Low Slope Roofs

A low-slope roof is any roof too flat to shed water the way shingles need it to, generally under about a 2:12 pitch, meaning it rises less than two inches for every foot of horizontal run. Most homes have some. Porch roofs, additions, dormers over a bedroom, sunroom roofs, flat sections behind a parapet, and the roof over an attached garage.

The reason it matters: shingles are not a waterproof surface. They shed water downhill. Below about 2:12 water no longer moves off fast enough, it works backward under the courses, and the roof leaks no matter how well the shingles were installed. A low-slope roof needs a membrane, a single continuous waterproof layer, and that is a different material and a different trade.

Three Membranes for Residential Low-Slope Roofs

EPDM

Ethylene Propylene Diene Terpolymer is a synthetic, black rubber membrane, most commonly used on commercial facilities. It’s a black membrane, so it isn’t recommended for residential use over a bedroom or living space because it will absorb heat, however it could be installed over a garage or non-living space. EPDM is extremely durable and is basically like having a bulletproof rubber roof system that lasts 25-30 years. 

TPO

Thermoplastic Polyolefin is a single-ply white membrane used in both commercial and residential roofing. Unlike EPDM, TPO’s white membrane reflects heat instead of absorbing it. If you have a flat roof or low-slope dormer over a bedroom, TPO is a great option because it will keep the room cooler, however if you have any windows overlooking the membrane, the white TPO will get much dirtier, much faster than black EPDM. While it’s not as durable as EPDM, you can still expect a TPO membrane to last 25 years – as long as it’s installed properly! 

PVC

Polyvinyl Chloride is also a single-ply white membrane used in commercial and residential roofing. PVC is also a great choice for any flat or low-slope roof over a living space or bedroom. Aside from both being white, PVC and TPO are very similar in their characteristics, except their chemical make-up. Even the installation process or almost identical. The only major difference is that PVC is a bit more flexible and has been around longer than TPO. Like TPO, you can expect to get 25 years out of a PVC roof as well, again as long as it’s installed properly. 

Compare this material to our other Roofing Types, or explore our full Commercial Roofing service line.

Modified Bitumen and Metal, the Other Two Options

Two more worth knowing about, because a single-ply membrane is not always the answer on a house.

• Modified bitumen. A multi-ply asphalt-based system, torch-applied, self-adhered or cold-applied. Its advantage on a residential low-slope roof is redundancy and toughness underfoot, which suits a roof deck or balcony that people actually walk on. Self-adhered versions avoid open flame near a house, which is the version we prefer on occupied homes.
• Standing seam metal. Rated for low slope down to a shallower pitch than shingles, and the right answer where the low-slope section is visible, a front porch roof, a bay window roof, a dormer facing the street. It also ties in cleanly to a shingle field. See metal roofs.

The Transition Is Where These Leak

Almost every low-slope leak we are called to on a house is not in the middle of the membrane. It is at the joint: where the low-slope section meets the steep-slope shingle roof, or where it meets a wall.

• The shingle-to-membrane transition. The membrane has to run up under the shingles far enough that water coming down the steep roof lands on membrane, not on a seam. Get this wrong by a few inches and the roof leaks in heavy rain only, which is why it takes years to diagnose.
• Wall terminations. The membrane turns up the wall and terminates under counterflashing or a termination bar. Sealant alone at that joint is a two-year solution.
• Kickout flashing where the roof edge meets a wall. Missing it sends water straight into the wall cavity.
• Inside corners and dead valleys. Where two planes meet and water has nowhere to go quickly. These need the most careful detailing on the roof and get the least.
• Penetrations. Vents, pipe boots, condensate lines and satellite mounts, all of which need to be flashed into the membrane rather than sealed on top of it.

Drainage: The Part You Cannot Coat Your Way Out Of

A low-slope roof still needs to drain. Water that sits for days after a storm accelerates membrane aging, adds weight, and eventually finds a lap. If your porch or addition roof ponds, that is a slope problem, and no membrane and no coating solves it.

The fixes are physical: tapered insulation to build slope, a cricket to divert water around a chimney or curb, additional scuppers or a drain relocated to the actual low point, and gutters and downspouts sized for the area. It costs more than a membrane replacement and it is the difference between fixing the roof and repeating it.

Common Problems We Find on Residential Low-Slope Roofs

• Shingles installed on a slope that cannot carry them. Extremely common on porch roofs and additions, especially on homes where a previous owner did the work. The roof was destined to leak from day one.
• Roll roofing nearing the end of its life. Older homes often have granulated roll roofing on low-slope sections, and it does not last like a modern membrane.
• Ponding behind an addition. Where a newer roof plane created a valley nobody engineered.
• Black EPDM over a bedroom. Works fine as a roof, makes the room hot. Detailed above.
• Failed transitions to the main roof: the leak that only appears in wind-driven rain.
• HVAC condensate and mini-split lines run across a membrane and sealed rather than flashed.

Cost and When to Do It

Low-slope sections are small in area and detail-heavy in labor, so cost per square foot is higher than the main roof while total cost is usually much lower. The efficient time to replace one is when the main roof is being replaced: the transition detail gets rebuilt properly, one crew handles both sides of the joint, and you pay for one mobilization instead of two.

If the main roof has years left and the low-slope section does not, it can absolutely be done on its own. Just have whoever does it rebuild the transition rather than tucking a new membrane under the existing shingle course and hoping. See roof replacement and financing.

Book a free roof inspection if you have a leak you cannot place, low-slope transitions are the most commonly misdiagnosed leak source on a house. Compare this material to our other Roofing Types, or explore our full Residential Roofing service line. For commercial buildings, see our commercial roofing systems and the full Commercial Roofing line.

Frequently Asked Questions