# Roof UL Assembly Question



## ETThompson1 (Mar 25, 2019)

Hi

Here's a good one for you guys...not sure how much you get into UL assemblies, but...

We do a lot of multifamily apartment projects which are Type VB, 3 stories, flat roofs with wood truss construction.  For a long time we've used certain 1-hour rated UL roof assemblies, so we can stop the fire partitions between the units (allowed by 420.2) at the ceiling.  The roof assembly for Type VB does not need to be rated but we rate it because that allows us to terminate the UL wall assembly at the rated ceiling, instead of having to run up to the underside of the roof sheathing.

We call for UL P538 or P522 and have gotten these approved and permitted for years.  An RFI brought up the issue of whether this assembly is OK for flat roofs.  The UL language (for P538, P22 is the same) starts off with "_Pitch or Parallel chord trusses, ..._." etc. which I guess without thinking about it too much I'd always taken to mean that flat roofs (parallel trusses) are OK.  However there is some language following which says this "_Where the truss intersects with the interior face of the exterior walls, the min truss depth shall be 5-1/4 in. with a min roof slope of 3/12 and a min. area in the plane of the truss of 21 sq/ft_."  (Full truss section below).  

A debate has broken out in our office if this means that the roof must be no less than 3/12 pitch, and cannot be flat.  My reading is no, that limitation only applies when you have the truss depth of 5-1/4", but the wording is confusing.  Those who think we are limited to 3:12 say that the parallel chord language could apply to pitched 3:12 roofs with parallel truss chords, which is possible.  We did consult the USG which sided with the 'must be 3:12' guys.

I'm just skeptical as we've built and gotten this design permitted for years with different plans examiners, but wondered what people here thought.

http://productspec.ul.com/document.php?id=BXUV.P538

"_2. *Trusses —* Pitch or Parallel chord trusses, spaced a max of 24 in. OC, fabricated from nom 2 by 4 lumber, with lumber oriented vertically or horizontally. Truss members secured together with 0.040 in. thick galv steel plates. Plates have 5/16 in. long teeth projecting perpendicular to the plane of the plate. The teeth are in pairs facing each other (made by the same punch), forming a split tooth type plate. Each tooth has a chisel point on its outside edge. These points are diagonally opposite each other for each pair. The top half of each tooth has a twist for stiffness. The pairs are repeated on approximately 7/8 in. centers with four rows of teeth per inch of plate width. Where the truss intersects with the interior face of the exterior walls, the min truss depth shall be 5-1/4 in. with a min roof slope of 3/12 and a min. area in the plane of the truss of 21 sq/ft._"


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## jar546 (Mar 25, 2019)

No takers?

I just got home and am exhausted, otherwise I'd take a stab at this one.


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## ETThompson1 (Mar 25, 2019)

LOL, guess not.  Rest up and we'll try again tomorrow.


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## steveray (Mar 26, 2019)

Min. 3/12 is what it says.....


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## ETThompson1 (Mar 27, 2019)

Well, interestingly enough, we had a contact/official at UL and they confirmed that we are able to use this design for a low-slope roof.  I might tell them they should hire some English majors!


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## kslifter (Thursday at 3:07 PM)

@ETThompson1 

This is a few years old, but I was looking into this very subject matter today and came across the P522 assembly in my searching for a 1 hour low slope roof assembly. Have you continued to have success using that assembly for low slope roofs? If so, is there some documentation or a person to contact to verify the use of this assembly?

Thanks


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