How to waterproof a 1/12 roof section that runs right under a window sill

Start with the slope. The IRC allows mineral-surfaced roll roofing down to 1:12 and asphalt shingles only from 2:12, while single-ply membranes (EPDM, TPO and PVC), modified bitumen and built-up roofs can go as low as 1/4 inch per foot, according to The Roofing Brief's summary of sections R905.2.2, R905.5.2 and R905.9.1 through R905.13.1. At 1/12, roll roofing sits exactly at its limit. A membrane has more margin on that slope.

The wall is the hard part. NRCA's detail MB(HC)-1 and the published details from GAF, Holcim and Sika all carry base flashing 8 inches or more up the wall, measured from the finished roof surface, with metal counterflashing lapping its top edge by about 4 inches, The Roofing Brief reports. A sill an inch or two above the deck leaves no room to turn a TPO sheet up the wall and terminate it. Base flashing that stops short is one of six install errors behind most parapet leaks on The Roofing Brief's list, because snow and wind-driven rain go over the top edge.

Liquid-applied flashing is made for this kind of spot. According to Sika, its PMMA resin goes on with a polyester fleece and hardens into one continuous reinforced membrane. It bonds without separate adhesive, and Sika says it handles tight clearances and odd shapes more easily than standard flashing products. The field membrane can stop short of the wall, and the reinforced liquid layer can carry the waterproofing up to the sill. Check that the resin is approved for the field membrane on the job.

Any upturn under 8 inches is outside those published minimums, so get the membrane maker's written approval of the detail before work starts. If the owner later agrees to raise the sill, a few more inches of wall make room for a standard upturn and counterflashing.