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Large overhangs without thermal bridging

Hey everyone,

I am designing a small commercial building for a client in Climate Zone 5A.
The design calls for largish overhangs on the south facade, and I am playing around with some ideas on how to build these.

Traditionally, the interior structure (in this case, timber frame) would bear on the interior, and the top chord of the truss or rafter tail would cantilever out to create the overhang. However, with this design there is automatic thermal bridging, and the building envelope is compromised.

In the past, I have used SIPs to create a sealed envelope, and then build up a separate structure over this to provide a cold roof, with the additional members needed to cantilever and create my overhang. The problem is that this is a lot of additional cost, roof height and waste.

Another method I have tried is to have two independent structures - one for the internal structure and an secondary system for the overhang, which would require additional columns / beams to carry this structure. This system has it's challenges as well, especially within areas of large glazing where attachment points can be few and far between.

Lastly, embedding outriggers in between SIPS panels has worked in the past, however I'm wondering if solid furring / rigid foam insulation on a roof system to eliminate this thermal bridging is an effective solution?

Sorry, I know this is wordy, but this question comes up a lot, and I would like to get tome feedback from folks who have dealt with this conundrum.

Thanks in advance!

e

Asked by Eric Whiting
Posted Thu, 04/10/2014 - 10:29

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2 Answers

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1.
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Eric,
One approach is to frame the building without any roof overhangs and to install rigid foam on the exterior side of the wall sheathing and roof sheathing. Then it's possible to build and install "applied overhangs." For further details on this method, see Airtight Wall and Roof Sheathing.

Of course, the wider the overhangs, the trickier the fastening details become.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 04/10/2014 - 11:02

2.
Helpful? 0

Yes, this is the system I mentioned in my second scenario above. More difficult with 48" overhangs. Hmm.

Answered by Eric Whiting
Posted Thu, 04/10/2014 - 11:39

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