Commercial roofing in Basingstoke

What we find

A sheeted roof is made of long profiled panels laid side by side and overlapping at the edges. Each overlap is called a lap, and each lap should be sealed with a strip of mastic, a soft sealant that stays elastic, so that wind-driven rain cannot be pushed up between the sheets. Along the length of the roof the sheets are held down by fixings, screws with a washer under the head, driven through the crown of the profile into the purlin below.

The drip lines you see inside tend to follow one of two patterns. Water that runs in a straight line down the fall of the roof is almost always a lap that has opened. The mastic has dried and shrunk, or the sheets have moved with the heat of the day and pulled the seal apart. Water that appears in spots along a purlin points to fixings. A screw that was driven too tight crushes its washer, one driven at a slant never seats, and over many winters the holes elongate as the roof flexes in the wind. We also find sheets that were laid with too short an overlap on a shallow pitch, which no amount of sealant will cure. Because the water travels along the underside of the sheet before it drops, the stain on the floor is often several metres from the real entry point.

What we find

Steel roof sheets are coated to protect them. Most carry a layer of zinc over the steel and a paint-type finish above that, often a plastisol, a thick PVC-based coating. That coating covers the face of the sheet well. It does not cover the edge where the sheet was cut to length, and a cut edge exposes bare steel and zinc to damp air. Rust starts there and creeps in underneath the paint, lifting it as a blister. This is cut edge corrosion, and it is one of the most common faults on older sheeted units.

It shows up first at the eaves, where cut ends sit in the gutter and stay wet, along the laps, where water is held by capillary action between two sheets, and around penetrations such as flue pipes and vents. Fixings rust too, and a rusted screw loses its grip on the purlin. Gutters that hold standing water can rot their way through the lower few centimetres of every sheet that overhangs them. Another cause is a sheet that has been scratched during fitting, or walked on with grit under the boots, which breaks the coating on the face and starts the same process from the top. Left alone, corrosion spreads along the sheet until the metal thins and holes appear. A sheet with a few orange edges is a maintenance job. One with holes through the profile is past that point.

What we find

Many business buildings put up between the 1950s and the 1990s around Basingstoke were roofed with corrugated sheets of asbestos cement, usually shortened to AC. The sheets are grey, often mossy, and brittle with age. Left intact and undisturbed, cement-bound asbestos is a lower risk than the loose fibre that people fear, but it is still a hazard when it is broken, drilled or scrubbed. If you own, manage or maintain a non-domestic building, the Control of Asbestos Regulations 2012 place a duty to manage asbestos on you: you need to know whether it is there, what condition it is in, and have a plan for it.

The roof tells its own story. Sheets crack where they have been walked on, and lichen holds moisture against the surface and slowly breaks the cement down. Fixings rust, so sheets lift in the wind. Rooflights set into the AC are often glass-reinforced plastic that has gone brittle. A common mistake is to pressure wash a moss-covered AC roof, which blasts the surface off the sheets and spreads fibre across the site and into the drains. Another is for an unprepared tradesperson to walk across it to reach a fault, and step through a sheet.

What we find

Rooflights in a sheeted roof are panels of translucent plastic, usually glass-reinforced plastic or polycarbonate, that let daylight into the building. They are formed in the same profile as the sheets around them and fitted into the same line of laps. They are also the weakest part of the roof, in two senses.

For leaks, the usual culprits are old and dried-out sealant at the laps, split fixings, and sheets that have gone brittle after years of sun. Plastic rooflights yellow, craze and lose strength as ultraviolet light breaks them down. Sometimes the first sign is simply a patch of cracks. In a heavy rain the lap above a rooflight can drive water in underneath, so the leak appears at the rooflight even though the fault lies in the sheet above it.

For safety, a weathered rooflight cannot be trusted to carry a person. A rooflight can look like a solid, flat part of the roof and give way without warning, and it is one of the main causes of serious falls through roofs. Moss, paint and a grey film of dirt hide the difference between a rooflight and a sheet. Even a rooflight with a mesh beneath it should be treated as fragile unless it is known to be rated otherwise. The Work at Height Regulations 2005 apply to anyone who goes onto the roof, whether to repair it, clean it or fit a sign.

What we find

Flat roofs on shops, offices, schools and small units are usually one of three things. A single-ply roof is a sheet of synthetic membrane, usually TPO or PVC, laid over insulation and joined at the seams by hot-air welding. A built-up felt roof is several layers of bitumen felt bonded together, usually with a mineral cap sheet on top. A liquid coating is a resin applied wet over an existing surface.

On single-ply, the usual trouble is at the details: the seam welds where a join was not fused properly, the corners and upstands where the sheet is folded and stressed, and the outlets, the drain points where water leaves the roof. A tear from a dropped tool or a fallen branch will do the same. On built-up felt, age is the enemy. The top layer cracks and loses its mineral grit, the laps open, and the layers beneath blister and let in water. In both, the root cause is often ponding: shallow standing water over months, which a flat roof laid without enough fall can never shed. If the insulation beneath has become wet, the roof loses its thermal performance and the weight of the water adds further load. The ceiling stain inside rarely sits below the opening, because water travels between the membrane and the insulation before it drops.

What we find

Many shops, offices and older industrial buildings have a gutter that is part of the building rather than hung from the edge. A parapet is a low wall that continues up past the edge of the roof. A box gutter is a channel set between two roof slopes or behind a parapet, lined in metal, asphalt or membrane. Both take the runoff from a large area of roof and funnel it to a few small outlets. They are the hardest-working parts of the building, and they do not forgive neglect.

The faults come in a predictable order. Leaves, silt and moss build up and block an outlet, and the gutter fills like a bath. A box gutter is built with a tight margin, so once water rises past the lining it spills over the top and runs down the inside of the wall, not away from it. Linings crack at the joints where metal has moved with the temperature. The pipes carrying water away fail at their joints and let it soak into the brickwork. On a parapet wall, the top coping stones or the flashing below them let go, and rain gets behind the lining. Because the water ends up inside the wall cavity or the structure below, the first sign can be damp on an internal wall some distance away.

What we find

The practical worry on most commercial jobs is not the roof itself but the disruption. A shop cannot close for three weeks. A food unit has to stay clean. A warehouse needs its loading bays clear and its racking dry. Occupiers also tend to ask a second question straight away: whose bill is this?

The answers depend on the building and the lease. Many commercial leases put the roof under the landlord's repair obligation, sometimes recovered through a service charge. Others make the tenant responsible for repair and the landlord for structure, or pass the whole building to the tenant on a full repairing lease. Which party arranges and pays for the work affects who we deal with, who gives access and who needs to approve the specification. We are roofers, not solicitors, and we do not give legal advice. Our answer on who repairs the roof, landlord or tenant sets out how these leases commonly divide it. Your solicitor or managing agent should read your lease before the work is ordered, since the answer is written there. A written survey with photographs and a fixed price helps either party see what is needed and what it costs.