Roofing Services
Flat and Low Slope Roofing
At a Glance
- Under about a two in twelve pitch, shingles do not belong
- Membrane is a sealed skin, not overlapping pieces
- Ponding water is the enemy and it is fixable
- Porch and addition roofs here are usually low slope
Plenty of Davenport houses have a flat section even though the main roof is steep. A porch, a back addition, a dormer top, a garage. Those sections leak more often than the rest of the house combined, usually because somebody put shingles on them.
Why Shingles Fail Up There
Shingles work by overlapping, and they rely on water running off quickly. Below about a two in twelve pitch water slows down, sits, and finds its way sideways under the laps. Wind driven rain does it even faster.
A low slope roof needs a continuous membrane with sealed seams, so there is no lap for water to work under in the first place.
The Three Systems Worth Knowing
| System | What It Is | Where It Fits |
|---|---|---|
| TPO | White welded plastic sheet | Commercial, and anywhere reflecting heat helps |
| EPDM | Black rubber sheet, seamed with tape | Small residential flats, simple shapes |
| Modified bitumen | Asphalt sheet in layers, torched or self adhered | Porches and additions, tough underfoot |
Each one has a full page in the materials library with lifespans and the honest weaknesses.
Ponding Water
Water still standing forty eight hours after rain is ponding, and it is a problem on any system. It works on every seam continuously, it grows algae, and in winter it freezes and expands inside whatever it is sitting in.
The fix is slope, usually built with tapered insulation to move water toward a drain, or a relocated drain. Adding another layer of membrane over a ponding roof just makes a newer puddle.
Questions About Flat and Low Slope Roofing
How long does a flat roof last?
Twenty to thirty years for a well installed membrane on a roof that drains. Considerably less on one that ponds.
Can you put a flat roof over the old one?
Sometimes, if the existing roof is dry and there is only one layer. If there is wet insulation underneath, covering it traps the water and rots the deck.
My porch roof leaks every spring. Why spring?
Snow sits on a low slope all winter and melts slowly into every seam. Spring is when a flat roof reports on its winter.
Get It Looked At First
No sales call. A roofer looks at the roof and tells you what it needs.