How to Flat Roof: The Slope
Positive drainage is the first and most important factor to consider with any low-slope roofing system. You don’t need much slope, just ¼”:12 is sufficient. That’s enough to keep water from ponding on the roof, which can cause leaks and premature roof failure. If the slope isn’t in the structure, install a tapered system between the roof deck and the roofing.
How to Flat Roof: The Base Sheet
The low-slope roofing system I use and recommend is the three-ply, self-adhering type. It gives you a professional grade roof without the hazards of torches and hot asphalt. I recommend a mechanically attached base sheet, so the roofing can be removed when the time comes. Be sure to follow the required nailing pattern, as the fasteners will be the only thing holding the roof down. If there is an adjacent sloped roof, remove enough shingles so you can run the base sheet about 16″ up the slope.
How to Flat Roof: The Smooth-surfaced Mid-ply
Be sure to use a mid-ply, so your roof doesn’t suffer from the inherent weakness of single-ply roofing systems. It gives you a protective back-up layer in case water penetrates a seam on the cap sheet. Start with a half-wide course, so the seams will end up staggered when you put the cap sheet on. That will prevent a big “hump” at the side laps, obstructing the smooth flow of water off the roof. If there is an adjacent sloped roof, run the mid-ply about 14″ up the slope so it doesn’t terminate at the exact same place as the base sheet.
How to Flat Roof: The Drip Edge
After you have the mid-ply rolled down nice and tight, it’s time to install the drip edge. Lap the metal at least 3″ and seal under the lap with asphalt roofing cement. After the metal is tacked on, go down to the ground to take a look. The drip edge is about all you see of the flat roof from the ground, so you want it looking good. Then nail the metal off with regular roofing nails spaced no more than 6″ apart. After the drip edge is nailed down, coat it with asphalt primer using a paint brush or small roller. Be careful it doesn’t drip down the face of the drip edge.
How to Flat Roof: The Granular-surfaced Cap Sheet
When installing the cap sheet, start with a full width course and let it overhang the drip edge by about ¼”. After it’s on, you can trim it off nice and flush with a straight blade in your utility knife. Be sure to use MB (modified bitumen) cement on the end laps, as regular flashing cement is not compatible. Terminate the cap sheet 12″ up any adjacent sloped roof. By staggering the three plies in this manner it avoids a hump under your shingles that could hold water and leak.