Flat and low slope roofs
Anything under 2:12. Not actually flat — a working low slope roof always has some fall built in — and it needs a membrane rather than shingles.
- Typical pitch
- Under 2:12
- How often we see it
- Occasional
Pitch ranges are trade convention, not code. Code sets minimum slope by material, not by roof shape.
Is this your roof?
- From the ground you cannot see a slope at all, though there should be a slight one.
- The surface is a continuous membrane, gravel ballast, or a coating, not overlapping shingles.
- Water leaves through internal drains or scuppers at the edge rather than a conventional gutter.
In Johnson County
This is the shape we get the most calls about in Olathe, Shawnee and Lawrence, and the one where we most often find work that was not done to the manufacturer's specification. Two Kansas-specific issues. Ponding: a deck with no positive fall will hold water for days after one of our short violent storms, and standing water finds every weak seam. Hail: it strikes a flat membrane perpendicular rather than at a glancing angle, so a storm a steep shingle roof shrugs off can bruise a membrane badly. And where a previous owner shingled a low slope section, that is a code problem, not just a wear problem.
What you can put on it
Material choice on this shape, and where code rather than preference makes the decision for you.
Asphalt shingles
Not permittedUnder the 2018 IRC, section R905.2.2, shingles require a minimum 2:12 slope. Below that they are not a permitted covering, whatever a previous owner may have installed.
TPO
Works wellA white single-ply membrane, heat-welded at the seams. Reflective, which helps with summer heat gain here.
EPDM
Works wellA black synthetic rubber membrane. Durable and long-proven; absorbs heat rather than reflecting it.
Modified bitumen
Works wellAsphalt-based sheet in multiple plies. Robust underfoot, which matters if the roof gets walked for equipment service.
What we check first
The failure points that are specific to this shape, in the order a roofer looks at them.
- Standing water 48 hours after rain, the usual industry benchmark for inadequate fall or blocked drainage.
- Every seam, especially at penetrations and around any rooftop unit.
- Drains and scuppers, for blockage.
- Flashing where the low slope section meets a taller wall.
- Any area that was shingled, which needs correcting rather than patching.
What moves the price
Square footage is a poor predictor on its own. These are the things that actually change the number on this shape.
- Membrane choice and the number of plies.
- Whether the existing build-up has to come off, and how many layers are on there.
- Penetrations — every vent, drain, curb and unit is a detail that must be flashed and a potential leak.
- Whether tapered insulation is needed to create positive drainage where the deck has none.
- Access, which on a commercial building can rival the roofing itself.
Common questions
Can you put shingles on a flat roof?
No. Under the 2018 IRC, section R905.2.2, asphalt shingles require a minimum slope of 2:12. Below that a shingle roof is not a permitted covering, because shingles shed water rather than seal against it and on a low slope water sits long enough to work back under the courses. A low slope roof needs a membrane such as TPO, EPDM or modified bitumen.
Why does my flat roof pond water?
Either the deck was built without positive fall, or the fall it had has been lost through deck deflection or a blocked drain. A properly built low slope roof is not level — it is pitched slightly toward its drainage points. Where the deck cannot be re-pitched, tapered insulation builds the fall back in. Water still standing 48 hours after rain is the benchmark the trade commonly uses to call it a problem, rather than a code test.
TPO or EPDM for a Kansas City flat roof?
Both are proven. TPO is white and reflective, which reduces summer heat gain and can help the cooling load on a building with a lot of roof area. EPDM is black, absorbs heat, and has the longer track record. For most Kansas City buildings the installation quality and seam detailing matter far more than which of the two you pick.
Do you repair flat roofs, or only replace them?
Both. A membrane roof with sound insulation and a localised seam or flashing failure is usually worth repairing. Once there is trapped moisture in the insulation, or the membrane has reached the end of its service life across the whole area, repairs stop being economic. An inspection should tell you which situation you are in, and a moisture survey settles it where it is genuinely unclear.
More answers on the roofing questions page, or see every roof shape side by side.
Often confused with
Shed and skillion
A single plane sloping one way, with no ridge. Common on additions, porches, dormers and modern infill homes.
Mansard
Four sides, each with two slopes, and a nearly flat deck on top. A French shape that turns the roof into a full extra storey.
Butterfly
Two planes sloping inward to a central valley, like an open book. A mid-century modern signature, and entirely dependent on that central channel working.
Ready to Protect Your Home?
Get a free inspection and estimate. EMTs, firefighters & veteran families receive $500 off!