A low-slope roof with parapets on all sides drains through interior roof drains. Where must the inlet of a secondary overflow drain be set under R903.4.1?

2 inches above the low point of the roof. R903.4.1 requires overflow drains the same size as the roof drains, with the inlet flow line 2 inches above the low point of the roof, discharging to an approved location and not connected to the roof drain lines. Overflow scuppers must be three times the drain size with a 4-inch minimum opening height.

How does the code define positive roof drainage?

A drainage condition that accounts for deck deflection and provides additional slope so the roof drains within 48 hours of precipitation. The MRC defines positive roof drainage as a condition in which loading deflections of the deck have been considered and additional slope provided to ensure drainage within 48 hours of precipitation. R908.1 uses the term in its reroofing exceptions.

What size must an overflow scupper be relative to the roof drains under R903.4.1?

Three times the size of the roof drains, with an opening at least 4 inches high. R903.4.1 requires overflow scuppers three times the size of the roof drains, with a minimum opening height of 4 inches, in the adjacent parapet walls, with the inlet flow line 2 inches above the low point of the roof. Overflow drains are the same size as the roof drains.

Low-Slope Drainage: Positive Drainage, Roof | Free Guide 2026

Key Takeaways

Unless a roof is sloped to drain over its edges, roof drains must be installed at each low point (R903.4).
Where parapets or raised perimeters would trap water, secondary overflow drains or scuppers are required. Overflow drains are the same size as the roof drains, with the inlet 2 inches above the low point of the roof.
Overflow scuppers must be three times the size of the roof drains, have an opening at least 4 inches high, and have the inlet 2 inches above the low point, and overflow drains may not connect to the roof drain lines.
Positive roof drainage means the roof drains within 48 hours of precipitation, allowing for deck deflection.
When an existing roof already has positive drainage, reroofing does not require adding slope to meet 1/4:12 or adding secondary overflow drains or scuppers (R908.1).

Why Drainage Matters

Standing water adds weight, about 5.2 psf per inch of depth, since water weighs 62.4 pounds per cubic foot. It speeds membrane aging, leaks through any small defect, and freezes and expands in Michigan winters. As a flat roof deflects, it collects more water, and deflection increases. On a roof with parapets, clogged drains can trap enough water to overload the structure.

Code Rules (2015 MRC)

Rule	Requirement
Membrane slope	BUR, modified bitumen, single-ply, spray foam, and liquid-applied roofs need a design slope of at least 1/4:12. Coal-tar BUR may use 1/8:12 (R905.9–R905.15)
Positive roof drainage (definition)	Loading deflections of the deck are considered, and additional slope is provided so the roof drains within 48 hours of precipitation
Roof drains (R903.4)	Unless roofs are sloped to drain over roof edges, roof drains are installed at each low point
Secondary (emergency overflow) (R903.4.1)	Where roof drains are required and the perimeter extends above the roof so water would be trapped if the primary drains back up, provide overflow drains or scuppers
Overflow drains	Same size as the roof drains, with the inlet flow line 2 inches above the low point of the roof
Overflow scuppers	Three times the size of the roof drains, with a minimum opening height of 4 inches, in the adjacent parapet, with the inlet 2 inches above the low point
Piping	Sized per the plumbing code (IPC Sections 1106 and 1108). Overflow drains discharge to an approved location and are not connected to roof drain lines
Reroofing (R908.1)	Existing roofs with positive drainage need not meet the 1/4:12 minimum, and need not have secondary drains or scuppers added

Installing a Roof Drain

Set the drain body at the low point, recessed if possible into a sump (a slight depression in the insulation or deck) so water enters the drain before it can pond around it. Secure it to the deck with the manufacturer's clamp or underdeck ring.
Strip in the membrane: extend the membrane over the drain bowl, set it in the manufacturer's sealant or mastic, and clamp it with the clamping ring, tightening bolts evenly. Cut the membrane to the drain opening without cutting under the clamp. Thermoplastic systems may use a weldable drain insert.
Install the strainer or dome to keep debris out.
Test by running water to the drain and confirming the connection below does not leak.

Scuppers and Edges

A scupper is an opening through a parapet or raised edge. Line it with a metal sleeve flashed into the membrane, and discharge into a conductor head and downspout, or away from the wall.
For roofs that drain over the edge, use a gravel stop or drip edge and a gutter sized for the area. Keep the membrane stripped in over the edge metal.

Correcting Ponding

Tool	How it helps
Tapered insulation	Factory-sloped boards (commonly 1/8 or 1/4 inch per foot) create slope to drains on flat decks
Crickets and saddles	Small sloped areas between drains, behind curbs, or at parapets that divert water toward drains or scuppers. Crickets are usually built steeper than the field so the valley lines where they meet the field still drain
Additional drains or relocated scuppers	Add outlets at persistent low spots
Structural correction	Sagging joists or beams need a structural fix first. Adding insulation only adds weight

Before reroofing a ponding roof, find the low spots with a level or laser, and design the tapered layout toward drains. Replace wet insulation. Remember R908.1's exception applies only when the existing roof already provides positive drainage. A roof that ponds should be corrected.

Where Low Slope Meets Steep Slope

On houses, the most common low-slope leaks happen at transitions, where a porch roof meets a shingled roof or a flat dormer roof meets the main slope:

Run the membrane up the steeper slope under the shingle underlayment and ice barrier far enough to stay above any water that backs up.
Flash pitch transitions with metal or membrane per the manufacturers' instructions, and flash wherever there is a change in roof slope or direction (R903.2.1).
Keep shingles off slopes under 2:12. Use a membrane there instead (R905.2.2).

Worked Example

A 20-by-30-foot flat porch roof has a parapet on three sides and one interior drain. Parapets trap water, so R903.4.1 requires a secondary overflow. The options are an overflow drain the same size as the primary drain with its inlet 2 inches above the low point, or an overflow scupper three times the drain size with an opening at least 4 inches high and its inlet 2 inches above the low point. Any overflow drain must run on separate piping to an approved discharge location, not tied into the primary drain line.

Michigan Roofing Contractor

10.3 Low-Slope Drainage: Positive Drainage, Roof Drains, Overflow Scuppers & Correcting Ponding