A roof too shallow for shingles needs a membrane, and a membrane lives or dies by drainage, seams and edges. Here is how the three common systems differ, when a coating or a recover is allowed, and what the coast adds.
Very few roofs are truly flat. The ones people call flat usually have a little pitch, a quarter of an inch per foot or so, and the whole design depends on that pitch moving water to a drain, a scupper or an edge before it sits. On houses they usually turn up over porches and additions, on shallow rear sections and dormers, and on garages. On storefronts and small commercial buildings they are often the whole roof.
This page covers where shingles stop and membranes start, the three membranes you will see on quotes, how water has to leave a low roof, when the code allows a coating or a second layer instead of a tear-off, what hurricane wind does at the edges, the details at curbs and drains, and the reviews that apply in historic and commercial districts.
The residential code South Carolina enforces draws the lines. Asphalt shingles may not go on a slope under 2 inches per foot, and metal shingles not under 3. Below that, the choices are a membrane or a standing-seam metal system, which the code allows down to a quarter inch per foot.
For membranes the code speaks of a design slope, and for modified bitumen, thermoset single-ply (EPDM) and thermoplastic single-ply (TPO and PVC) alike it is at least 1/4 inch per foot, about 2 percent (Sections R905.11.1, R905.12.1 and R905.13.1). That is the slope water needs to keep moving.
The trouble spots on houses are the places in between: a porch roof pitched at 1.5:12 that someone shingled anyway, or a shallow section where a steep main roof dumps its water. Shingles on a slope that low let wind-driven rain in, because they are made to shed water, not hold it out. If you have one of those, the fix is a membrane or standing seam, not more sealant. The metal roofing page covers the standing-seam option.
All three are recognized by the code, each against its own ASTM standards:
| System | What it is | Code section and standards |
|---|---|---|
| TPO (and PVC) | A thermoplastic sheet whose seams are welded with hot air | R905.13: ASTM D4434, D6754 or D6878 |
| EPDM | A synthetic rubber sheet with seams joined by tape or adhesive | R905.12: ASTM D4637 or D5019 |
| Modified bitumen | Asphalt reinforced and modified with polymers, in rolls set in adhesive, torched or self-adhered | R905.11: ASTM D6162, D6163, D6164, D6222, D6223 or D6298 |
Which one fits depends on the roof more than on a brand: how big it is, how many walls, curbs and pipes break it up, whether anyone walks on it, and what sits underneath. On a re-roof, matching what is there can also make review easier: on the Charleston peninsula, preservation staff can usually approve a TPO-for-TPO replacement over the counter.
A quote should name the membrane by maker and product, its thickness, how it is attached and how its seams are made. Ours name every material by brand and product.
Most low-slope leaks start where water stands. A membrane can tolerate a lot, but a puddle that stays for days keeps every seam and fastener under it wet, and any small opening there gets a steady supply.
Rain here does not give a low roof much time. NOAA's estimates for downtown put a one-hour storm that comes once in ten years above 3 inches, and the gutters page has the short-burst figures. A low roof has to pass that water through its drains, scuppers or edge gutter as fast as it falls.
The code is practical about older buildings. On a re-roof, a roof that already drains positively is exempt from the quarter-inch design slope (R908.1, Exception 1), and it need not gain the secondary emergency drains or scuppers a new roof would need (Exception 2). The words that matter are positive drainage. Where water ponds, the re-roof is the time to add slope with tapered insulation or a cricket, move a drain to the low spot, or open a scupper, and our quotes list that work line by line, including drain and scupper work.
Low roofs give more options than shingle roofs, and the code spells them out:
Section R908.3.1.1 then takes the recover off the table in three cases: the old roof is wet or worn out, it is slate or tile, or two layers are already there. The same sense applies to a coating: it cannot rescue a roof that is already soaked.
Design wind across the three counties runs from 133 to 150 mph by address. On a low roof, wind pulls hardest at the edges and corners, which is why FEMA's post-Ike advisory on metal roof systems, low-slope and steep alike, asks designers to state separate uplift loads for the field, the perimeter and the corners of a roof, along with how wide that perimeter is. On a small roof, it suggests using the corner load everywhere.
What that means on a quote:
The City of Charleston asks for the maker's specifications with any elastomeric roof permit, and Folly Beach asks for roofing specifications that prove its 150 mph rating. Either one is a good test of whether a quote was written for your address.
On a low roof the membrane itself rarely fails first. The details do: the curb under an air conditioning unit, a pipe boot, a drain bowl, the joint where a low roof meets a brick parapet or a wall of siding. The code is short on these points but clear. Under Section R908.5, a re-roof replaces any flashing, edge metal, outlet or vent that shows rust, damage or decay, and R908.6 has new flashing built to the maker's instructions, with metal primed wherever bitumen will be stuck to it.
Near salt water, metal edges, drain rings and scupper boxes deserve the same question as any other metal on the roof: what are they made of, and what are they fastened with.
Equipment adds its own paperwork. Moncks Corner, for one, asks for an engineering letter on the roof and a roof-anchor installation packet before it permits rooftop solar. Rooftop units need curbs tall enough to flash properly and a clear path for water around them. We flash curbs and penetrations as part of the membrane work, not as an afterthought, and each one shows up on the written scope.
Low roofs draw a different kind of review than steep ones:
Every town page on this site lists who issues the roofing permit there; we pull it.
We have not found a current published price for low-slope roofing in this region that we would stand behind, so here is what changes a real number:
Modified bitumen, EPDM and TPO roofs need at least a quarter inch of slope for every foot under the residential code, about 2 percent. On a re-roof, an existing roof that already drains positively is exempt from that rule.
Neither wins everywhere. The choice turns on the roof's size and shape, how many walls, curbs and pipes break it up, foot traffic and what is underneath. On a historic building in Charleston, replacing like with like, such as TPO for TPO, usually keeps the review simple.
The code allows a new coating over most existing membranes without a tear-off, and Charleston wants the maker's specifications with an elastomeric roof permit. A coating only works on a dry roof, though. Over wet insulation it traps the water.
Sometimes. A recover is allowed when the new membrane's maker approves it, but never on top of a wet or worn-out roof, or one that is already two layers thick. Then everything comes off to the deck.
Often because it is too shallow for what is on it. Shingles are not allowed below 2:12, and on very low slopes wind-driven rain gets under them. A membrane or a standing-seam metal roof is the usual fix, along with the flashing where it meets the house.
In most places here, yes. Most of the city, town and county rules researched for this site list roofing as work that needs a permit, commercial buildings may need design review first, and each town page lists who issues the permit. We pull it.
Flat roofing across greater Charleston, from the sea islands and beach towns to Summerville, Goose Creek and Moncks Corner. Each city has its own page with the permit rule and code edition that apply there:
Sources: 2021 International Residential Code as adopted in South Carolina, Chapter 9: R905.2.2, R905.4.2, R905.10.2, R905.11-R905.13, R908.1, R908.3, R908.3.1, R908.3.1.1, R908.5, R908.6 (UpCodes) · NOAA Atlas 14 precipitation-frequency estimates (intensity), downtown Charleston · FEMA, Metal Roof Systems in High-Wind Regions, Hurricane Ike Recovery Advisory (uplift loads by roof zone) · Charleston County Building Inspection Services, Roof Underlayment Form (2021 code, commercial note) · City of Charleston, roof permit guideline (elastomeric roofs: manufacturer's specifications) · City of Charleston, Board of Architectural Review (staff approvals, Historic Corridor District) · Town of Mount Pleasant, Old Village Historic District Guidelines · City of Folly Beach, Building and Planning · City of Goose Creek, building permits · Town of Moncks Corner, building permits and inspections (rooftop solar) · Berkeley County Codes Enforcement (133 mph minimum). Codes, programs and prices change; confirm the current rule with your city before you rely on it.
Free inspection, a written scope, and photos you keep either way.
Book my free inspection →
