How do you survey an industrial roof?

Nothing gets measured until the roof can be reached safely, and on an industrial building that is rarely a ladder against a gutter. Most warehouse and factory roofs are fragile by default. Profiled fibre cement, older GRP rooflights and thin single-skin sheeting will not hold a person, and a fragile rooflight painted over years ago looks much like the sheet next to it from above. So the first part of any survey is deciding how you get on, where you can put your weight, and what stops a fall through a panel that has gone brittle.

In practice that means walkways or crawl boards spanning the purlins, edge protection along any open side, and a plan for the fragile lights themselves. On a low pitch you may work off the roof directly. On anything steeper, or where the sheeting is suspect, the sensible survey happens from a mobile platform or from inside on a scaffold tower looking up at the underside. The underside view is worth as much as the top view, because that is where you read the structure the finished roof is hiding.

Industrial roofs come in a handful of families, and which family you are standing on changes every decision after it. The common ones are built-up felt on a metal deck, single-ply membrane over insulation, profiled steel sheeting with a separate liner, standing-seam aluminium, and composite insulated panels with the insulation bonded between two skins. Each carries load differently and each takes a rooflight in a different way.

Age matters as much as material. A roof from before the mid-1980s may contain asbestos cement sheeting or asbestos-based rooflights, and that has to be identified and handled under the right controls, not disturbed on a hunch. Part of a proper survey is simply establishing what era the covering is from and flagging anything that needs testing before work is planned around it. You are not there to disturb it. You are there to know it is present so the specification accounts for it.

While you are reading the build-up, note the condition of what is already up there. Yellowed, crazed GRP rooflights that have lost their light and gone brittle are common on older units, and replacing a run of failed lights with new units is a straight swap that reuses the openings already cut into the structure. That is a replacement, and it is the cleanest kind of industrial rooflight job there is, because the holes and the flashings are already located for you.

The numbers that decide a rooflight scheme are structural. Measure the bay spacing, the purlin centres and the sheeting profile, because an industrial rooflight has to sit into the same module as the sheet it replaces or sits beside. Most metal roofs work to a set profile pitch and a set purlin spacing, and a rooflight that ignores that module leaves you cutting and trimming steel that was doing a job.

Take the roof pitch with a level, not by eye. A great many industrial roofs sit at a shallow pitch, sometimes only a few degrees, and pitch drives both the drainage and the type of rooflight that suits. Record the direction of fall, where the gutters and valleys run, and how water leaves the roof, because dropping a new opening into a drainage path stores up trouble. Then measure the openings you are proposing against the structure around them: a rooflight that spans between two purlins behaves differently from one that needs a purlin cut and trimmed out, and the second is a structural alteration that a competent person signs off, not a like-for-like change.

• Bay and purlin spacing. Sets the module every rooflight has to respect.
• Sheeting profile and gauge. Determines the flashing and the compatible rooflight type.
• Pitch and direction of fall. Governs drainage and rules out the wrong rooflight forms.
• Existing opening sizes. Tells you what is a swap and what is a new cut.

Which way the roof slopes is not a footnote on an industrial building, because the glazed area is large and the roof is shallow enough to catch a lot of sky. A south-facing slope of profiled rooflights over an office mezzanine or a packing area can turn that space into a greenhouse by early afternoon in summer, and the people working under it feel it long before anyone checks a thermometer. A flat or near-flat industrial roof is worse still, because horizontal glazing sees the high summer sun almost square on and orientation stops helping you.

So a survey records orientation and the reason for it, the same way it records spans. Down here on the Southend seafront the estuary faces south with an open horizon and reflected light off the water, which is exactly the exposure that pushes a glazed roof into overheating. That is why the survey feeds a glass specification and not just a size: solar-control glazing with a low G-value on the sun-facing slopes, clearer glass where the roof faces north and the daylight is welcome. Getting that split right is the heart of our commercial skylight installation work, and it is the same physics that drives every energy-efficient skylight installation we specify. Measure the roof, then measure the sun on it.

A survey that stays as a set of numbers has not finished. The point of it is to arrive at a scheme: how many rooflights, at what size, in what glass, sitting into which bays, reached and installed by what safe method, and notifiable to Building Control where the thermal or safety-glazing rules apply. The good surveys write down the constraints as plainly as the dimensions, so the person specifying is not guessing at the fragile bays or the drainage runs after the surveyor has climbed down.

If you are weighing up rooflights across a warehouse, factory or trade unit, start with the roof itself rather than a product. Tell us the roof type, the rough pitch and which way it faces, and we will look at the whole roof rather than one opening in it. Ring 01702 898232 or request a quote, and if you want to understand the numbers first, our skylight installation costs page sets out where an industrial scheme tends to land.