How to Plan Flat Roof Drainage: Slope, Ponding, Drains, Scuppers and Overflows
A flat roof is a drainage system with a membrane on it. Plan where the water goes before choosing what goes on top.
- Advanced
- A few hours to survey and plan
- Updated
The short answer
Plan flat roof drainage by giving every part of the roof a path to a drain, scupper or gutter: a minimum slope of 1/4 inch per foot, drains at the low points, crickets to split water between drains, and a separate overflow set about 2 inches above the primary drain. Water that stays more than 48 hours after rain is ponding and should be corrected, not tolerated.
Flat roofs are not flat. They are low-slope roofs that rely on a designed slope and a designed set of drains to move water off the surface. Every other decision about a flat roof, from membrane to insulation to edges, is made on top of that drainage plan. When the plan is wrong, the membrane sits in water, dirt and algae collect, the structure carries extra weight, and leaks appear at every detail.
Most leaking flat roofs in the Fraser Valley are not failing because the membrane is bad. They are failing because water is not leaving the roof. A long wet winter means months of rain arriving on surfaces that may be a fraction of an inch out of level, and the smallest depression becomes a permanent pond. Planning the drainage properly is the cheapest thing that can be done for a flat roof.
This guide covers how to read a roof, decide where water should go and choose between drains, scuppers and gutters. The details of each are in how to install a flat roof drain, how to install a scupper and how to install tapered insulation and crickets.
Tools
- Long level or laser level
- Tape measure
- Chalk or marker
- Garden hose for water testing
- Camera
- Graph paper or drawing software for a slope plan
Materials
- Roof plan with dimensions
- Structural information for the deck and framing
- Rainfall design data for the location
- Drain, scupper and gutter product data
Safety first
- A flat roof with a parapet can hold a great deal of water if drains block. One inch of water weighs about 5.2 pounds per square foot, so several inches can overload a structure. Clear blocked drains promptly.
- WorkSafeBC requires fall protection where a fall could be 3 metres (about 10 feet) or more, and at any height where a fall could cause serious injury. Low parapets do not make a roof safe.
- Do not stand in or walk through deep ponded water. It hides soft deck and open drains.
Step by Step
Step 1: Survey the existing slope and find the ponds
Put a long level or use a laser to shoot elevations at a grid of points across the roof, and find where water is going and where it stops. Mark the ponds on a sketch of the roof. After a rain, go back and see how long the water stays. Water still standing 48 hours after rain is generally treated as ponding, the common threshold in membrane warranties.
Look for dirt rings, algae, stained membrane, a sag in the deck and a drain that is higher than the water around it. A drain set on the high side of a depression leaves the pond in place no matter how large the drain is.
Step 2: Set the target slope
Most membrane manufacturers require at least 1/4 inch per foot for their warranties. That is about 2 percent, or a 1/4 inch drop for every 12 inches of run. Structures settle and decks deflect, so designing for a little more than the minimum gives a margin.
Slope can be built into the structure with sloped framing, or added on top with tapered insulation. On a reroof, tapered insulation is the usual way to fix a roof with too little slope. The plan sets the taper, which is explained in how to install tapered insulation and crickets.
Step 3: Place drains at the low points
Interior drains sit at the lowest elevation of the local area. A drain placed next to a wall or at the high part of a bay never gets water. Structural columns and framing often decide where a drain can go, so coordinate with the building structure. See how to install a flat roof drain.
The number and size of drains are set by the rainfall intensity for the location and the roof area each drain serves, and follow the plumbing code. Sizing should come from a designer or plumber, not a guess. A drain that is blocked by debris or sized too small causes water depth on the roof to climb quickly.
Step 4: Use crickets to divide water between drains
A cricket is a small, sloped, two-sided ridge that splits water around an obstruction, such as a curb or a wall, or carries it from the middle of a roof toward the drains. A typical cricket slope is about 1/2 inch per foot, which is steeper than the roof slope so it pushes water along.
Without crickets, water stops behind an upslope curb or in the middle of a roof between two drains and stays there. Plan them on the drawing, and build them with tapered insulation.
Step 5: Provide overflow drainage
A roof with parapet walls on all sides can trap water, and a blocked drain can quickly load the roof. Plumbing codes generally require secondary drainage on roofs like this, and the structure must be designed to carry the water depth at the overflow. The common arrangement is an overflow scupper or drain with its inlet about 2 inches above the primary drain surface.
The overflow must discharge to a visible place so a blocked primary drain is noticed. See how to install a scupper. Check the exact requirement against the current building and plumbing codes.
Step 6: Decide between drains, scuppers and gutters
Interior drains lead water down through the building to storm pipes. They keep the roof edge clean and suit large roofs, but they need plumbing and maintenance access. Scuppers discharge through the parapet wall onto a downspout or to grade, and they are simple and visible. Edge gutters suit roofs without parapets, such as a low-slope house roof.
On a low-slope roof with an edge gutter, water flowing off the membrane must enter the gutter without overshooting, and the membrane should overlap the gutter edge metal correctly. See how to install TPO-coated edge metal for one edge detail.
Step 7: Draw the plan and check it
Mark every drain, scupper and cricket, draw arrows for water flow and write the slope on each plane. Check each area: does water have a path that never crosses a curb, a seam or a detail? Does the water from each section reach a drain in the expected time?
Consider maintenance as part of the plan. Drains need access and strainers, and somebody must clear them after leaf fall and storms. A drainage design that depends on clean drains but has no way to reach them will fail in practice.
Water Weight and Structure
Standing water is heavy. One inch of water on a roof weighs about 5.2 pounds per square foot, so two inches is over 10 pounds per square foot on top of the roof's other loads. A roof that ponds can deflect, which makes the pond deeper, which adds weight. This is called progressive deflection and it is one way flat roofs collapse.
If a roof has been ponding for a long time, have the structure checked before adding insulation or new membrane. A deck that has sagged will need to be corrected, not hidden.
Fraser Valley Notes
- Rain volume: winter rainfall is high and persistent. Drains must be sized for intense bursts as well as long wet spells.
- Debris: needles, leaves and moss clog strainers under trees. Plan for regular clearing and keep overhanging branches cut back.
- Freeze-thaw: water in and around drains can freeze at the edges, especially near Hope and the canyon. Keep drain bowls and scupper openings unobstructed.
- Heat: summer heat can make ponded water hot and soften some membranes. Another reason to drain the roof.
Common Mistakes
- Drains on the high side. A drain set beside the low area leaves the pond exactly where it was.
- No crickets behind curbs or between drains. Water stops against the obstruction and never reaches a drain.
- Relying on a single drain. One blocked strainer and the roof fills, with no overflow to show it.
- No overflow or a hidden overflow. Nobody notices the blockage until water reaches the top of the parapet or a ceiling stain appears.
- Designing exactly to 1/4 inch per foot. Settling and deflection eat the margin and leave ponds.
- Ignoring structure. Adding slope and new roofing over a sagging deck keeps the problem hidden and adds weight.
When to Call a Professional
Drainage planning mixes roofing, structure and plumbing. The rainfall design, drain sizing and overflow requirements belong to the building code, and the structure has to carry the water depth at overflow. On anything larger than a small shed roof, a designer or an experienced roofer should do the plan.
Call a roofer if water stays on the roof for more than two days, if the deck sags or feels spongy, if drains back up or if the ceiling below is stained. Those roofs need diagnosis, not just a new membrane.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Flat Roofing and Torch On service or the towns we cover.
How to Plan Flat Roof Drainage: Questions
What is the minimum slope for a flat roof?
Most membrane manufacturers require at least 1/4 inch per foot, about 2 percent, for their warranties. Less than that and water tends to pond in any low spot. Structures settle and decks deflect, so designers often aim for slightly more. Tapered insulation is the usual way to add slope on a reroof.
How long can water sit on a flat roof?
Water that remains on a flat roof more than about 48 hours after rain is generally treated as ponding. Short-lived puddles are expected, but standing water ages the membrane, collects dirt and algae, adds weight to the roof and often voids warranties. Ponding should be corrected with slope, crickets or additional drains.
Do flat roofs need an overflow?
Roofs with parapets or raised edges that could trap water generally need a secondary drain or overflow scupper, commonly set about 2 inches above the primary drain. It lets water escape if drains block and gives a visible warning. Requirements come from the current building and plumbing codes, so confirm them for the specific building.
Can you put gutters on a flat roof?
Yes, on low-slope roofs without parapets. Water runs off the membrane edge into an edge gutter or gutter apron, and the membrane is lapped over the edge metal so water enters the gutter. The gutter must be sized for the roof area, sloped to its downspouts and kept clear of leaves and needles.
How do you fix ponding water on a flat roof?
The fix depends on the cause. Options are to add tapered insulation and crickets to create slope, move or add drains at the low points, clear blocked drains, or correct a sagging deck. Coatings and patches do not cure ponding. Have the structure checked first if the deck has sagged.
Related Guides
All Flat Roof Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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