Wood Truss Flat Roof w/ Parapets

[phamENG]

Quick truss question. I have an occupied flat roof with parapets on a house that I'm working on. 12" deep trusses with a 3' parapet. I could balloon frame it, but I don't love that idea. How reasonable is it to put that kind of a parapet into the truss itself? Seems like it should be doable, but thought I'd bounce it off the group. I've done smaller parapets like this before, but never with guardrail type loading.

Braces would be nice, but the occupied area is small and braces would reduce it to an unusable space.

 

[dylansdad]

Pham,
I just finished one for a commercial project, however this roof was not occupied.

 

[jayrod12]

I would say it's totally doable that way.

However, the building envelope people where I practice would lose their minds. The parapet would have a "chimney" effect. I've found myself having to come up with convoluted ways to make the parapet work via straps, bolts, hold downs, etc. Just so the roof air-vapor barrier could go straight underneath the parapet wall and tie in directly with the wall AVB.

 

[kissymoose]

I believe incorporating it into the truss is standard practice around here.

 

[phamENG]

Thank you both. Nice to have some confirmation.

As for the building envelope...maybe blocking at the roof level with closed cell foam in the parapet and AVB going up and over? Beats me. I find building science fascinating but don't seem to ever find the time to learn much beyond the specifically structural stuff I need for the day to day...

 

[XR250]

Why not just nail 2x4's or 2x6's to the side of the truss to form the parapet? I would worry about damage during handling if it is built-in. Seems like with the backspan and repetition, the fastener demand would not be that onerous.

 

[phamENG]

XR - probably straps. It's about 680# at each chord for guardrail loading, but the wind loading will probably control anyway, now that I think about it. Good point on the handling - the contractor is already on board so I'll run it by him and see what his preference is in the field.

 

[dik]

I've never done it, but can you have the parapet members as part of the truss package? Have to watch out for rotation/deflection issues and building envelope.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Aesur]

We have had this discussion for non-occupied spaces with a local truss engineer (actual engineer) and on the projects we have, we have been able to achieve up to about 5' parapets with 2' deep trusses before bracing or another solution is required. This worked for a few locations, unfortunately the 12' tall parapets the architect wanted required bracing which screwed up their mechanical locations... go figure...

One thing to note about parapets though is that when you specify the parapet to be integral to the truss, I have been noticing that the parapet deflection is not being held to the same deflection limits as the building and therefore you may have cracks in the finishes if missed.

 

[phamENG]

dik - that's the original question. I've done smaller parapets as part of trusses, and similar sized parapets on larger trusses before...just wasn't sure about these proportions and the application of the 200# live load.

Aesur - thanks for the heads up on deflections. I'll be sure to make sure my specification is clear.

 

[dik]

Thanks pham... I've not done it, but don't think it would be difficult to incorporate.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[RontheRedneck]

I do parapets as a part of a truss all the time. They work fine depending on the wind load and the height of the parapet.

But never on 12" deep trusses. I assume those would be built as 4X2 trusses? I doubt it would work that way.

 

[phamENG]

Thanks, Ron. I appreciate the feedback. It is likely that these would end up with chords on the flat as 4x2. I suppose they'd have to do (3)2x4 turned to align with the chords and plated together periodically up the height.

Then would you suggest something like what XR mentioned above? Scabbing the parapet stud on the side of the truss and then fastening it to chords? Like I said, to do that would probably require a twist strap or something similar to develop sufficient strength.

 

[RontheRedneck]

phamENG, I'm not sure what would work. The truss manufacturer my be able to plate a 4X4 or 4X6 vertical into the truss to form a parapet.

Scabbing alongside the truss would still present the same problem with the ratio of the height of the parapet relative to the truss. I'd assume that adequate fastening would be a problem.

Again, it depends on your wind loading requirements.