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Project record · Fibre-cement encapsulation

A 2340 m² asbestos-cement roof, kept in place and sealed from below.

A single-storey commercial building under a profiled asbestos-cement roof. The sheet stayed exactly where it was; 50 mm of closed-cell foam went on from the underside. Here is what we found, what we specified, and what it did.

2340m²
Roof treated
50mm
Closed-cell, from below
≈0.53W/m²K
Treated-roof U-value, calculated
≈9°C
Warmer roof underside in winter

Footage and photographs

The roof, seen from below.

The layer going on, and the roof sprayed with the stock still in place. Our own footage and photograph from the job.

Project recordEncapsulated from beneath
Inside the building under its asbestos-cement roof: the steel truss roof sprayed from beneath in a continuous cream closed-cell layer, polythene screening hung across the bay and stock kept at the far end.
Project recordSprayed, stock in place

All project records

01The building

A working roof, made of a material you do not touch.

A single-storey commercial building under profiled asbestos-cement sheet. Thin, cold and fragile, it is exactly the kind of roof that sweats in winter and cannot be walked on, cut or taken down without a great deal of care and cost.

Why these roofs run wet. Fibre-cement sheet has very little insulating value of its own. On a cold night the underside drops below the dew point of the air inside, moisture condenses on it, and the building gets drips that look, at first, like a leak. Stock, racking and anything else beneath the roof take the consequences.

Why nobody wants to touch the sheet. Older sheets may contain asbestos, so the roof is not walked on, drilled or removed if there is any way to avoid it. Stripping and replacing a covering of this size also means scaffold, downtime and a skip full of regulated waste.

The sensible move. Leave the covering where it is and work from beneath it. The roof was to be kept: the brief was to remove the condensation risk and improve its thermal performance without replacing it. The rest of this page is what that meant in practice.

Building
Commercial, single-storey
Roof
Profiled asbestos-cement sheetKept in place throughout
Treated from
The underside
Application
50 mm closed-cell, Enverge NexSeal LE
Completed
Completion pack issued 15 June 2026
Client
Unnamed

02What we found

1950 m² on plan. 2340 m² of face to cover.

A profiled sheet has more face than the plan shows, because the layer follows every rib. We survey the plan area, apply the sheet's stated profile allowance, and price and report on the face we actually treat.

Section, not to scaleThe allowance is the sheet's stated figure.

03What we specified, and why

50 mm closed-cell on the underside, so the sheet is sealed and the roof stays.

One continuous layer, sprayed to the underside of the sheet, that insulates and seals in the same pass. Nothing on the roof is walked on, cut or removed.

  • The system: Enverge NexSeal LE closed-cell, at 50 mm. A BBA-certified product, tested to BS EN 14315-1. The certification belongs to the product; we apply it.
  • Why from below. Encapsulation keeps the covering in place and encloses the sheet and its fixings in one continuous layer. The fragile face stays untouched from above, and the building keeps its roof.
  • Why closed-cell. It bonds to the sheet and encloses every fixing and lap, so the cold underside stops being the surface the room's air touches. The layer is the insulation and the air and vapour seal at once.
  • Why 50 mm here. The depth was set for this building and this brief. It says nothing about the depth your roof needs; that comes from your objective and your build-up. How thickness follows the objective.
Product
Enverge NexSeal LE, closed-cell
Depth
50 mm
Substrate
Profiled asbestos-cement sheet, underside
Certification
BBA-certified product; tested to BS EN 14315-1
A cut section of closed-cell spray foam on a stone bench under raking light: the pale foam body follows the profile of the sheet it is bonded to, with a steel rule and a probe laid beside it.

What 50 mm of closed-cell looks like in section. A rigid, fine-celled body that follows the profile of the sheet it is sprayed onto, rather than bridging it.

04How the work ran

Survey, estimate, application, completion pack.

Four steps, each one written down. The roof stayed on the building from the first visit to the last.

  1. Site survey

    The roof measured, from inside.

    1950 m² of plan area, the sheet profile identified, and the underside inspected for what the layer would have to follow.

  2. Estimate

    Priced on the face, not the plan.

    The profile allowance applied and shown: 2340 m² at 50 mm closed-cell, with the treated area stated on the estimate.

  3. Application

    Sprayed from the underside.

    Enverge NexSeal LE applied in continuous passes to the sheet, its laps and its fixings. Nothing walked on, cut or removed above.

  4. Completion

    Completion pack issued.

    The record of what was applied, where and at what depth, issued on 15 June 2026.

05The outcome

A warmer underside, calculated. A drier roof, as reported.

The thermal figures are calculated for this build-up from the site survey and the completion record, not read from a meter. What a building actually uses depends on how it is run. What the customer noticed is reported in their own terms.

Treated-roof U-value

≈0.53W/m²K

Calculated for 50 mm closed-cell on this asbestos-cement build-up.

Conductive heat transfer through the roof

≈91 to 92%less

Against the untreated sheet.

Internal roof-surface temperature

≈9 °Chigher

Under representative winter conditions: the underside sits well above where it used to.

Roof heat-loss reduction potential

≈616 to 677MWh a year

On the surveyed 1950 m² envelope. A potential for the roof element, not a bill.

Where the figures come from

Calculated, for this roof. The U-value, the heat-transfer reduction, the surface temperature and the MWh potential are derived from the site survey and the completion record for 50 mm Enverge NexSeal LE on this build-up.

Yours will differ. A different sheet, depth or objective gives different numbers. How we run a condensation risk analysis.

06Where this record leads

If this looks like your roof.

The problem most fibre-cement roofs start from, the method this one was treated under, and what a roof like it costs per square metre.

Your next step

Your building.
Your brief.
A specialist.

If you already know the U-value or thickness you need, tell us and we price to it. If you do not, tell us what you know about the building and the problem, and we work it out with you. Someone who does this every day reads it and comes back to you.

Start a project brief About five minutes. Photographs help; nothing else is needed.

Or pick up the phone

Telephone . Before 3pm on a working day, you are contacted the same day. After 3pm, the next working day. A commitment to contact, not a quotation.

Want a figure before you talk to anyone? What it costs: the cost guide