Commercial roofing in Bath

A single-ply roof is a flexible sheet unrolled over insulation on a flat or gently sloping deck and joined to itself at the edges. The sheet is usually PVC, TPO or EPDM. EPDM is a synthetic rubber supplied in wide sheets and normally stuck down or held by ballast. PVC and TPO are plastics whose overlaps are welded with hot air, so the seam becomes part of the sheet rather than a glued strip on top of it. Because the membrane itself is thin, the roof depends far more on what lies beneath it and on how it is finished at every edge than on the sheet alone. Our guide to single-ply roofing covers the systems in more detail.

On industrial units and warehouses the first job is reading the deck. A single-ply sheet can be laid over an existing roof only if the old covering is sound, dry and flat enough to carry it. Wet insulation under an old membrane is the usual reason an overlay is ruled out. We find it by looking at where water has been getting in, lifting a small area at a time, and feeling for soft or sodden board. If the insulation is damp, it comes out. Laying a new sheet over wet board only seals the problem in, and the new roof inherits the old one's leak.

Where single-ply fails

Wear in the middle of a large membrane is rare. Trouble shows up at the details. Upstands at walls and plant bases need the sheet turned up and sealed, with a termination bar or a flashing to stop water getting behind the top edge. Flashing means the metal or membrane that seals the junction at a wall. Pipe penetrations need purpose-made collars, since a sheet cut around a pipe and daubed with sealant will open again as the pipe expands and contracts. Seams that were not properly cleaned before welding or adhesive may show as a thin dark line, and a run of failed seams usually points to a contaminated surface or a rushed day on the original job rather than a sheet at the end of its life.

Foot traffic is the other cause. Roofs that are walked by engineers servicing air handling units or cleaning rooflights pick up scuffs, dropped tools and cut corners at the edge of a path. Where the traffic is regular, a walkway of heavier membrane or paving pads is laid so that boots never touch the thin sheet. It is a small addition that saves a repair later.

Repair, patch or recover

A single failed seam or a punctured area can usually be cut out and patched with matching membrane, welded or adhered to a cleaned surface. Chemical compatibility matters, because a patch of the wrong material will not bond to the sheet around it. Where the membrane is crazed, shrunk away from the upstands or leaking in several places, patching becomes a poor use of money and we tell you directly. The usual choice is then a recover over sound insulation, or a strip back to the deck and a new build-up. Commercial roofs are priced on survey, because the system, the insulation, access and edge details all change the figure.

Many single-ply roofs on older units sit on a flat deck with very little fall. Standing water that remains for days after rain is a sign the deck has dished or the outlets are set too high. Neither is cured by the membrane. The build-up questions are the same ones we deal with on domestic flat roofs, only on a larger scale and with heavier equipment on top.

If you manage several units with the same membrane, tell us. The seams and details on one building often predict what we will find on the next, and a survey of one roof can inform a sensible order of work across the estate. We can also compare the condition of each roof on a single page, so spending is aimed where the need is greatest.

Most industrial sheds, workshops and farm-style commercial buildings are clad with profiled metal sheeting, usually steel with a factory-applied coating, sometimes aluminium. Sheets are lapped along their sides and ends and held down by fixings through the crown or the trough. A built-up system has a liner sheet inside, an insulation layer and an outer sheet, the whole supported on purlins, the horizontal rails running across the roof frame. The metal is only as durable as its coating, and the coating is weakest wherever the sheet has been cut. Our cut-edge corrosion guide explains the process stage by stage.

Cut-edge corrosion begins where a sheet was cut to length or around a rooflight, a flue or a vent. The edge exposes bare steel under the paint, and moisture creeps under the coating from there. First you see a rust-coloured line along the edge. Then the coating blisters and lifts, and the corrosion spreads in from the edge. In the worst case the steel perforates and the sheet leaks through a hole rather than at a joint. Sheets exposed to condensation from below, or to standing water in the overlaps, deteriorate faster than sheets that dry out quickly.

Fixings, laps and sealants

Fixings are the second pressure point. A self-drilling screw with a sealing washer seals the hole it makes, but washers harden with age and the sheet moves with temperature. Over time the screw backs out slightly and the washer cracks, leaving a tiny path for water. Sealant at the laps dries and shrinks. Neither failure shows from the ground, so we check them on the roof, sheet by sheet, and mark what we find on a roof plan.

The cure depends on the stage reached. Early corrosion can be treated: the edge is cleaned back to sound metal, a rust-inhibiting primer and a compatible coating applied, and the lap resealed. Where it has spread, a restoration system of treatment, coating and new fixings can give a roof another stretch of service. If the sheets are perforated across large areas, or the profile has lost its strength, new sheets are the sensible choice. Overcladding, fitting a new outer sheet over the old, is an option on some buildings, though it adds weight to the purlins and means looking at the structure first.

Old sheeting and asbestos

Cement sheets on older buildings need extra caution. Some corrugated cement sheeting installed in earlier years contains asbestos, and it must not be drilled, broken or scrubbed. If we meet it, we stop and explain the options. The HSE draws a line between asbestos cement sheets lifted whole and sheets that will be substantially broken up, and the rules differ on each side of it. Our page on asbestos on industrial roofs sets this out and links to the HSE guidance.

Roofs of this type often also carry rooflights and gutters, which fail at the same laps. We survey the whole line from ridge to eaves so the treatment works as one scheme rather than a series of isolated patches, and the access we set up once serves every stage of the work.

Commercial buildings rarely shed water from a roof edge straight to the ground. A parapet is a low wall that rises above the roof edge, and behind it sits a parapet gutter, which is a channel faced in lead, asphalt or membrane that gathers rainwater and leads it to an outlet. A box gutter does the same job in the angle between two roof slopes, often found on the long, double-span roofs of workshops, shops and school buildings. A valley forms where two slopes meet at an angle, so both shed water into a single run. These are the parts of a roof that carry the most water in the smallest space, so a small fault there has a large effect. We write about them in more detail in our page on industrial valley gutters.

Most failures start with blockage. Leaves, moss, litter and silt collect at outlets and in the low points, so water backs up and finds the weakest point in the lining. A gutter laid without enough fall keeps water standing after every shower, which ages the lining faster. Lead linings can split at the drips, the small steps built into a long gutter to carry water down, because lead expands and contracts with temperature and long sheets creep. Our lead roofing page covers how lead is detailed to cope with that movement. Asphalt linings craze and crack, and membrane linings fail at the seams or the upstands.

Flashings, copings and overflows

At a parapet, the flashing tucks into a chase cut in the masonry and folds over the edge of the lining. When the mortar joint above has eroded, water gets behind the flashing and runs down the back of the wall. Copings, the stone, concrete or metal cap on top of the parapet, can lose their jointing and let water straight into the wall below. A leak that appears on the inside of a parapet wall months after rain has often travelled a long way from the point of entry.

An overflow is a safety outlet set slightly higher than the main outlet. If the main drain blocks, water leaves through the overflow before it reaches the height of the doorway threshold or the flashing. A parapet gutter with no overflow depends on the outlet staying clear every time it rains. Checking that outlets and overflows are the right size, clear and set at the right height is quick work and is on every inspection.

Rainwater goods at the eaves

On buildings with open eaves, the gutters and downpipes are the other half of the drainage story. Sagging brackets, open joints and corroded cast iron or aluminium all show up as stains down a wall. Where the building is tall or the gutters long, access can be the main cost of the job. We bring the gutter work into the same programme as the roof work so the access is set up once, and for repair or replacement of the rainwater system itself see our gutters, soffits and fascias page.

Because a blocked outlet is the commonest root of these problems, clearing outlets on a regular visit protects the lining and the wall beneath it. It belongs in a maintenance schedule as much as in any repair, and a clear photograph of each outlet on every visit shows how quickly debris is building.