Plywood as a nailing surface roofing substrate

[Examiner]

Construction Type II-B: Section 603.1; 1., 1.3

Can plywood that is attached to the top surface of a structural metal roof deck be allowed as a non-combustible item in the composition of the roof? It appears that Item 1.3 of the above referenced section is regarding the fire retardant wood (that is permitted in roof construction for Construction Type II-B) as a structural component of the roof. If the plywood is only there for a nail base (blocking) and is not a structural item then it seems that fire retardant wood is not required. It would be no different than strips of wood furring for attachment of a standing seam roof system or combustible allowed blocking in a wall.

The ICC has approved the Tectum sandwich panel as a non-combustible roof deck. The Tectum panel is composed of cementious wood fiber, polyiso in a varity of thicknesses and OSB board on top of the polyiso. The OSB is the nail base for the finished roof covering.

Input welcome.

 

[barlovian]

Re: Plywood as a nailing surface roofing substrate

I just found my book with the formal interp from BOCA 1996. The committee determined that it was permitted.

The exact question was whether combustible wood structural panels are allowed in roof construction of noncombustible buildings for the purpose of fastening a roof covering.

The answer was “Yes,…as long as a noncombustible roof deck is provided as the structural element, then foam plastic insulation, wood structural panels, nailing/furring strips, and roof coverings may be applied.”

This isn't an exact fit with the IBC in 603.1 because Item (4) allows combustible roof coverings with a Class A, B, or C classification but there is no separate Item to address insulation or furring that could be installed between the roof deck and the Class C roof covering.

After that interp came out, I just looked at roof assemblies to see if the roof deck met the noncombustibility requirements and that the specs showed that the roof covering had the correct classification. When IBC was published, this part was no clearer than it was in BOCA.

 

[Gene Boecker]

Re: Plywood as a nailing surface roofing substrate

The question is whether or not the plywood is simply a nailing base or if it has any structural qualities. If it has structural qualities, then it is acting as part of the "roof" and needs to meet the criteria for non-combustible (as allowed or exempted). If the plywood has no structural basis on the roof but is simply fastened to the metal structural deck, the the plywood can be viewed as part of the "roof covering" which is allowed to be whatever it can be as long as it is Class A, B or C as required.

 

[brudgers]

Re: Plywood as a nailing surface roofing substrate

The question is whether or not the plywood is simply a nailing base or if it has any structural qualities. If it has structural qualities, then it is acting as part of the "roof" and needs to meet the criteria for non-combustible (as allowed or exempted). If the plywood has no structural basis on the roof but is simply fastened to the metal structural deck, the the plywood can be viewed as part of the "roof covering" which is allowed to be whatever it can be as long as it is Class A, B or C as required.

So long as the plywood isn't required to support gravity loads or resist lateral loads, that makes sense to me.

At that point it's as much of an underlayment as roofing felt or a slipsheet and I don't believe those are required to be non-combustable.

 

[morroof]

Brudgers is right. You won't need to think about the plywood so much since it is just for a nail base.