Wood Roof Deck in Type II Construction

[MKALLAY]

I am looking at a project where a wood roof deck and walkway is proposed on a Type IIA Construction Class building. The project is under DC Building Code (IBC 2015). I believe this would be permitted per 603.1 exception 1.3, provided the deck is constructed entirely with fire retardant treated wood - is this correct? Any thoughts or comments?

 

[RLGA]

My interpretation is that the rooftop decking becomes a "roof covering" by IBC definition and becomes a part of the "roof assembly." Thus, the rooftop decking must comply with Section 1505 and be classified per ASTM E108 or UL 790, or, for wood, tested per ASTM D2898.

 

[MKALLAY]

RLG, would that be the case if the walking surface and structure was raised entirely above the (existing) roof membrane?

 

[RLGA]

RLG, would that be the case if the walking surface and structure was raised entirely above the (existing) roof membrane?

Yes. The tests in the code determine a roof covering's susceptibility to flame spread due to exposure from burning brands (i.e., falling embers). Since rooftop decking covers more roofing membrane than leaves exposed, it is more likely that the rooftop decking will be more exposed to burning brands than the roofing membrane underneath.

Although serving different purposes, the roofing standard, ASTM E108, is similar to the interior finishes standard, ASTM E84, in that they are testing materials for flame spread due to fire exposure. Although not equal, you may be able to justify a wood material tested per ASTM E84 and has a flame spread of 75 or less (Class B or better) as being compliant with Table 1505.1 for a Class B roof covering (minimum required for Type IIA construction).

 

[steveray]

How high is the roof? Not sure what 2015 had for occupied roof vs. high rise.....

 

[MKALLAY]

How high is the roof? Not sure what 2015 had for occupied roof vs. high rise.....

The building is not a high rise - the deck in question is at a lower roof, approx 20' above grade plane.