Flat Roofing in Portland, Maine

Flat is never quite flat

A flat roof is built with a slight pitch toward its drains. The 2021 code sets the minimum design slope for each low-slope membrane at a quarter inch per foot, 2 percent: built-up roofing (R905.9.1), modified bitumen (R905.11.1), thermoset single-ply such as EPDM (R905.12.1) and thermoplastic single-ply such as TPO or PVC (R905.13.1). Coal-tar built-up roofs are the exception, allowed down to an eighth of an inch.

The drainage rules (R903.4) follow from that:

• unless the roof sheds over its edges, it needs a drain at each low point;
• where a parapet or other edge could trap water, it needs overflow drains or scuppers as a backup;
• overflow drains may not be tied into the main roof drain lines.

Re-roofing gets two allowances (R908.1): an existing roof that already drains need not be reworked to reach the quarter-inch pitch, nor gain backup drains or scuppers it never had. Water that stands long after a rain is a sign the existing roof does not drain, and worth fixing while it is open.

EPDM, TPO and modified bitumen

Most flat roofs here are one of three systems, each named in the code's roof chapter:

• EPDM. A synthetic rubber sheet, black or white, laid in wide pieces with taped or glued seams. It is the material in this page's photo.
• TPO and PVC. Thermoplastic sheets, usually white or gray, whose seams are welded together with hot air.
• Modified bitumen. Asphalt-based rolls, torched, mopped or self-adhered, often finished with a granule-surfaced cap sheet.

Older built-up roofs, layers of felt and hot asphalt under gravel, still turn up on commercial blocks. The code bars putting gravel and other aggregate surfacing back on during a re-roof (R908.5).

The choice depends on the deck, the edges and penetrations, the rooftop equipment, and what the building's insulation is doing. On every system, the details carry the job: edge metal, wall flashing run up under siding or counterflashing, curbs for vents and units, and the drain bodies themselves.

Snow and ice on a flat roof

The emergency management agency's figures, passed on from the Insurance Institute for Business & Home Safety, show how fast weight adds up on a roof that cannot shed it. Ten to twelve inches of fresh snow, or three to five of packed snow, is about five pounds per square foot; an inch of ice counts as much as a foot of new snow; a four-foot pack, half old and half fresh, can reach 60. Against that, the agency puts a typical roof's spare capacity at about 20 pounds per square foot.

On a flat roof the danger signs inside are a ceiling that sags or cracks, doors on the top floor that start to stick, and new leaks. Get people out and have the building checked if you see them.

Shoveling a membrane roof is work for someone who knows where the seams and flashings are; a shovel edge can cut rubber. The ice storm of 1998 is a reminder of the stakes: the Weather Service counted a roof collapse among its deaths.

Which code and which contract law apply

Two lines cross on buildings like these, and they fall in different places.

• The building code. Maine's residential code is its rule for one- and two-family dwellings. A three-unit triple-decker or a mixed-use block answers to the commercial side of the Maine Uniform Building and Energy Code instead, which adopts the 2021 International Building Code and Existing Building Code. Ask the town's code office which applies before the scope is written.
• The contract law. Maine's Home Construction Contracts Act covers a residence of three or fewer living units, so a triple-decker owner gets its protections: a written contract for work over $3,000, signed by both sides, with no more than a third paid up front. A building of four or more units, or one used for business, falls outside the Act, and the terms are whatever the contract says, which makes reading it closely more important.

Maine adopted the 2021 energy code without changing its rules for existing buildings or roof replacements, and Efficiency Maine puts the attic level under the current codes at R-60. Where insulation on a flat roof sits above the deck, ask the code office whether the replacement has to add to it, and plan the edge and flashing heights for the added thickness.

Permits, historic districts and roof shapes

• Portland: Munjoy Hill and India Street, where many triple-deckers stand, are among the city's historic districts. Work that cannot readily be seen from a public way at pedestrian height is exempt from preservation review, which can include a flat roof hidden behind a parapet. A structural change, such as new joists, needs a building permit.
• York: the historic district guidelines allow rubber membrane roofing only where it is minimally visible.
• Kittery: in the Kittery Foreside mixed-use zone, main buildings take gable, gambrel, saltbox or hip roofs; flat or nearly flat shed roofs are not allowed on new buildings or on big expansions.
• Richmond: new principal buildings in the Village District need roofs pitched 6 in 12 or steeper, and flat or shed roofs are not normally accepted.
• South Portland: a property in a flood hazard area needs a flood hazard development permit before any building permit.

Rooftop units, solar and new curbs add electrical or mechanical permits in most towns.

Repair, recover or replace a flat roof

Signs a flat roof needs attention: open or lifting seams, blisters, cracks along the edge metal, flashing pulling off walls and curbs, water standing for days, and soft spots underfoot that mean wet insulation below.

• Repair fits a sound roof with local damage: a seam, a flashing, a puncture by a dropped tool.
• A coating is one of the recovers the code allows over an existing roof without a tear-off (R908.3.1), on a roof that is dry and sound.
• Replacement is needed when the roof or its insulation is wet through, or when two roofs are already in place; the code bars a recover in either case (R908.3.1.1).

The code will not let a new roof go over a soaked one, so wet insulation found in a cut test or from below comes out. What moves the price: the area and the number of penetrations, how much wet insulation and deck is replaced, edge and wall flashing, drains and scuppers, rooftop equipment that has to be lifted, and access on a busy street.

Common questions about flat roofing in Portland

Where we work

Flat roofing across greater Portland and southern Maine, from Casco Bay and the York County coast to the Sebago Lake towns. Each city has its own page with the permit rule and code edition that apply there: