The short answer
Before winter, clear every outlet, gutter and valley, check the laps, sealants and flashings, and look at the underside for early signs of damp. During cold weather, watch for blocked drainage, ice in gutters and snow drifting against upstands. Do not walk on the roof to clear snow; call a roofer instead.
Why winter shows up the weak points
A commercial roof spends most of the year coping. Winter adds standing water, freezing and thawing, and sometimes a load of snow, all at the same time, and a roof with a small fault will usually tell you about it then.
Water that has collected in a lap or a hairline crack freezes and expands, which opens the gap a little wider. When it thaws, more water gets in. Repeat that a few times and a seam that looked sound in October is letting water through by January.
None of this needs a dramatic event. It is the ordinary, repeated cycle of cold nights and milder days that does the work, which is why a short check beforehand is cheaper than a repair afterwards. Our page on how long a commercial re-roof takes explains why a failing roof is better dealt with in settled weather.
Outlets, gutters and valleys come first
Drainage is the number one winter job. Leaves from nearby trees collect in outlets and valley gutters through autumn, and by the time the first hard frost arrives they have packed into a plug. Water then backs up behind the blockage, the roof holds a shallow pond, and a pond is heavy and sits against every seam.
Clear gutters, hoppers, downpipes and outlet grilles, and check that overflow pipes are open at the wall. An overflow is there for the day the main outlet freezes or blocks, and it only works if it is clear.
Old box gutters and parapet gutters deserve a second look, because a worn lining holds water in the very place where it should run away. If a lining is cracked, our page on industrial gutter lining sets out the usual repair routes.
When snow is lying on the roof
Snow does not always lie evenly. Wind moves it, and it tends to build up in drifts against parapets, upstands, plant housings and the taller part of a stepped roof. Those drifts add weight to a limited area, and when they melt in a hurry they send a lot of water to outlets that may be partly blocked.
If snow is lying, the safest approach is to leave it and keep the drainage clear at ground level. Do not send anyone onto the roof to shovel it off. Snow hides edges, rooflights and soft spots, and a rooflight that looks like roof surface under a white blanket is how people fall through. Our guide to fragile roofs and safety explains why.
If you are worried about the load on an older building, get a structural engineer, or a roofer who can recommend one, to assess the frame. We will tell you if the structure needs a specialist.
Ice, frozen drainage and what happens when it thaws
Ice causes two separate problems. The first is a frozen outlet or pipe, which stops water leaving the roof and leaves it sitting on the covering. The second is ice at the eaves or in a gutter, which holds meltwater behind it so that it is pushed under laps and up beneath flashings.
Both tend to appear on the first cold mornings, then ease as the day warms up, so a roof that leaks only on certain days is a hint to check the drainage as well as the covering.
Do not hack at ice with tools. Sheet metal, single-ply membranes and rooflights are all easily damaged. Warm the outlet from inside or from ground level where that can be done safely, and call for help if it will not clear. Prevention is easier: a clean, free-running outlet seldom freezes solid.
Checks from the ground and from inside
Much of a winter check needs no ladder. From the yard or car park you can see whether gutters are sagging, whether pipes are discharging properly, and whether any sheets have lifted at the ridge or eaves. A pair of binoculars helps.
Inside, look for fresh stains on ceilings or insulation, drips onto racking, and rust marks around fixings. A damp patch that dries and returns is useful evidence, so note where it is and take a dated photograph. Our page on composite roof panels explains how to read damp signs on an insulated panel roof, where the water can travel before it shows.
Compare what you see after each cold spell with the photos from before. Slow changes are easy to miss if you rely on memory. A short note of the day, the conditions outside and the spot where you saw damp takes a minute and gives a roofer a useful timeline if a leak does appear.
When to call and what we do
Call if an outlet will not clear, if water is coming through, if you see a sheet or panel lifting, or if a rooflight has cracked. We give leaks and storm damage priority, and any water coming in is dealt with before we plan the rest.
For a planned winter visit we clear drainage, check laps, sealants and fixings, photograph the condition and send a written quote covering anything that needs attention. A routine like this suits a business with several buildings or a landlord with a unit let out, and it is the idea behind a maintenance plan.
The aim is always the lightest repair that actually cures the problem, and workmanship on our repairs is guaranteed in writing for 10 years. Our commercial roofing page lists the other services we offer to industrial, retail and agricultural buildings across Swindon and the surrounding towns.