Based on my experience, I have seen a lack of construction documents that actually recognize rooftop conditions. Let me provide some examples:
The Exterior Structural Steel Stud Wall/Parapet
My recent involvement in more than a few legal cases has led me to believe that one of the most egregious design issues is at the roof edge, involving the use of structural steel studs in the wall and extending them up above the roof deck to become the parapet. This development, the apparent result of continuous insulation and air barrier requirements, has created a litany of concerns.
First, few are blocked off and the ones that are blocked off are improperly sealed. Now you have created a perfect air plenum, bringing warm, moist air up to the parapet where condensation is occurring when the exterior temperature is right. This results in deteriorated sheathing, mold growth and the re-emulsification of those water-based adhesives or the deterioration of the facer on the substrate. Now the adhesive bond to resist wind forces is gone and— you guessed it— up comes the roof.
Air movement up stud plenum aside, there is another issue and that is the securement of the roof membrane at the wall. Depending on design, it might be possible to drive the screw into the roof deck (possibly damaging the vapor retarder) but often the screw goes horizontally. The vertical substrate boards are not strong enough to resist the pull-out and now a condition has been created that is at high risk for anchor pull-out and substantial roof damage. Hint: Install a 16-gauge plate on the exterior side of the stud, behind the sheathing, into which the screw can be installed.
My overall recommendation would be to avoid the stud wall/parapet. It’s too risky. However, if you design a stud wall/parapet, be sure to seal between the studs, exterior facing substrate and the roof-deck level. The seal is to prevent any air moving from the conditioned space to the parapet.
Before we begin discussing freezer buildings (those buildings kept at -20 F), let’s remember the effects of extreme cold on building materials. You don’t remember? I see that all the time! The physics of freezer buildings are that the warm, often moist exterior air is trying to be pulled into the cold, dry freezer building with great force. Thus, the exterior building façade is the vapor retarder. If compromised, the effects are quite dramatic:
- Insulation becomes ice-locked and icicles form on the interior and they can be quite large.
- Roof systems can become blocks of ice and condensation runs amuck.
A 4-by-8-foot board of polyisocyanurate insulation shrinks 1/2 inch under -20 F temperatures. On a roof of 96 feet, that is 6 inches of gapping, allowing cold air to permeate up and resulting in a thermal short costing dollars.
Understanding how a building will function— in this case as a freezer building— and the effects on the building materials might lead a design professional to realize he or she should design for building contraction, air-intrusion resistance, seal the roof edge and incorporate redundancy to account for design mishaps and construction tolerances.