Economical Roof Sheathing with Trusses at 4 ft oc 1

[bones206]

I had good luck specifying cold-formed roof trusses at 4 ft oc. Contractor's like having less trusses to deal with and the wider spacing that makes things way easier for the trades and allows scissor lifts to access the attic space, etc.

The trade off seems to be having to install metal decking as the structural sheathing layer, with another layer of plywood just to facilitate roofing materials. Depending on the roof geometry, the decking coordination and installation can be a real pain.

Are there any roof sheathing options for 4 ft truss spacing besides metal deck? Specifically for 40 psf and up snow loads.

 

[DoubleStud]

You can run 2x rafters running perpendicular to the trusses. Perhaps 2x4 or 2x6 with plywood on top. No metal decking.

 

[jayrod12]

What about T&G plank decking? Those can certainly do what you're asking. Commercial grade Northern lumber (the weakest in my design manual) is good for around 200 PSF at only 1 1/2" thick at a span of 48" for strength. For deflection it appears to be good for around 135 PSF at 48" spans.

 

[bones206]

Just went back to look at a recent project and the architect had called out UL P523 for the roof assembly.

http://superior-truss.com/wp-content/uploads/2018/06/design-p523.pdf

Looks like that assembly allows for either metal deck or plywood. If plywood is used, min. 20 Ga purlins or hat channels can be used to reduce sheathing span to 24". I feel like purlins would be tricky to install unless they prefabbed truss module groups on the ground first. With metal deck at least they have a work platform to stand on as they go.

I'll poke around UL and see if there are other assemblies that use wood purlins or timber decking with light gauge metal roof trusses.

 

[XR250]

Maybe 1" plywood. Probably would have to buy 1 unit min. though

 

[Aesur]

@jayrod12 beat me to it, but I was going to suggest 3/4 or thicker sheathing if needed, check the APA span tables, I would think there is a standard plywood that would work for that loading and span.

 

[bones206]

The 1" sheathing does look viable. For diaphragm design, do you just use the SPDWS capacity values given for 3/4" sheathing?

 

[XR250]

Probably. I think most 3/4" sheathing is rated for 48" O.C. but not likely for those loads.

 

[bones206]

For what it's worth, I found an older thread with pretty much the exact same question and answer:
thread507-447037

 

[RontheRedneck]

We often do wood trusses 4' O.C. They typically use 2X4 purlins laid flat and spanning over 3 or 4 trusses.

I recognize that doing 2X4 purlins with metal trusses is not the same. But it might be an idea worth considering.

 

[bones206]

The feedback I got from one metal truss manufacturer is that the overall cost of metal decking + plywood versus metal purlins/hat channels + plywood is a wash. So my thinking was that I might as well get a stronger roof and go with the decking + plywood approach.

Just seems like there should be a product that can better fill this 4 ft spacing niche.

 

[XR250]

Why use CFS trusses art all? Fire thing? If so, the plywood is not an issue?

 

[IceNine]

I imagine attaching the plywood to the metal trusses is a nightmare.
Why not have them design the wood trusses for a 4' spacing?

 

[ALK2415]

Could you use two opposite "corrugated sheets" as composite section ?
without the concrete of course.

 

[bones206]

I think it’s architect driven. They needed non-combustible construction for some reason. But the UL they referenced does allow plywood sheathing.

 

[XR250]

Makes sense. I thi9nk I have run into to that before.

I imagine attaching the plywood to the metal trusses is a nightmare.

Depends what gage the top chord is. The winged Tek screws are pretty easy but need like a 16ga top chord to work. I think someone makes some pneumatic pins for this purpose as well. Otherwise, pre-drilling the plywood is the only other way I know. If you don't pre-drill and try to use standard Teks, the plywood can jack up and not pull tight.

 

[bones206]

ALK2415 - that corrugated sheet is the same thing that I referred to as metal decking. Here are some photos of a recent project with this roof system. First photo shows the metal decking being installed and the second photo shows the plywood layer being installed.

Maybe this is the best roof option available for this type of building system, but it would be nice if there were some alternatives that could span 4 ft. I'll ask the contractors I know what they think about the 1" plywood option. The predrilling requirement sounds like it would be a significant drawback.

 

[ALK2415]

Transverse CRS C-shape might works ?
the connection between Truss/joist (top chord) will be a problem though ..
using continuity connection [splice/overlapping] between each two-spans [like third picture] might solve this !