08 · Flat and low-slope
Under 2:12,
shingles are
the wrong answer
Lanais, porch roofs, carports and additions are often built with almost no fall at all, and then shingled anyway because the rest of the house is shingled. On a low slope, water sits and moves sideways, and a shingle roof is a lapped system that assumes water only ever runs down.
Low-slope
- Shingle limit
- 2:12 with doubled underlayment. Below that, no shingles
- System
- Two-ply self-adhered modified bitumen, granulated cap
- Flashlight or kettle
- Neither. Self-adhered only
- Wall termination
- Membrane up at least 8″, into a reglet or behind siding
- Transition
- Shingles above lap over the membrane, never under
- Ponding
- Water still standing after ~48 hours means fall, not membrane
- Penetrations
- Boot, curb or filled pitch pan. Not mastic
- Coatings
- A system on a flat roof. Paint on shingles
Where the line actually is
Shingle manufacturers allow their product down to 2:12 — two inches of rise per foot — and only then with a doubled underlayment or a fully self-adhered base. Below 2:12 the instructions do not permit shingles at all, so a shingled low-slope roof is out of warranty from the day it is finished, whatever anyone told the homeowner.
Plenty of Florida porch roofs are 1:12 or flatter. If yours is shingled and it leaks in every heavy rain rather than in every rain, this is usually why: water backs up at the laps, gets under them and travels.
What we use instead
Self-adhered modified bitumen, in two plies: a base sheet and a granulated cap sheet, fully bonded, with the laps rolled. It is quiet, there is no flashlight and no kettle anywhere near your house, and the granulated surface takes Florida sun far better than a smooth membrane. On a small lanai it is a day's work.
Where a single-ply makes more sense — a bigger deck, a roof somebody walks on — we will say so, and if it is a size or a detail that wants a commercial crew we will tell you that instead of improvising on your house.
The transition, which is where these leak
Almost every low-slope failure we are called to is not out in the field of the membrane. It is at the point where the flat section meets the shingled roof above it, or where it dies into a wall.
Done properly, the membrane runs up the vertical face at least eight inches and is terminated — either into a reglet cut in the masonry, or behind the siding, or under a counterflashing — and the shingles above lap down over the top of it, not the other way round. Done quickly, the membrane is cut off at the joint and a bead of sealant is run along it. That joint is the lowest point of the whole assembly and the sealant is the only thing holding it.
Ponding
Water standing on a flat roof more than about forty-eight hours after rain is ponding, and it is a structural and drainage problem rather than a membrane problem. Membrane will tolerate it for a while, but it grows algae, it accelerates the surface, and it is dead weight where the deck already sags. The fix is either tapered insulation to build fall into the surface, or a drain or scupper in the right place. Recovering a ponding roof with a new membrane and no change to the fall just buys you time at a poor exchange rate.
Penetrations on a flat roof
Every pipe, conduit, condenser stand and cable through a low-slope roof needs a proper detail: a boot, a curb, or a pitch pan filled and topped. A pitch pan is legitimate and it is also the one detail that needs looking at every couple of years, because the filler shrinks. Anything that has been made watertight with a mound of black mastic is a leak that has not happened yet.
Coatings
On a flat roof, a coating is a real system with a real specification — a sound substrate, proper preparation, a stated dry film thickness and a warranty from the coating manufacturer. It can be a sensible way to extend a membrane that is intact but tired. It is not a way to fix a roof that is already leaking, and it is not something to spray on a wet or ponding surface, whatever the man with the truck says. On shingles, as we say elsewhere, a coating is simply paint.
Other work
The rest of what we do
- 01 Tear-off and re-roof Old roof to the deck, deck repaired and re-nailed to code, dried in, flashed and shingled. The only version of a new roof we sell.
- 02 Roof repairs A repair when a repair is honest, and a straight answer when it is money thrown at a roof with two years left in it.
- 03 Leak diagnosis Attic first, roof second, water test third. The stain on your ceiling is almost never under the hole.
- 04 Flashing and pipe boots Step, counter, apron, kick-out, valley metal and boots. Where roofs actually leak, and where caulk pretends to be a repair.
- 05 Ventilation, intake and exhaust Ridge vent is the easy half. The soffit intake feeding it is the half that is usually missing.
- 06 Storm inspection What we look at after a blow, what we will document, and the two things Florida law says a roofer cannot do for you.
- 07 Gutters, fascia and drip edge Gutter apron, hanger spacing, pitch and outlet count. Most rotten fascia is a drip edge problem wearing a gutter costume.