Commercial buildings share a family resemblance from the road, but their roofs do very different jobs. A distribution shed needs a big, simple weathering skin that goes on quickly and sheds water without fuss. A workshop with machinery and a loading door wants the same, plus attention to noise, condensation and daylight. An office building, often with a parapet wall, plant on the roof and people working underneath, needs a roof that is quiet, well insulated and tidy at the edges.
Three systems account for the roofs we are asked to survey: profiled metal sheeting, single-ply and felt built up in layers. Each has a place, a typical lifespan range and a set of ways it fails. None is best in the abstract. The right one turns on the slope of the roof, the structure beneath it, how the building is used and how much attention the owner will give it afterwards.
This guide sets out how the three compare and what we look at when we survey a unit or an office roof. If you already have a roof you want to understand, our page on types of commercial roof shows what you are probably standing under.
The building decides the roof, before the material does
Start with what happens under the roof, because that sets the demands on it. In a storage warehouse the main need is to keep rain out at low cost over a very large area. Heat and cold matter less, and a plain pitched shed with metal sheets suits it. In a food or electronics workshop, the interior may need a stable temperature and a roof that does not drip condensation onto the work, so insulation and sealing carry more weight. In an office or showroom, comfort and appearance join the list.
Pitch and structure then narrow the choice. A steel portal frame with light purlins is designed for a lightweight covering and will not take heavy loads without checking. A concrete deck or a timber deck suits a flat roof system. A pitched roof of low angle may take sheeting but not tiles. Before any system is chosen, the existing structure has to be known, which is one reason the survey includes the inside of the building.
Pitch, falls and drainage
Water has to leave the roof. On a flat roof, the design fall is 1:40 to reach a finished minimum of 1:80, according to IKO citing the 2018 edition of BS 6229 (since revised), which leaves room for building tolerances and sag between supports. A metal-sheet roof drains by its profile, but it still depends on its gutters, outlets and, at the end, downpipes that can take a storm.
Wind exposure is another factor on this coast. Sheets, membranes and their fixings take the full force of sea weather, so the fixing pattern, the edge trims and the way the laps are sealed all deserve more attention than on a sheltered inland site. We say what the fixings are and where, not just what the covering is.
It is worth deciding what the roof must do before choosing how it does it. The sections below take each system in turn.
Profiled metal sheet: the usual warehouse roof
Profiled metal sheeting is the covering that comes to mind for an industrial unit. Steel or aluminium sheets are rolled into a ribbed or corrugated shape that gives them strength across the span, and they are fixed to the purlins with screws and sealed at the laps. They are light, fast to lay on a large roof, available in many colours and easy to combine with rooflights.
There are two broad versions. A single-skin roof is one sheet with, at most, a liner and some insulation hung underneath. A double-skin or built-up system has an inner liner, a layer of insulation held on spacers and an outer sheet, which is better for temperature control and for reducing condensation. Insulated composite panels combine the layers in one factory-made unit and go on quickly.
How metal roofs fail
The weak points are well known. Fixings can back out, corrode or lose their sealing washers. Laps open where sheets have moved with heat and cold. Cut edges, where a sheet was trimmed on site and the protective coating was broken, can start to rust and creep under the paint. Gutters between two sheeted slopes collect leaves and silt, and when they leak they do so into the building. Rooflights, usually plastic or glass-reinforced panels set in the sheet pattern, age faster than the sheets around them and can become fragile.
Coastal air narrows the margin. Salt-laden damp settles on metal and works at any weakness in the coating, so inspecting fixings and edges earns its keep, and we look closely at both.
A well-fixed, well-maintained sheeted roof can run for many years, and a refurbishment, such as a coating or overcladding, often costs less than stripping and starting again.
Single-ply membrane on flat and low-pitch roofs
Single-ply means one factory-made layer of waterproofing, laid on insulation over a deck. The two families you meet most are EPDM, a synthetic rubber, and PVC-based membranes, which are welded at the seams with hot air. They are used on flat roofs, on low-pitch roofs and over complicated shapes where long sheets can be cut and folded into place with few joins.
The way the sheet is held down varies. It can be stuck to the insulation with adhesive, held with mechanical fasteners along the seams or laid loose and weighted with ballast, usually gravel or paving, which also protects it from sunlight and foot traffic. Each suits different roofs. Ballast needs a structure that can carry the added weight. Mechanical fixing needs a deck that holds fasteners. Sticking it down needs a clean, dry, sound surface, and it is rarely an option on a roof with wet insulation underneath.
Lifespan and what to expect
GreenSpec puts the expected service life of EPDM at 20 to 30 years, while other guides go higher, so the safe wording is 20 to 30 years or more. Flat roofs generally run for 15 to 30 years, according to MyBuilder (updated 21/08/2026). Those are ranges, never promises, and installation quality counts as much as the membrane.
Details do the work. Upstands, where the sheet climbs a wall or parapet, along with corners, the outlets and the penetrations for pipes and plant are where leaks begin, not the open field of the sheet. A roof with many penetrations is more prone to trouble than a clear one, so the position of plant matters.
On an office block, single-ply is often the preferred answer because it can sit under a roof terrace or around services and takes a neat finish at parapets. On a big warehouse it competes with metal sheet, and the deck and the budget usually settle it.
Built-up and torch-on felt
Built-up felt is the older flat-roof system: layers of bitumen-impregnated felt, usually a base layer and a cap sheet, stuck together and to the deck. Torch-on felt is the version where the layers are heated with a gas torch to melt the bitumen and fuse them. A well-laid multi-layer roof is a very different thing from the single thin layer that gives felt a poor name.
The attraction is familiarity and repairability. Felt roofs can be patched, re-capped or overlaid by most flat-roofing trades, and the material copes with awkward shapes. The drawbacks are the joints. Every lap, upstand and outlet is a seam that can open as the roof moves, and a sloppy job fails early.
How long felt lasts
Published lifespans disagree, which tells you something. Less gives 10 to 20 years for torch-on felt, adding that a well-laid roof can reach 20 years, whereas a badly laid one may give out before 10. CORC suggests torch-on that is looked after properly can last 30 years or more. The gap between those figures comes down to the quality of the work and the upkeep, and it is the reason we describe a lifespan as a range with conditions.
Safety matters when felt is laid. Torching on a commercial roof means naked flame, so the plan has to cover fire watch, access, what is stored nearby and how the roof edges and combustible materials are handled. A contractor should be able to explain the arrangements, and some owners choose a cold-applied or mechanically fixed alternative to avoid the question. We discuss method with the building owner and, where it applies, with their insurer.
Felt remains a sensible choice for small flat roofs on workshops, extensions and plant rooms where cost and simplicity count. For a large, heavily trafficked roof, we would look at the alternatives first and say why.
Office buildings: parapets, plant and people underneath
An office roof puts more detail into a smaller area. Parapet walls rise above the roof edge, and the joint where roof meets wall is a flashing detail that has to survive years of movement. Rooftop plant, such as air-handling units, extract fans and aerials, brings penetrations, cable routes and foot traffic. Rooflights and smoke vents break the surface. Roof drainage is more likely to run through internal outlets and pipes that are hidden inside the building, which makes blockages harder to see and leaks more disruptive.
Because people work underneath, comfort and condensation matter. A warm roof has its insulation above the structure and the weatherproof layer on top of that, keeping the deck warm and cutting the risk of condensation forming inside. A cold roof has insulation below the structure and relies on ventilation to carry moisture away. The two behave differently when they get wet, and a refurbishment has to respect whichever was built.
Access and planned upkeep
Plant engineers walk office roofs more often than anyone else, and foot traffic wears membranes. Walkways and protection mats on the routes to the plant are a small cost that saves damage. Gutters and outlets need clearing on a routine, ahead of the autumn leaf fall and before winter storms, and the same applies to parapet gutters.
Plenty of office roofs also carry or may later carry green roof planting or solar panels, which add weight and penetrations and need careful detailing of the membrane beneath. If either is in prospect, tell the roofer before the design is settled.
The page on commercial roof maintenance plans sets out what an inspection should cover and how often it is worth doing.
Matching the system to the building: a quick guide
The table below is a starting point for conversation and nothing more. Every roof has its own history, and the survey decides. It shows what each building type tends to need, which systems usually suit it and what we check first.
| Building | Systems that often suit | First checks |
|---|---|---|
| Storage warehouse | Profiled metal sheet, composite panels | Fixings, laps, gutters, rooflights |
| Workshop or factory unit | Insulated metal system, single-ply | Condensation, wet insulation, outlets |
| Small flat-roofed unit | Built-up felt, EPDM | Falls, ponding, upstands |
| Office block | Single-ply, warm roof build-up | Parapet flashings, plant, internal drains |
Whichever system is chosen, three questions help. What is the structure beneath it, and can it carry the load? What happens at the edges, the penetrations and the outlets, since these are where roofs fail? And who is going to look after it, and how often?
Planning and approvals
New work on a commercial roof can raise planning or building control questions, such as a change of appearance, a rooflight array or a raised plant structure. Our page on commercial roof planning sets out what to check and with whom. When you choose a contractor, our guide to choosing a commercial roofing contractor lists the questions to ask.
For a unit like yours we start with a survey, photograph what we find across the roof, and then write up one total price. If you are weighing a change, commercial roofing is where to start, and the quote form takes only a few minutes.
