The short answer
A commercial roof drains badly for a handful of reasons: blocked outlets and gutters, falls that are too shallow or have settled, outlets set too high or too few in number, and gutters that are undersized or sagging. The result is ponding water that ages the covering and then soaks through any open lap or split. Clearing comes first, then the cause is found and corrected.
Outlets, gutters and what blocks them
The simplest cause is checked first. Leaves, moss, silt, feathers, windblown litter and the grit shed by ageing felt collect at outlets and in gutters until water has nowhere to go. One clogged outlet can leave a whole bay under standing water after a wet spell.
Box gutters, parapet gutters and valley gutters behave the same way. They are narrow channels between roof planes or behind a wall, and they take the run-off of a large area. A little debris is enough to make them overflow into the building, which is why many interior leaks on commercial buildings trace back to a gutter and not to the sheet.
Outlets also fail from damage. Cracked outlet bodies, loose leaf guards, and the seal between the membrane and the outlet are all common weak points. Down pipes can be blocked or crushed lower down, so clearing the top does not always fix the flow.
Brett Martin gives a rule of thumb for gutters, 1mm of fall for every 600mm of run. A gutter that has sagged on its brackets, or was set level, holds water along its length. Our guide to valley gutter lining shows how channel repairs are done.
Ponding and what causes it
Ponding is water that stays on the roof after the rain has stopped. It is a sign that the surface is not falling to the outlets as it should. Causes include a roof laid with too little fall in the first place, a deck that has deflected under the load, insulation that has compressed, and outlets set above the lowest point of the roof.
Flat roofs are not truly flat. BS 6229:2025 sets a finished fall of at least 1:80, and IKO advises designing for 1:40 so the finished roof still achieves it. The standard is for membrane roofs pitched at up to 10 degrees. Roofs built to a lower fall are likely to hold water.
Standing water ages the covering. It carries dirt and algae, it loads the deck, and it is under every seam and lap for longer than the roof was designed to cope with. A small puddle left for years is not a cosmetic matter, since the membrane under it stays wet.
Ponding is not always dramatic. A shallow patch that holds water for a day after rain is the sign to look for, and a survey should note where and how deep it lies.
Fixes, from clearing to new falls
The remedy depends on the cause. If the problem is blocked outlets and gutters, the job is to clear them, flush the down pipes and fit guards, and to set up a regular check so they stay clear. That is a modest job, and it comes first.
If the cause is an outlet in the wrong place, a new outlet can be formed at the low point and connected to the system. Where a roof lacks fall, tapered insulation can be laid under a new membrane to create one, and a strip-and-renew is the point at which a proper fall can be built in.
Gutters that have sagged can be re-hung to fall correctly, and linings can be renewed where the old ones have failed. Undersized gutters or too few outlets mean the system cannot take the flow, and the answer there is more drainage capacity.
Where the membrane has already been damaged by ponding, patching the surface without fixing the fall only postpones the problem. We tell you which applies, with photographs, and we set out the repair and the renewal side by side. Our page on commercial roof maintenance describes the checks that keep drainage clear.
What we check on a drainage survey
We start from the lowest points and work upwards. We look at every outlet, gutter and down pipe, test the flow from a safe position, and note any ponding after rain. We photograph the stains and tide marks that show where water has stood.
We also look at what the water is doing to the covering: lifted laps, blistering, algae, rusted fixings and softened insulation at the edge of a puddle. The condition of those areas tells us whether a clear-out is enough or whether the surface needs more.
Where the roof is out of reach or fragile, access is arranged first with edge protection and proper platforms. Nobody should walk across a roof lit up in wet weather to look at it, least of all one with fragile roof lights.
The result is a plain account of where the water goes, where it does not and what it would take to fix. Once surveyed, the roof is priced as a single written total, access included. See the commercial roofing page for the rest of the service.
