Industrial Flat Roofing

Why an industrial roof is a different problem

Most guidance written about commercial flat roofing quietly assumes a concrete or timber deck on a building of modest span — an office, a retail unit, a school block. An industrial roof is rarely that. It is usually profiled metal sheet spanning purlins across a portal frame, covering an area measured in thousands of square metres rather than hundreds, and it behaves differently in almost every respect that matters.

The deck flexes. A long span deflects under snow, under maintenance traffic and under its own build-up, and that deflection is what turns a roof that was laid to fall into a roof that ponds mid-bay. Drainage is usually internal — valley gutters running between bays, or perimeter gutters discharging to internal downpipes — so a gutter failure puts water inside the building rather than down an external wall. Thermal movement over a fifty or eighty metre run is measured in tens of millimetres, and any detail that does not accommodate it will eventually fail at the fixing. And the sheet itself is a working surface that people have to get onto, which raises questions of fragility that simply do not arise on a concrete deck.

That is why a credible industrial quotation starts with a survey rather than a rate. Two roofs of identical area on the same estate can need entirely different work — one a £30/m² coating job with fifteen years left in the substrate, the other a full strip because the liner tray is corroded through and the purlins need checking.

The four routes, and when each one is right

Almost every industrial roof decision resolves to one of four options. They are not interchangeable, and a contractor who only ever proposes one of them is selling a product rather than surveying a roof.

Localised repair is right where the covering is sound and the failures are discrete and identifiable — a handful of failed laps, a cracked upstand, corroded fasteners in one bay. Indicatively £25–£70/m² over the treated area, buying perhaps five to fifteen years. It is the wrong answer when you have been back three times in two winters, because at that point you are funding the same defect repeatedly without addressing it.

Liquid over-coating — cold-applied PMMA or polyurethane taken over the existing profile, laps and fasteners — suits a metal roof whose sheet is structurally sound but whose coating has weathered and whose laps and cut edges are beginning to corrode. It is a genuine option on roofs where access, occupancy or cost rules out a strip, and cold-applied systems avoid hot works entirely, which matters where the insurer has restricted them. Site preparation is the whole job: the coating is only as good as the cleaning, the treatment of cut-edge corrosion and the detailing at fasteners and laps.

Over-sheeting installs a new profiled outer sheet on a spacer system above the existing roof, with new insulation in the void. Because the old sheet stays put, the strip-out and disposal cost falls away and the building usually stays in use throughout — which is why it is often the pragmatic choice over an occupied production floor. It is also the standard route where the existing sheet is asbestos cement and encapsulation is preferable to removal. We cover the method and its limits in detail on the over-sheeting and over-cladding page.

Full strip and re-roof is the honest answer where the liner tray or purlins are corroded, where the existing build-up cannot carry another layer, or where the roof is being reconfigured. It is the most expensive and most disruptive route and the only one that lets you verify and repair the structure underneath. It is also the point at which the thermal-element rules almost certainly apply.

Falls, ponding and the gutters that actually fail

BS 6229:2025 is the current standard for flat roofs of continuously supported construction. It replaced the 2018 edition at the end of 2025 and it changed the design approach: the old rule of thumb that you designed to 1:40 to finish at 1:80 was removed. What the standard now requires is that the finished fall is no flatter than 1:80, with the design fall established from a structural analysis and a level survey of the actual deck rather than from a rule of thumb. On a long-span industrial deck that distinction is the whole point, because the deflection you are designing around is real and measurable.

A zero-fall roof is a defined design case, not an accident: it sits between 0° and 1:80 and it must be laid without back-falls, with a build-up specified for permanent water contact. It is a deliberate choice, and it should appear on a drawing rather than emerging from a roof that was supposed to fall and does not.

In practice, the failures we are called to on industrial roofs concentrate in the drainage rather than the field of the sheet. Internal valley gutters silt up, the outlets block, the water backs up above the level of the sheet laps and then finds its way inside — often a long way from the actual defect, because it tracks along a purlin before it drops. Gutter lining is frequently the single highest-value intervention on an industrial roof, and it is routinely omitted from quotations that price the field area and stop there. Ask specifically what is proposed for the gutters, the outlets and the overflow provision.

Wind uplift: why perimeters and corners fail first

Wind does not load a roof evenly. Airflow separating over the edge of a large, exposed industrial roof generates suction that is several times higher at the corners and along the perimeter than it is in the middle of the field. Calculation to BS EN 1991-1-4 divides the roof into those zones and sets a fixing pattern for each, which is why a properly specified mechanically-fixed roof has a visibly denser fastener pattern round its edges.

This is the most common place we see a specification quietly value-engineered. A uniform fixing pattern across the whole roof is cheaper to price and quicker to lay, and it will survive ordinary weather perfectly well for years — until the storm that peels the covering back from a corner. On a building of any exposure, ask to see the uplift calculation and the zoned fastener schedule that came out of it. On a large industrial roof it is not a formality; it is the difference between a covering that stays on and one that does not.

Part L, U-values and when the rules bite

Re-roofing an industrial building is usually a controlled thermal-element job. The trigger is renovation of more than 50% of the surface of the individual element — the roof — or more than 25% of the total building envelope. Cross either threshold and the element has to be upgraded to the current standard, which for a re-roof is around 0.18 W/m²K, established by calculation for the actual build-up rather than assumed.

On an industrial roof this is a genuine design constraint rather than a box to tick. Insulation thickness sufficient to reach 0.18 W/m²K adds height at every abutment, upstand and rooflight kerb, and it adds load. On an over-sheeting job the insulation sits in the void created by the spacer system, so the spacer height is set by the thermal calculation, not chosen for convenience. Getting that sequence the wrong way round — picking a spacer and then discovering the U-value will not be met — is a common and expensive mistake.

Building control notification matters too. A contractor registered under a competent person scheme such as CompetentRoofer can self-certify notifiable re-roofing work and issue a Building Regulations Compliance Certificate, which saves a separate building control application. We are a lead-generation service rather than the installing entity, so that is a capability we look for in the contractors we connect you with, not a registration we hold ourselves.

Asbestos, fragility and working on the roof

Industrial roofs built before 2000 may contain asbestos cement sheet or asbestos in the rooflights, and a great deal of British industrial stock falls in that window. The duty to manage under the Control of Asbestos Regulations means a survey and a management plan sit ahead of any physical work, and the finding materially changes the options: encapsulation or over-sheeting becomes considerably more attractive than a strip that generates licensed waste.

Fragility is the other constraint that surprises building owners. Weathered profiled sheet and, especially, aged GRP rooflights will not reliably support a person. Falls through fragile roofs and rooflights remain one of the most persistent causes of serious injury in construction, and the practical consequence for you is that any quotation must include the access, staging and edge protection to work safely. A price that looks unusually keen on a large industrial roof is often a price that has not costed that properly.

Can the roof carry solar?

Large industrial roofs are the obvious candidates for rooftop PV, and the question we are asked most often is whether the structure will take it. A ballasted, penetration-free array typically imposes somewhere around 15–25 kg/m² on the roof, and that can rise towards 30 kg/m² in exposed locations where more ballast is needed to resist uplift. Whether that is acceptable is a structural question about the existing frame and purlins, answered by an engineer, not by a roofer.

The sequencing point is more important than the number. Mounting an array on a covering with eight years of life left commits you to lifting and reinstating the entire array to re-roof underneath it. If the roof is anywhere near the end of its service life, do the roof first and specify it knowing what it will carry — the fixing pattern, the ballast distribution and the walkway provision all change. Our guide to whether a commercial flat roof can carry solar panels works through the assessment in more detail.

What industrial flat roofing costs

We publish indicative supply-and-fit ranges rather than a single headline rate, because on an industrial roof the spread between a light-touch coating and a full strip is larger than the difference between any two membrane choices. Full ranges are on the commercial flat roofing cost page; the figures relevant to industrial work are these.

Mechanically-fixed single ply on a warm deck runs around £90–£160/m², with a service life of roughly 25–35 years and a manufacturer guarantee commonly in the 20–30 year range where an approved contractor installed it. Built-up felt and reinforced bitumen systems sit around £90–£150/m². Cold-applied liquid and GRP systems run around £100–£180/m². Localised repair and overlay work sits at £25–£70/m² over the treated area. Over-sheeting typically lands between the repair band and a full single-ply re-roof, because the existing sheet stays in place and the strip-out and disposal cost comes out of the job.

VAT is charged at the standard 20% on commercial roofing work (VAT Notice 708), recoverable as input tax by a VAT-registered business. On the capital-allowances side, insulation added to an existing building is generally a special-rate expenditure item rather than a main-pool one, and the practical route is usually the Annual Investment Allowance — confirm the treatment of any particular scheme with your accountant before you rely on it.

Set against those figures, the number worth calculating is what the current roof costs you. An industrial roof being patched reactively can absorb an indicative £6,000–£9,000 a year without ever getting better, before you count a single ingress over stock, plant or a production line. Over a ten-year horizon that comparison usually decides the question on its own.

How an industrial roof survey works

A useful survey produces four things: a condition assessment of the covering, the laps, the fasteners, the gutters and the rooflights; a level survey establishing the residual falls and where water actually stands; a view on the deck and structure including any asbestos finding; and a written recommendation that names the system, the build-up and the guarantee it carries. Anything less than that is an estimate dressed up as a survey.

What you should get back is an itemised written proposal — the system and manufacturer named, the insulation specification and the calculated U-value, the zoned fixing pattern, the gutter and outlet works, the access and safety provision, the programme, and the guarantee term with what it covers and what voids it. If a proposal will not name the system, it cannot be compared with another one.