A commercial roof is a system, not just a covering. The visible sheet or membrane sits on a deck, usually with insulation under or over it, and it depends on edges, outlets and upstands that are fixed to the building around it. When a shop, unit or office roof fails, the cause is as often one of those supporting parts as the covering itself. Choosing a system therefore starts with the building: its shape, its structure, the fall of the roof, how often people walk on it and what happens beneath it. This guide describes the families of system we see on commercial buildings in and around Bath, and how they differ in construction, behaviour and upkeep. It is an overview, so it points to our more detailed pages where a topic deserves one. The right system for your roof is settled on site, after we have looked at the deck and the drainage and photographed what is there.
How commercial roofs divide into families
Almost every commercial roof falls into one of four families, and knowing which one you have is the first step to making a sensible decision about it. The first is the flat or low-slope roof finished with a continuous waterproof layer, which may be a single-ply membrane, built-up felt or a liquid-applied coating. The second is the profiled metal roof, where long sheets span between purlins on an industrial shed or warehouse. The third is the standing-seam or formed metal roof, in zinc, copper or aluminium, more common on offices, civic buildings and the upper floors of period properties. The fourth is the pitched roof in slate or tile, which turns up on older shops, pubs, chapels and converted mills.
These families behave differently because they shed water differently. A pitched roof throws water off by gravity and overlap, so its covering is a series of small units that are allowed to leak a little at the laps. A flat roof has no such margin. It relies on one unbroken layer and on every join, upstand and outlet in it being sound, so a small defect can turn up as a brown patch on a ceiling well away from the real fault.
It also helps to separate the covering from the build-up beneath it. A warm roof puts the insulation on top of the deck with the waterproofing over it, so the structure stays at room temperature. A cold roof puts the insulation beneath the deck and leaves a ventilated void above it. Many older commercial buildings in Bath have had one or both altered over the years, so what you see from the street may not match what is under it. We find out on the survey, because the build-up governs what can be laid on top of it.
Single-ply membranes: PVC, TPO and EPDM
Single-ply is the term for a factory-made sheet, laid in one layer, that forms the waterproofing. The three you will meet are PVC, TPO and EPDM. EPDM, a synthetic rubber, arrives in very large sheets, so a big roof can have few joins. PVC and TPO are plastic-based membranes whose seams are normally joined by hot air welding, which fuses the two sheets into one. The detail is covered on our page about single-ply membranes on commercial roofs.
The attraction is speed, light weight and a modest number of seams. A membrane may be screwed down through the insulation into the deck, stuck with adhesive, or held in place by a layer of ballast, and the deck, the wind loading and the use of the roof decide which. Mechanical fixing needs a sound deck that will hold a fastener. Adhesive needs a clean, dry, compatible surface. Ballast needs a structure that can carry the added weight, which is a question for an engineer, not a roofer's guess.
Typical lifespans are published as ranges that depend on the product, the exposure and the quality of the work, and they are not a promise. For EPDM the sources we rely on give 20 to 30 years or more. For PVC the range is about 20 to 30 years, with some sources going higher, and for TPO it is commonly 20 to 35 years depending on the product. The certificate for the specific membrane will tell you what has been tested, so ask to see it.
Where single-ply is weaker is at the details. Corners, pipe penetrations, rooflight upstands and parapet junctions are made on site, with pre-formed pieces or hand-worked patches, and that is where most leaks start. A good roof of this type is judged on its edges more than its field of membrane.
Built-up felt and asphalt
Built-up felt is the traditional multi-layer system: a base layer, one or more further layers and a protective cap sheet, stuck down with hot bitumen or by melting the underside with a flame. It is widely used on shop extensions, rear flat roofs and older warehouses, and it has the advantage of being familiar to every roofer and straightforward to patch. The full story is on our page about built-up felt on commercial buildings.
Its weakness is that bitumen ages. The surface dries, the mineral finish scours and the layers can lift at laps and upstands, so roofs of this type tend to show their age as splits, blisters and cracks around the edges. The sources we use give a typical life of 10 to 20 years, longer for a well-made three-layer system, and note that a well-laid roof can reach 20 where a poorly laid one may fail much sooner. Torch-on work also demands careful fire-safety planning, and we check the deck, the adjacent materials and the working method before a flame is used near a commercial building.
Mastic asphalt is a related material, laid hot in two coats over a separating layer. It sets as one continuous surface and is often used for balconies, walkways and the flat roofs of older buildings, including listed ones. It is heavier than felt, needs specialist plant and skills, and is slower to lay, so it is chosen where a continuous surface and a traditional appearance are worth that effort.
For owners, the useful point is that felt and asphalt are repairable. A defect can often be cut out and patched, which is a sensible choice while the rest of the roof still has life in it. When cracking has spread over every slope, patching stops paying for itself and a new covering is the sensible answer.
Profiled metal sheet and standing seam
Profiled metal sheeting is the commonest covering on industrial sheds and warehouses. Steel sheets, usually with a plastic coating, are formed into ridges and troughs that give the sheet stiffness, and they are fixed to purlins, the horizontal beams that carry the roof. Some roofs are single-skin, with insulation hung beneath. Others are built up from a lining sheet, spacers, insulation and an outer sheet, which holds the insulation in a controlled cavity. Insulated composite panels are a third option, supplied as a factory-made sandwich that goes up in a single operation.
Life is long when the coating survives. Supplier and trade sources give wide ranges, from 15 to 25 years before the first maintenance to well over 40 years overall, and coastal or industrial atmospheres shorten it. The main enemy is cut-edge corrosion, where the steel core is exposed at a cut or a fixing hole and rust creeps under the coating. Our page on cut-edge corrosion on metal sheet roofs goes into it. Sealed laps and failing fasteners are the other usual leak points.
Standing seam is a different kind of metal roof. The sheets, in zinc, copper, aluminium or stainless steel, are folded together along upstanding seams, so there are no screws through the surface, and the roof can flex with the heat. Zinc is reported to last from 60 to well over 100 years, and copper at least 80 years where it is well laid and supported. It is a premium option and is detailed carefully at eaves, ridges and abutments, so we would not recommend it where nobody is going to maintain the gutters.
Lead sheet, which is the traditional metal for flashings and gutters, is also used as a main covering on some flat roofs and dormers. We explain it where it appears, and give the sourced figures in the materials page.
Pitched commercial roofs in slate and tile
Not every business works from a flat roof. In the centre of Bath and in towns such as Frome, Radstock and Bradford on Avon, many shops, pubs, offices and workshops sit under pitched roofs in slate, clay tile or stone. These are built like a house roof: a covering fixed to battens, with an underlay (the membrane under the battens) and rafters beneath, with lead dressed at the chimneys, valleys and walls. Two terms recur below. Two slopes that meet in an inward corner form what roofers call a valley. The lead strips that seal the roof against a wall or stack are flashings.
Commercial buildings often add features that a house does not have. There may be a parapet gutter, a channel hidden behind the front wall, or a box gutter running between two roofs. These are the hidden parts of the system. Water collects in them, outlets block, and a crack in the lead where one length steps down shows as damp in the room below. The lead is specified by code, the standard grading of sheet thickness, and relaying a gutter in a higher code is routine work on a building that has had long service.
A listed or protected building brings the look of the covering into the decision, not only its performance. Ask B&NES planning before any change of material, since permission for that is decided apart from the roofing. We say more on that in our guide to commercial roofs on listed buildings.
When a pitched commercial roof is re-covered, the same Building Regulations thresholds apply as to any other roof. Small repairs fall below the threshold at which building control gets involved, namely less than 25 per cent of the envelope and less than 50 per cent of the roof. Above that, a thermal upgrade may be needed, and so may a structural check if the new covering is 15 per cent or more heavier.
Matching a system to the building and its use
The best system is the one that suits the building underneath it, so we start with five questions. What is the structure, and what load can it carry? What is the existing build-up, and is it dry? What falls does the roof have, and does water leave it? How often does anyone walk on it, whether for plant, cleaning or access to rooflights? And what happens inside: heat, steam, dust, food or people?
| Building and use | Systems usually considered | Main point to settle |
|---|---|---|
| Industrial unit or warehouse | Profiled metal, insulated panels, or a single-ply overlay | Fixings, laps and rooflights |
| Shop with a flat rear roof | Built-up felt, single-ply or asphalt | Outlets, upstands and access |
| Office with plant on the roof | Single-ply, with walkways | Foot traffic and penetrations |
| Period or listed premises | Slate, tile, lead, asphalt or zinc | Consent and matching materials |
| Block of flats | Single-ply, felt or pitched covering | Warm-roof build-up and who pays |
Falls deserve a word. The standard for flat roof design, BS 6229:2018, recommends a minimum finished fall of 1:80, and designers aim for 1:40 so that tolerances and deflection still leave a working fall. A roof that ponds water for days after rain is carrying a load the designer did not intend, and it will age faster.
An overlay is sometimes the right answer, where a sound old roof can take a new covering and avoid stripping it. We cover the conditions in our guide to overlay systems, and how long each system tends to last in our answer on commercial roof lifespan. For a comparison of the materials themselves, read our guide to commercial roofing materials.
What a commercial roof survey settles
A survey turns a general choice into a particular one. We go onto the roof, lift or cut back the covering where we need to see the layers, and photograph your own roof, so that you can see the insulation, the deck and the drainage for yourself. We note the falls, the outlets and the state of the upstands, and whether the existing insulation is wet. That tells us whether the roof can be repaired, overlaid or stripped, and what system is worth specifying.
After the survey, a fixed price arrives in writing. It lists the system, the insulation, the edge details, the access and the way the work is phased around your trading. Where a repair will do, the repair is what we recommend and price. Where work affects the building envelope, we explain the Building Regulations side and deal with the notification. If the building has an asbestos register, have it to hand for the survey, since older commercial roofs often hide it in sheets or felt.
Commercial roofs also come with obligations outside the roof itself. A tenant may be responsible for repair under a lease, a managing agent may need to consult leaseholders, and an insurer may expect a record of inspection. Those subjects have their own pages, and the survey report is written so it can be handed on to whoever needs it. If you are weighing systems for a particular building, the next step is a survey of that building.
The wider commercial roofing service page sets out the repair, replacement and maintenance work available for business premises.