Roof Over-Sheeting and Over-Cladding

What over-sheeting actually is

Over-sheeting — also called over-cladding when the same idea is applied to walls — is a re-roofing method rather than a material. A spacer system is fixed through the existing profiled sheet into the purlins below. New insulation is laid into the void that the spacers create, and a new profiled outer sheet is fixed to the spacers above it. The old roof stays exactly where it is, doing nothing structural but no longer exposed to weather.

The attraction is commercial rather than technical. On a large industrial roof the strip-out and disposal is a substantial share of the cost and almost all of the disruption: it opens the building to the weather bay by bay, it requires temporary protection over whatever is below, and on an occupied production floor it often forces shutdowns. Over-sheeting removes that phase entirely. The roof goes on above a building that carries on working.

The method belongs to profiled metal roofs and to low-pitch industrial construction. It is not a technique for a flat membrane roof on a concrete deck — the equivalent there is an overlay or a strip-and-recover, which we cover on the commercial flat roof refurbishment page. If your building is a warehouse, factory or distribution unit with a trapezoidal metal roof, over-sheeting is likely to be on the table. If it is an office with a flat membrane, it is not.

When over-sheeting is the right call

The condition that matters most is not the outer sheet — it is everything beneath it. Over-sheeting suits a roof where the structure is sound and the weathering surface has reached the end of its life. That is a very common combination: the outer sheet takes the UV, the rain and the thermal cycling, so it degrades decades before the purlins do.

It is a strong choice where the building cannot be taken out of use. Cold storage, continuous production, occupied distribution — anywhere a phased strip would mean lost operating days — the arithmetic usually favours over-sheeting even when the raw construction cost is comparable, because the downtime does not appear on the roofing quotation but does appear on the P&L.

It is also the standard route where the existing sheet is asbestos cement. Leaving the material undisturbed under a new skin avoids generating licensed waste and avoids the controlled removal that goes with it. That decision has to be taken with a competent asbestos surveyor and recorded in the management plan, but the roofing logic is straightforward: encapsulation is very often proportionate where removal is not.

Finally, it is a natural opportunity to fix the thermal performance. The spacer void is where the insulation goes, so a roof that had 25 mm of degraded board — or nothing at all — can be brought up to current standards in the same operation.

When over-sheeting hides a problem instead of solving one

This is the part that gets left out of an over-sheeting sales pitch, so we will be direct about it. Over-sheeting is a bad answer in four situations.

The liner tray or purlins are corroded. The spacer system fixes into the purlins. If the purlins have lost section, or the liner is perforated, you are fixing a new roof to a structure that is going. It will be considerably more expensive to correct once it is buried under a new sheet.

There is no residual structural capacity. The spacers, insulation and new sheet add dead load, and the assessment has to include what else the roof is expected to carry — including any future solar array at roughly 15–25 kg/m², rising towards 30 kg/m² in exposed locations. That is an engineer's calculation on the actual frame, not a rule of thumb.

The roof is already wet inside the build-up. Sealing saturated insulation under a new roof traps the water. It will keep corroding the deck and the fixings and you will not be able to see it happening. A moisture survey answers this before the decision is made, not after.

The falls are wrong. Over-sheeting broadly follows the existing geometry. If the roof ponds because it was laid without adequate fall, or because the deck has deflected, a new sheet on spacers will pond in the same places. Correcting falls means a build-up that can be tapered — which usually means a different method.

Falls, drainage and the gutter question

BS 6229:2025 requires a finished fall no flatter than 1:80, with the design fall established from structural analysis and a level survey rather than from a rule of thumb. On an over-sheeting job the practical question is whether the existing geometry already delivers that once the new sheet is on, and the honest answer on a deflected long-span roof is often that it does not without intervention.

The gutters need a decision of their own, and they are the most frequently omitted item on an over-sheeting quotation. Raising the roof plane by the depth of a spacer system changes the relationship between the sheet and the existing valley or perimeter gutter. Either the gutter is lined and brought up to suit, or it is replaced, or the new sheet discharges into a gutter that is now sitting too low and too small for it. Since gutter failure is the single most common cause of water getting inside an industrial building, this is worth pressing on before you compare prices.

Thermal performance and Building Regulations

Over-sheeting a roof is normally a renovation of a controlled thermal element. Once the work covers more than 50% of the roof surface — or more than 25% of the total building envelope — the element must be upgraded to the current standard, which for a re-roof is around 0.18 W/m²K, demonstrated by calculation for the actual build-up.

The sequencing point matters more here than on any other method. The insulation sits in the spacer void, so the thermal calculation sets the spacer height. Choosing a spacer system first and then discovering it cannot accommodate the insulation depth required is a genuinely expensive mistake, and it is one we see. Ask for the U-value calculation and the spacer specification together, as one decision.

Interstitial condensation deserves a mention too. You are creating a new void above an old roof, and where the existing sheet is relatively vapour-tight the risk profile is different from a conventional warm deck. A condensation risk analysis belongs in the design. Where the work is notifiable, a contractor registered with a competent person scheme such as CompetentRoofer can self-certify and issue a Building Regulations Compliance Certificate — a capability we look for in the contractors we introduce, rather than a registration we hold.

What over-sheeting costs

We do not publish a single rate for over-sheeting, because the spread is driven by the spacer height the thermal calculation demands, the condition of the substrate and how much gutter and rooflight work comes with it. What we can say honestly is where it sits relative to the alternatives on our commercial flat roofing cost page.

Over-sheeting typically lands between the localised repair and overlay band of £25–£70/m² and a full mechanically-fixed single-ply warm-deck re-roof at £90–£160/m². It is more than a coating and less than a strip, and the reason is structural: you are buying a complete new outer envelope and new insulation, but not paying to remove and dispose of the old one. Where the existing sheet is asbestos cement, that avoided disposal cost is the single largest line item you are saving.

VAT is charged at the standard 20% on commercial roofing work (VAT Notice 708) and is recoverable as input tax by a VAT-registered business. Insulation added to an existing building generally falls to the special-rate pool rather than the main pool, with the Annual Investment Allowance the usual practical route — confirm the treatment of your scheme with your accountant.

Over-cladding walls, and where we stop

The same principle applies to walls: a new outer skin on a spacer system over failed or thermally poor cladding, without stripping what is there. It is frequently specified in the same programme as a roof over-sheet, because the access and the scaffold are already up and the junction between the two has to be detailed as one.

We are a flat and low-pitch roofing specialist, so wall over-cladding is not where our depth is. For a project led by the walls rather than the roof, Commercial Roofing Quotes covers industrial cladding properly. We would rather point you somewhere useful than pretend a roofing page is a cladding page.