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Looking for advice on best practices flat roof soffit venting and sealing

keithhoffman22 | Posted in General Discussion on August 4, 2022 07:55pm

Hi FHB forum users,

I’m looking for some building science advice about best practices for whether and how to vent soffits on a flat roof.

Background:
Flat roofed attached garage had poorly constructed soffit tops resulting in moisture penetration.  I believe lack of venting exacerbated the problem since any moisture penetration was pretty trapped.  TJIs rotted and are being entirely replaced by GC per engineer with pretty much the exact same assembly as before.  Roof is EPDM pitched to center plumbing style drain (to french drain) via EPS cricket.

The current questions which I disagree on the GC regarding is sealing the TJI end blocks and venting the soffit. 

The GC says the soffit shouldn’t be vented because it part of the garage assembly and shouldn’t have the unshaped truss end blocks foam sealed so that the soffits and garage insulation can ‘breathe’. 

My opinion is that the soffit ends should be continuously vented and the block ends should be sealed with canned foam to prevent vapor and air intrusion.  But I’m just a DIY homeowner.

Assuming you had TJIs, OSB decking, EPDM roofing, overhanging soffit ends, fiberglass insulation (probably 7″ in 12″ of space, another battle I’m losing), what would you do regarding truss end blocking and soffit venting.

Thanks a bunch for any advice you can provide.

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Replies

  1. oldhand | Aug 05, 2022 01:12pm | #1

    You need to add your location to get any meaningful advice.

  2. User avater
    unclemike42 | Aug 05, 2022 03:36pm | #2

    You also could provide a few photos or sketches.

  3. keithhoffman22 | Aug 05, 2022 04:02pm | #3

    Thanks for the replies guys. I'll see about some photos.

    As for climate, front range of Colorado (5b?). Hot, mostly dry summers. Mixed cold-warm mostly dry winters. Design temps 0F, 91F though this structure is currently not planned for conditioning.

    A WUFI model is under consideration but in today's busy environment I don't know anyone who could do one on short notice.

    Good southern sun on the roof in the winter, some tree shading in summer. But long-term, I'm sure someone is going to cover the black EPDM with a flat roof decking product (not in this project budget or design) so I wouldn't expect a lot of solar gain.

    Let me add one more question:
    One of my largest concerns is construction material drying if the top, bottom, and sides are closed soon and impermeable. If anyone has experience in what kind of drying times are required for TJIs and OSB, I'd be interested. I don't yet have moisture meter readings (this weekend).

    Thanks!

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