A TPO roof is a continuous welded sheet with a hole cut in it at every drain. The hole is sealed by a prefabricated flashing, usually a moulded TPO drain flashing or a target patch, clamped down onto the drain body with a ring and bolts. Everything that goes wrong with drainage on a single ply roof goes wrong at that joint or at the slope leading to it.
That is worth knowing before you read a quote, because drainage is where specifications are thinnest. A quote can name the membrane, the thickness and the attachment and say nothing at all about what happens at the outlets, which is the detail most likely to leak and the most expensive to put right afterwards.
How water is supposed to leave the roof
| Element | What it does | What goes wrong |
|---|---|---|
| Slope and crickets | Moves water towards the outlets | Flat areas, deck deflection, insulation laid without fall |
| Sump | Gives the membrane positive fall into the drain | Omitted, or too shallow to work |
| Drain flashing and ring | Seals the membrane to the drain body | Uneven clamping, damaged flange, debris under the ring |
| Strainer | Keeps debris out of the leader | Missing, broken or buried in silt |
| Leader and internal pipe | Carries the water away inside the building | Undersized, blocked or disconnected |
| Overflow or scupper | Relieves the roof when the primary outlet blocks | Set too high, or absent |
The sequence matters. A drain cannot do anything about water that never arrives, and a perfect slope is wasted on an outlet with a blocked leader beneath it.
The drain detail itself
The membrane is turned down into the sump, the flashing is welded to it, and the assembly is clamped to the drain body so that compression, not sealant, makes the seal. Three things decide whether it holds. The flange has to be clean and flat, because grit, an old sealant ridge or a distorted ring leaves a channel under the clamp that water finds. The bolts have to be tightened evenly, so the ring pulls down as a plane rather than as a series of points. And the membrane has to lie in the sump without bridging, which is why the sump is formed before the membrane goes down rather than squeezed afterwards.
Where a drain is reused on a recover, an extension ring or a retrofit insert brings the clamping surface up to the new membrane level. A drain left low under a thicker new assembly is a permanent puddle with the most vulnerable detail on the roof at the bottom of it.
Strainers, and the maintenance nobody books
A strainer is a cage over the outlet that stops leaves, roofing debris and insulation offcuts entering the leader. It is also the first thing lost to a storm or to somebody servicing a rooftop unit, and a missing strainer is how a blockage moves from the roof into the pipe, where clearing it is a plumbing job.
Check that every outlet has one, that it is the right one for the drain, and that it is secured. Then put the outlets on a schedule. Maintenance of this kind is usually a condition of the membrane warranty anyway, and drainage is the item on that schedule which most reliably pays for itself.
Overflows and secondary drainage
A primary drain blocks. That is not a failure of maintenance so much as a fact about roofs, which is why codes generally require a secondary route for the water. It may be an overflow drain next to the primary one with its inlet set above the roof surface, or a scupper through the parapet at a set height.
The height is the point. An overflow set at the surface drains the roof continuously and tells you nothing. An overflow set correctly stays dry until the primary outlet stops working, and water coming out of a scupper is a visible signal that something needs clearing today.
Retrofit drains, and when they are the answer
Where an existing outlet is damaged, undersized or sitting at the wrong level, a retrofit drain is inserted into the existing pipe and sealed to it, bringing a new flange and clamping ring up to the finished roof. It avoids opening the ceiling below, which on an occupied building is most of the cost and all of the disruption. Two cautions: the insert slightly reduces the bore of the pipe, so capacity is checked rather than assumed, and the existing pipe has to be sound where the seal is made.
What to require in writing
- A drain detail drawing showing the sump, the flashing and the clamping arrangement.
- The number and position of primary and secondary outlets, with the design rainfall figure behind them.
- Whether existing drain bodies are being reused, extended or replaced.
- New strainers at every outlet, listed as an item.
- A tapered insulation layout where fall has to be created.
- A water test or a flood test of the outlets before handover.
Point six is the one most often argued about and the cheapest to carry out while the crew is still on site.
Where to go next
If the membrane itself is still being chosen, TPO roofing explained covers thickness, attachment and where the sheets fail. If the problem is water that already stands on an existing roof rather than a detail on a new one, roof insulation explains how tapered layouts are used to create fall without touching the structure.
Common questions
How many drains does a TPO roof need?
Enough for the rainfall intensity used by your local code, the area each drain serves and the depth of water the structure can carry. The number is a calculation rather than a rule of thumb, and it is done by whoever designs the roof, not on the day of the work.
- Local code sets the design rainfall intensity
- Drainage area per drain comes out of that figure
- Pipe size and depth of flow matter as much as count
- Secondary or overflow outlets are usually required too
- Reroofing on an existing layout inherits the old answer
The part owners most often meet is the inherited layout. A recover or a reroof normally reuses the drains that are already there, which means it also reuses whatever was wrong with the original design. If the roof has a history of standing water or of water reaching the parapet in heavy rain, say so before the system is specified, because that is the moment when extra outlets, larger leaders or a changed insulation fall are cheap. Afterwards, adding a drain means cutting the deck, running pipe through occupied space and patching finished membrane.
What is a drain sump, and does every drain need one?
A sump is a shallow recess formed in the insulation around the drain so the membrane slopes down into the outlet rather than meeting it flat. Most low slope roofs benefit from one, and on a roof with little fall it is the difference between a drain that empties and a puddle.
- Formed by tapered or cut insulation around the outlet
- Gives the membrane a positive fall into the drain
- Keeps the clamping ring below the surrounding surface
- Depth is limited by the insulation thickness available
- Prefabricated sump panels are made for this
Without a sump, the drain flange sits proud of the membrane around it and the last part of an inch of water never leaves. That shallow film is where dirt collects, where algae grows and where freeze and thaw works on the detail. It also makes a leak at the clamping ring far more likely, because the water is standing on the one part of the roof with the most joints in it. On a thin assembly there may not be enough insulation depth for a proper sump, in which case the honest answer is a crickets and tapered insulation layout that drives water towards the outlet instead.
Why does water stand around a drain on a TPO roof?
Either the water cannot get to the drain or it cannot get through it. The first is a slope problem, usually a flat area, a deflecting deck or insulation laid without fall. The second is a blockage, a strainer buried in debris or an undersized leader downstream of the roof.
- Check the strainer and the leader before blaming the slope
- Deck deflection between structural members holds water
- A drain set too high leaves a permanent film
- Compressed or wet insulation creates a hollow
- Ponding adds weight exactly where it is least wanted
The order of investigation matters because the cheap cause is the common one. Clear the strainer, run water and watch how fast it leaves. If it drains freely and still ponds between storms, the problem is geometry rather than plumbing, and the answer is tapered insulation, additional outlets or in some cases structural. Standing water also accelerates everything else: the seams in that area are the ones that fail first, and wet insulation under the pond stays wet. [Flat roof systems](/articles/flat-roofs/) covers where the fall on a low slope roof is supposed to come from.
Can a new TPO roof be installed over the existing drains?
Usually, and the drain is the detail that decides how much of the old assembly can stay. The membrane is clamped to the existing drain body, so the body has to be sound, accessible and at the right height relative to the new insulation thickness above it.
- The existing drain body must be sound and reachable
- New insulation raises the surface relative to the drain
- A drain extension ring or a retrofit insert may be needed
- Cast iron bodies are often reused, thin metal ones rarely
- Clamping bolts and rings are replaced, not reused
Adding insulation on a recover lifts the finished surface, and if nothing is done at the outlet the drain finishes below the new roof in a well of its own. The usual remedies are an extension ring on the existing body or a retrofit drain inserted into the pipe, which brings a new flange and clamping ring up to the new level. Both are ordinary work when they are allowed for in the specification and awkward when they are discovered mid project. Ask for the drain detail drawing with the quote. [TPO installation](/articles/tpo-roof-installation/) covers the rest of the sequence.
How often should roof drains be cleared?
At least twice a year on a clean site, and after every heavy leaf fall or storm on a roof with trees or dust nearby. Clearing is the cheapest work done on a roof and the only maintenance item that prevents a load the structure was not designed to carry.
- Twice yearly as a minimum on any low slope roof
- After storms and heavy leaf fall as well
- Strainers are lifted and the outlet checked, not just swept
- Note how fast water leaves while you are there
- Photograph each outlet to build a record
The useful habit is to record what you found rather than just to clear it. An outlet that silts up every visit is telling you water is arriving there with debris and sitting long enough to drop it, which is a slope or a drainage capacity question rather than a cleaning one. A blocked drain in a heavy storm is also the one roof fault that can threaten the structure instead of the finishes, because standing water weighs a great deal and the deflection it causes collects more of it. Overflow outlets exist for exactly that reason and need checking at the same time.