Low-slope residential
The part of the house where shingles do not work
Plenty of houses here have one section that is not steep enough for shingles. It is usually a back addition, a porch roof or a dormer top, and it is usually where the leak is.
Why shingles fail on a low slope
A shingle roof sheds water downhill from one overlapping course to the next. It is watertight because gravity moves the water off it faster than the water can work sideways. Reduce the slope and that stops being true: water moves slowly, wind drives it back up under the course, and the laps become a path in rather than a barrier.
The result is a roof that looks fine and leaks in every long, heavy rain. Putting more sealant on the laps does not change the physics.
What goes on a low slope instead
| System | Where it suits | What to know |
|---|---|---|
| Modified bitumen | Small residential flat sections, porch roofs, rear additions | Torch-applied or self-adhered; a proven, repairable system that tolerates the detail work an old house demands |
| TPO or PVC single-ply | Larger flat sections and additions with simple geometry | Heat-welded seams rather than adhesive; a welded seam is as strong as the sheet |
| EPDM rubber | Simple rectangles with few penetrations | Well proven and easy to repair; seams and terminations are where attention goes |
| Standing seam metal | Porch roofs and dormer tops on visible elevations | Works down to lower slopes than shingles and looks right on historic elevations |
The details that decide it
- Where the low slope meets the steep slope. The transition from shingles to membrane is the most common failure point on this kind of roof, and it needs to be built so the shingle water runs over the membrane, never under it.
- Terminations at a wall. A membrane has to be mechanically terminated and counterflashed, not glued to the siding.
- Drainage. A low-slope section needs somewhere for the water to go, and that outlet needs to be bigger than the debris that will find it.
- Penetrations. Every pipe, conduit and support post through a membrane needs a proper boot or a fabricated curb.
If the flat section on the back of your house has been "repaired" three or four times, check what is actually up there. A shingle roof on a low slope cannot be repaired into working, and that is the most common finding on this kind of call.
Have a back addition or porch roof that keeps leaking?
Request a free inspectionHow do I know what pitch my roof is?
Pitch is the rise over twelve inches of horizontal run. A roof you could comfortably stand on without holding anything is usually 4:12 or less. If a section looks flat from the ground, it is almost certainly below the shingle minimum.
Can the flat section be done at the same time as the main roof?
Yes, and it usually should be. They are different systems and different materials, but the transition between them is a single detail and it is best built by one crew at one time.
Is a rubber roof still a good option?
EPDM is a mature system that is easy to repair and performs well on simple shapes. On roofs with a lot of penetrations and transitions, a heat-welded membrane usually details better.
All work is performed by licensed and insured local roofing professionals.