What is the minimum slope for a flat roof?

1/4 in of rise per foot, a 2 percent slope, for built-up, modified bitumen, EPDM, thermoplastic single-ply, sprayed foam and liquid-applied roofing under IRC R905. Coal-tar built-up roofing is allowed at 1/8 in per foot. The code calls it a design slope and ties it to drainage.

How much fall does a 16 ft flat roof need?

At least 4 in from the high edge to the low edge or drain, because 16 ft at 1/4 in per foot is 4 in. A 20 ft run needs 5 in and a 24 ft run needs 6 in.

Can I put a new flat roof over the old one?

R908.3.1 permits a recover when the new covering follows the manufacturer's approved instructions. It is not allowed if the existing roof is water soaked or deteriorated, if it is slate, clay, cement or asbestos-cement tile, or if it already has two or more layers.

Does an old flat roof have to be rebuilt to 1/4 in per foot when reroofing?

No, if it provides positive drainage. R908.1 exempts reroofing from the design slope minimum in that case, and also from adding secondary overflow drains or scuppers.

What laps does a self-adhered modified bitumen roof use?

For Flintlastic SA, CertainTeed gives base side laps of 2 in and end laps of 4 in, cap side laps of 3 in and end laps of 6 in, with end laps in neighboring courses offset at least 3 ft.

How cold is too cold to install a flat roof?

It depends on the product. CertainTeed allows Flintlastic SA down to 35°F, with cold weather procedures from 35°F to 49°F. Elevate's EPDM guide adds precautions below 40°F and stops adhesive work if condensation forms.

Do flat roofs need drains?

R903.4 requires drains at each low point unless the roof is sloped to drain over its edges. Where a parapet or similar edge would trap water, R903.4.1 adds overflow drains or scuppers set 2 in above the low point.

Can a homeowner install a flat roof membrane?

CertainTeed states that its Flintlastic roofing products are intended for use by professional roofing contractors only. The code requires installation under the manufacturer's instructions, so those instructions are the standard the work is judged against.

How to Install a Flat Roof

Installing a flat roof is mostly deciding things before any membrane is unrolled: how the roof will fall, whether the old roof comes off, and which manufacturer's instructions the job will follow, because the code defers to them. This is that sequence, with the figures from the code and from two published installation documents.

By Lucas Pradella, builder and editor. Checked 2026-09-16.

A flat roof has a required slope, and it comes first

The finding that shapes every later step: no membrane in the International Residential Code may be laid dead level. Each low-slope covering in Section R905 carries a minimum design slope. Built-up roofing, modified bitumen, EPDM, thermoplastic single-ply such as TPO, sprayed polyurethane foam and liquid-applied roofing all sit at 1/4 in of rise per 12 in of run, which is a 2 percent slope. The sections are R905.9.1 for built-up, R905.11.1 for modified bitumen, R905.12.1 for EPDM, R905.13.1 for thermoplastic, R905.14.1 for foam and R905.15.1 for liquid-applied. Coal-tar built-up roofing is the single exception at 1/8 in per foot.

Several of those sections add the words "for drainage", and that is the point of the number. The fall is what moves water off the sheet. The membrane is only asked to keep out whatever water is still passing over it.

So installing a flat roof starts with a question about geometry rather than product: does the deck already fall a quarter inch per foot toward an edge or a drain, and if it does not, where is the fall going to come from? Framing, sloped sleepers and tapered insulation are the usual answers. The flat roof calculator works the tapered insulation volume for a given run.

What a quarter inch per foot adds up to

The minimum sounds trivial until it is multiplied by a run. This site's arithmetic on the R905 figures:

Fall from the high edge to the drain line, by run and minimum slope
Run to the low edge or drain	At 1/4 in per ft (modified bitumen, EPDM, TPO, built-up, foam, liquid-applied)	At 1/8 in per ft (coal-tar built-up only)	At 1 in per ft (above this, CertainTeed requires back nailing)
8 ft	2 in	1 in	8 in
10 ft	2.5 in	1.25 in	10 in
12 ft	3 in	1.5 in	12 in
16 ft	4 in	2 in	16 in
20 ft	5 in	2.5 in	20 in
24 ft	6 in	3 in	24 in

A 16 ft garage roof draining to one edge needs its high side at least 4 in above its low side. Those inches have to exist before the first roll comes out of its wrapper. The last column is there because the CertainTeed instructions cited below change their fastening rule once the slope passes 1 in 12, so a roof built between the two figures stays in the simpler case.

Where the water goes is also code. R903.4 requires roof drains at each low point unless the roof is sloped to drain over its edges. Where drains are required and a parapet or similar edge would trap water, R903.4.1 adds secondary overflow drains or scuppers, with the overflow inlet 2 in above the low point and scuppers at three times the size of the roof drains with openings at least 4 in high. A roof that sheds over an open edge into a gutter avoids that whole layer; the gutter calculator sizes the receiving end.

The code hands the method to the manufacturer

Chapter 9 says remarkably little about how a membrane goes down. R903.1 requires roof assemblies to be installed following the code and the approved manufacturer's instructions, and each low-slope section repeats it: R905.11.3 for modified bitumen, R905.12.3 for EPDM and R905.13.3 for thermoplastic sheets. What the code does fix is the material. Modified bitumen membranes must meet one of the ASTM standards in Table R905.11.2, EPDM must meet ASTM D4637 or D5019, and thermoplastic sheets ASTM D4434, D6754 or D6878.

That means the installation instructions for the product you buy are, in effect, part of the standard the work is held to. This guide therefore follows two published documents rather than general advice: CertainTeed's residential booklet for Flintlastic SA, a self-adhered SBS modified bitumen system, and Elevate's application guide for RubberGard EPDM. The first is titled for low-slope residential roofs. The second is a commercial specification and reads like one.

Neither is aimed at homeowners. The back page of the CertainTeed booklet states that Flintlastic products are intended for use by professional roofing contractors only, and the Elevate guide routes anything outside its specification to the manufacturer's technical services.

Tear off or cover over: Section R908 decides

On an existing roof the first decision is whether the old covering stays. R908.3 defines a roof replacement as removal of existing layers down to the deck. A recover, meaning new roofing over old, is permitted by R908.3.1 where the new covering is installed under the manufacturer's approved instructions, and item 4 of that list allows a new protective coating over existing built-up, modified bitumen and single-ply roofs without tear-off.

R908.3.1.1 then rules a recover out in three cases: the existing roof is water soaked or too deteriorated to act as a base, the existing covering is slate, clay, cement or asbestos-cement tile, or the roof already carries two or more applications of any covering.

Two exceptions in R908.1 matter on older flat roofs. Reroofing does not have to meet the 1/4 in design slope where the roof provides positive drainage, and a recover or replacement on such a roof does not force secondary overflow drains to be added. The Elevate guide adds its own condition: where the soundness of an existing roof cannot be verified, it calls a full tear-off to the structural deck good roofing practice, and it puts removal of wet insulation and wet substrate on the owner or designer.

Deck and weather limits, as the documents print them

CertainTeed lists the substrates Flintlastic SA base plies may be self-adhered to. For wood that is APA rated exterior grade plywood or OSB, and priming is required. The booklet adds three cautions for wood decks: direct adhesion is not permitted by code in Miami-Dade County, a membrane adhered straight to a wood deck means replacing the deck when the roof reaches end of life, and self-adhering to plank decks of resinous woods such as pine is prohibited. On a nailable deck such as plywood, the booklet calls mechanically attaching a base sheet, Flintlastic SA NailBase, best practice.

Every surface must be free of dust, dirt, oil, debris and moisture, and application is not to be attempted with ice, snow, moisture or dew present. The weather rules are specific. The self-adhered sheets bond well at 50°F and rising. Between 35°F and 49°F the cold weather procedure applies, which brings in Arctic Edge lap tape and a hot air welder. Rolls are stored indoors on upright pallets, and in cold weather at 60°F or more. The booklet says never install cold rolls and never heat them with a torch. Cut lengths relax unrolled for at least 15 minutes at a minimum of 50°F.

Elevate's EPDM guide sets its own threshold at 40°F, below which condensation can form on solvent-based adhesives and primers; if it does, work stops. Ponded water, snow, frost and ice must be cleared, and the substrate must be relatively even, clean, dry and smooth.

Base layer: start at the low point

Both documents begin at the lowest part of the roof and work upward so that laps shed water. CertainTeed's wording is that no side lap should face against the flow.

Mechanically attached base sheet. Let the NailBase overhang perimeter edges by 2 in and turn it 2 in over the fascia. The minimum fastening pattern is every 9 in on center along side laps and every 18 in in two staggered rows across the field. Side laps are at least 2 in, end laps 4 in, and end laps in neighboring courses are offset by at least 3 ft.

Self-adhered base ply. Sweep the deck, fold the sheet back halfway along its length, pull the split release film, press that half down, then repeat on the other half. Roll the whole surface with a heavy weighted roller, working from the middle of the sheet out to its edges. The booklet says not to roll lengthwise because it can stretch the sheet. Laps and offsets match the NailBase figures.

Where end laps meet side laps, the booklet has the corners cut on a diagonal, roughly 5 1/2 in long, with a dab of FlintBond adhesive at the cut, to keep water out of the T-seam. It also says not to leave a base sheet exposed to weather: the cap goes on the same day, and all plies of the assembly should be installed in one day.

Courses and laps across the roof

The booklet's field details for a two-ply system start the base with a half-roll course of 19 11/16 in and the cap with a full 39 3/8 in course, so their side laps never line up. Base side laps are 2 in and cap side laps 3 in. Here is this site's arithmetic on those figures for common roof depths, counted along the fall:

Courses of 39 3/8 in Flintlastic SA sheet from the low edge to the high edge, two-ply system
Low edge to high edge	Base courses (half-roll starter, 2 in side laps)	Trimmed off the last base course	Cap courses (full-roll starter, 3 in side laps)	Trimmed off the last cap course
8 ft	4	35.8 in	3	16.1 in
10 ft	4	11.8 in	4	28.5 in
12 ft	5	25.2 in	4	4.5 in
16 ft	6	14.6 in	6	29.2 in
20 ft	7	3.9 in	7	17.6 in
24 ft	9	30.7 in	8	6.0 in

The count leaves out the 2 in base overhang at the edges, which on some widths adds a course. The booklet lists coverage as 2 squares per NailBase or PlyBase roll and 1 square per cap roll without saying whether laps are already deducted, so the order is easier to settle by counting courses than by dividing area. The 3 in cap lap alone takes about 7.6 percent of every sheet's width. For squares and waste on the whole roof, the roofing calculator does the area.

Edge metal, then the cap sheet

On a two-ply system with no interply, CertainTeed has a 9 in strip of base ply adhered flush to the roof edge first. Edge metal of at least 26 gauge is set in a troweled bed of FlintBond 1/8 in to 1/4 in thick and fastened 4 in on center in two staggered rows. Oil is cleaned off the metal with vinegar and water, and the top face is primed with FlintPrime Aerosol before any membrane goes over it. The roof flashing calculator counts the linear feet.

The cap sheet starts at the low point, flush with the roof edge, selvage edge toward the high side. Remove each selvage release strip only as the next course arrives to keep that adhesive clean. Side laps are at least 3 in, end laps 6 in, offset 3 ft, with the same diagonal corner cuts. Wherever granule surface laps over granule surface, such as an end lap, the booklet requires one of two methods: a full 1/8 in to 1/4 in troweling of FlintBond across the 6 in lap, or a hot air welder set between 900°F and 1,100°F with a silicone roller, taking three passes on a 6 in lap. Between 35°F and 49°F the welder method is required.

Once bonded, every lap gets checked. If any area can be lifted, the booklet says it is not properly adhered, and a seam probe finds voids. On slopes over 1 in 12 the sheets must also be back nailed, and sheets run parallel to the slope are blind nailed at end laps.

If the membrane is EPDM

The Elevate guide's fully adhered sequence runs differently. Unroll the sheet without stretching, leaving enough for edges and seams, and let it relax for at least 30 minutes. Fold it back on itself, sweep dusting agent and dirt off both mating surfaces, and apply bonding adhesive to both at the same time, keeping it out of the seam area.

The adhesive is ready when the touch-push test passes: a clean dry finger pushed forward at an angle comes away without wet or stringy adhesive. Roll the membrane into the adhesive from the fold, then broom it down and repeat on the other half.

Seams use splice tape on primed membrane. The primer goes on at least 1 in wider than the tape, the top sheet is trimmed so 1/8 in to 1/2 in of tape shows at the finished seam, the release liner is peeled at 45 degrees while the seam is broomed, and the seam is rolled with a silicone hand roller. Patches go over every point where three sheets meet. The guide also warns against oil-based or bituminous roof cement on RubberGard, and against drying adhesives with heat guns or open flame.

What a hired install is priced at

HomeAdvisor's flat roof cost guide gives all-in installed rates per square foot of $4 to $8 for modified bitumen, $4 to $13 for rubber (EPDM), $4 to $14 for TPO and $4 to $10 for built-up tar. Removing the old roof adds $1 to $5 per square foot, and a permit $250 to $600. On a 16 by 20 ft garage roof of 320 sq ft, this site's arithmetic puts a modified bitumen roof at $1,280 to $2,560 before tear-off. Last price check: 2026-09-13. Source: HomeAdvisor flat roof cost guide.

The roof replacement cost calculator runs the same rates on your own area.

Where this guide stops

Framing is outside its scope, and a new slope built with sleepers or added weight is a structural change. R908.2 requires the roof structure to carry the covering and the material and equipment loads of installing it, and that is a question for someone who can see the framing.

It does not choose between documents for your product. The figures here belong to Flintlastic SA and RubberGard EPDM as published; another manufacturer's laps, primers and temperatures will differ, and its instructions govern its roof. Neither document describes torch application, and this guide does not cover it.

And the code text is Seattle's printing of the 2018 IRC. Local adoption can differ in edition and in amendments, so the building department is the place to confirm slope, drainage and whether a permit is needed.

Run the numbers

Frequently asked questions

What is the minimum slope for a flat roof?: 1/4 in of rise per foot, a 2 percent slope, for built-up, modified bitumen, EPDM, thermoplastic single-ply, sprayed foam and liquid-applied roofing under IRC R905. Coal-tar built-up roofing is allowed at 1/8 in per foot. The code calls it a design slope and ties it to drainage.
How much fall does a 16 ft flat roof need?: At least 4 in from the high edge to the low edge or drain, because 16 ft at 1/4 in per foot is 4 in. A 20 ft run needs 5 in and a 24 ft run needs 6 in.
Can I put a new flat roof over the old one?: R908.3.1 permits a recover when the new covering follows the manufacturer's approved instructions. It is not allowed if the existing roof is water soaked or deteriorated, if it is slate, clay, cement or asbestos-cement tile, or if it already has two or more layers.
Does an old flat roof have to be rebuilt to 1/4 in per foot when reroofing?: No, if it provides positive drainage. R908.1 exempts reroofing from the design slope minimum in that case, and also from adding secondary overflow drains or scuppers.
What laps does a self-adhered modified bitumen roof use?: For Flintlastic SA, CertainTeed gives base side laps of 2 in and end laps of 4 in, cap side laps of 3 in and end laps of 6 in, with end laps in neighboring courses offset at least 3 ft.
How cold is too cold to install a flat roof?: It depends on the product. CertainTeed allows Flintlastic SA down to 35°F, with cold weather procedures from 35°F to 49°F. Elevate's EPDM guide adds precautions below 40°F and stops adhesive work if condensation forms.
Do flat roofs need drains?: R903.4 requires drains at each low point unless the roof is sloped to drain over its edges. Where a parapet or similar edge would trap water, R903.4.1 adds overflow drains or scuppers set 2 in above the low point.
Can a homeowner install a flat roof membrane?: CertainTeed states that its Flintlastic roofing products are intended for use by professional roofing contractors only. The code requires installation under the manufacturer's instructions, so those instructions are the standard the work is judged against.

Check these numbers yourself

2018 Seattle Residential Code, Chapter 9 Roof AssembliesSeattle Department of Construction and Inspections, printing IRC Chapter 9. R903.1 and R903.4 on pages 515 and 516; minimum slopes R905.9.1 to R905.15.1 and material standards on pages 527 to 529; R908 reroofing on page 531. Read 2026-09-16.
CertainTeed, Flintlastic SA Self-Adhered Roof Systems for Low-Slope Residential RoofsCertainTeed installation booklet, Code No. 00-00-907-NA-EN, dated 06/25, hosted by BuildSite. Substrates and priming, temperatures and storage, NailBase fastening, lap widths, starter courses, edge metal, cap end lap methods, back nailing over 1 in 12. Read 2026-09-16.
Elevate, RubberGard EPDM Application GuideElevate (Amrize), July 2025, pages dated 7/7/2025, hosted by BuildSite. Cold weather below 40°F, substrate preparation and recover, 30 minute relaxation, bonding adhesive and touch-push test, splice tape seams. Read 2026-09-16.
HomeAdvisor, how much does a flat roof replacement costWritten by HomeAdvisor staff writer, edited by Ryan Noonan, 20 June 2026. Installed rates per sq ft by material, removal and permit. Read 2026-09-13.

Figures here last checked against the source documents on 2026-09-16. Codes are amended locally; confirm against the edition your jurisdiction enforces.

About the author

Lucas Pradella builds and maintains Nails & Numbers. He is not a tradesperson: every figure on the site is traced to a named, dated document, and the page links it so you can check it yourself. More about how these pages are made.

Every guide on this site: Guides.