Flat Roofing in St. Louis

Where flat roofs show up in St. Louis

In the city, 98,061 of the 174,310 housing units counted by city-data.com from census figures were built in 1939 or earlier. The National Park Service's file on the Shelley House, a 1906 two-family flat on Labadie Avenue that it calls typical of the bulk of the city's historic housing, describes a flat asphalt roof behind parapet walls, plus two flat-roofed additions at the back. The same file notes that whole blocks of two- and four-family flats were often built at once, differing mainly in roof type, windows and detail; the St. Louis City page covers the permit side.

Parapets change the drainage. Water cannot spill over the edge, so the residential code wants a drain at each low point unless the roof is sloped to drain over its edges (R903.4), and an overflow drain or scupper wherever a raised edge would trap water if the main drain clogs (R903.4.1). Those citations are to the 2018 IRC the city enforces; Missouri has no statewide code, and Brentwood, for one, has moved on to the 2024 edition.

In the suburbs, a common flat roof is the porch, carport or rear addition tied into a sloped roof, and too often it was shingled at a pitch too low for shingles. The code draws hard lines:

EPDM, TPO or modified bitumen

EPDM. The EPDM Roofing Association says the membrane can be fully adhered, mechanically fastened or ballasted, and that even aged EPDM can be repaired with simple tools. It also reports that EPDM passes FM 4470 testing, which includes hail, and that hail warranties covering 1-inch and 2-inch stones are sold.

TPO. Carlisle, a TPO maker, says a properly heat-welded seam ends up the strongest part of the membrane, and that dirt or adhesive overspray in the seam area causes weak bonds it calls cold welds. The seam is only as good as the welding.

Modified bitumen. By ARMA's account, a torch lets crews install it in any season, yet over a combustible deck the torch method calls for a separator sheet underneath; the cold-adhesive and self-adhering versions need 45 to 50°F and climbing. Its multiple plies give a backup layer the single-ply sheets do not.

Slope, drains and ponding water

A flat roof should be dry within a couple of days of a storm. The building code's definition of positive roof drainage is a roof that drains within 48 hours of precipitation (2018 IBC, Chapter 2), and the residential code builds membranes to a quarter inch of fall per foot. Water that stays longer is ponding, and ponding finds every weak seam.

Storms here can dump water faster than a small drain can take it. By NOAA's Atlas 14 figures, a ten-year storm in the city drops 2.10 inches in a single hour. That is why the code wants a backup outlet behind a parapet: an overflow drain the same size as the main drain with its inlet 2 inches above the low point, or a scupper three times the drain's size and at least 4 inches tall (R903.4.1).

Re-roofing gets some slack. A replacement or recover on a roof that already drains within that 48-hour window does not have to be rebuilt to the quarter-inch slope or gain new overflow drains (IRC R908.1; IBC 1511.1). The EPDM Roofing Association adds that ponding alone will not void an EPDM warranty, but that roofs should be designed to drain, since standing water raises repair costs and lets more moisture in through any puncture.

Roof coatings: when one is worth buying

The code lets a new protective coating go over an existing coating, modified bitumen, EPDM, TPO, built-up roofing or metal without a tear-off (2018 IRC R908.3.1). Whether it is worth doing depends on the roof underneath. The Roof Coatings Manufacturers Association's own answers set the limits:

• Do not expect a coating to locate a leak or close it. Leaks get traced and repaired, and the roof dried out, before any coating goes on.
• Not every coating tolerates ponding water; check that the product is rated for it if your roof holds any.
• Water-based products want an air temperature of at least 50°F that is climbing, and a dry day ahead.
• Gravel-surfaced and ballasted roofs are not usually coatable without special treatment.
• A properly applied coating can add up to 10 years or more of roof life, and the roof should be checked twice a year and after major storms.

The EPDM Roofing Association notes that acrylic coatings can go directly on a prepared EPDM surface, and advises asking the membrane maker before using other paints so the warranty stays intact. InterNACHI's chart gives an aluminum coating 3 to 7 years. A coating makes sense on a sound, dry roof with years left in it; on a roof with wet insulation or failing seams, it is money spent hiding the problem.

Repair, recover or replace

A single open seam, a split around a pipe or a loose edge is repair work, and EPDM in particular takes repairs well into old age, per its trade association. A new roof is the better buy once the trouble is spread across the whole surface, the insulation under the membrane is wet, or the roof has outlived the InterNACHI estimates in the table above.

The code lets a second membrane go over the first only if the new product's instructions allow it. It bars that outright when the existing roof is soaked or worn out, or when two layers are already up there (IRC R908.3.1.1). A full replacement strips everything down to the deck (R908.3), and rusted or damaged flashings, edgings and drain outlets get replaced either way (R908.5).

Permits follow the material. St. Louis City requires one whenever the new roof differs from the old, so switching a tired modified bitumen roof to TPO needs one, and Maryland Heights requires a permit for TPO roofs outright. In the Fox Park historic district, the district standards treat replacing a flat roof as routine maintenance, though the building code still applies. We file for the permit wherever one applies. The price on a flat roof turns on its size, the membrane, how much wet insulation or decking has to come out, and how many drains, scuppers and rooftop curbs need flashing.

Commercial flat roofs

Storefronts, warehouses and larger apartment buildings come under the International Building Code's roofing chapter rather than the residential code. The 2018 IBC carries the same quarter-inch minimum slope for modified bitumen (1507.11.1), EPDM (1507.12.1) and TPO (1507.13.1), and adds commercial-only rules:

• Roof drains and overflow drains sized under the plumbing code (1502.1, 1502.2), with scuppers sized so ponding never exceeds what the structure was designed to carry (1502.3).
• Metal edge on low-slope built-up, modified bitumen and single-ply roofs designed for wind and tested to ANSI/SPRI ES-1 (1504.5); gutters are excepted.
• Ballasted single-ply roofs designed to ANSI/SPRI RP-4 (1504.4).
• Any penetration wider than 30 inches, a rooftop-unit curb included, gets a cricket on its upslope side to split the water around it (1503.5).
• On a re-roof, existing secondary drains or scuppers stay unless replaced by a code-compliant system (1511.1).

Across the river, the license changes too. Illinois' Roofing Industry Licensing Act limits a contractor with a limited license to residential buildings of eight units or fewer; commercial and industrial roofing takes an unlimited license, backed by a $25,000 bond instead of $10,000 (225 ILCS 335). Ask to see the license number before work starts on a Metro East building, whether it is a Collinsville storefront or a Belleville warehouse.

Common questions about flat roofing in St. Louis

Where we work

Flat roofing across greater St. Louis, from the city and St. Louis County out to St. Charles, Jefferson and Franklin Counties and across the river into the Metro East. Each city has its own page with the permit rule and code edition that apply there: