PVC over car decking use ice&water shield?


I’m installing a Mechanical Fastened heat welded PVC roof on a small 40 x 40 office building flat roof that currently has torch down over hot tar on a car decking deck. Car decking is 2x6’s T&G, very strong. My plan is to tear off the existing roof and go with sloped 4 x 4 insulation panels and slope the middle (which now has standing water) to the drains/scuppers. My concern is inside the building has no attic, the ceiling is open and exposed. The tenant plans to use a drop ceiling and put batt insulation over the drop ceiling, or maybe in the bays of the ceiling joists where the car decking can be seen from the underside.
I don’t see much reason to vent, because there’s no air flow, it’s all open underneath. I’m worried about any condensation/water drip on the underside of the PVC sitting top of the foam panels on top of the car decking.

?? Should I use ice/water shield under the foam panels to prevent any water condensation from warm air rising up on the underside and cold air on the topside/outside? -Also we do get some snow in Seattle. Would that cause moisture on the underside? Vent vs. no vent? Maybe there’s another material I can put on the car decking to start off with?


I would not want a vapor barrier such as ice shield under the insulation, it is liable to create more condensation issues than it prevents. It would basically be the same thing as not tearing the old roof off.

If you ever have a roof leak you will most likely not notice it until the insulation is fully saturated. When insulation becomes saturated the entire roof and the insulation will have to be removed.

I would put the insulation on top of the drop ceiling and leave an air space bewtween the insulation and the roof deck so it can breathe.


The other possibility is to calculate the dew point and make sure you have enough insulation on top of the roof to eliminate condensation problems.


Thanks Don. I think we will do the foam panels 3 inches in the middle and then slope it down to the 1/2 inch fan fold insulation panels around the edges. No Ice and water. Then the mech attached PVC over that. That’s what my installer thought also. I had forgotten we did one like that on a house with rigid insulation panels on top of the car decking, which was the exposed ceiling inside that house. After we tore off the old nasty tar roof and we did a PVC mechanical fasten on that one and no call backs for a year or more now, so it must be ok. There was one loose seam from the walker machine and that caused a couple drips right after we completed the job. We found it and welded it, a little 4 inch part of a long seam that maybe the guy started and stopped or was moving too fast. DOUBLE CHECK YOUR SEAMS is the lesson learned there, and I will this time -have them run the sharp tool over all of the seams to double check.