) those new 2x12 need to bear on something at either end.
For more information on this type of work, see How to Install Rigid Foam On Top of Roof Sheathing.
Thanks for the valuable info. I'll pass this along to our contractor. I believe the 2x12s are nailed to the 2x4s but they are not resting on top of the wall. They were nailed into the wall at the bottom.