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The four pitched roofs of Global House after the work, seen at dusk: a continuous pale coated surface over every slope, the rooflights and the valley gutter included.
Project recordCompleted, at dusk

Project record · Asbestos-cement roofs · External encapsulation

Global House, Surbiton: 1575 m² of asbestos-cement roof sealed from above.

A business park of four pitched asbestos-cement roofs and an office roof: worn, stained and suspected of microfractures. Removal was ruled out, with homes and businesses within 30 m. Instead, a low-pressure biocidal wash, 25 mm of closed-cell roofing foam in lifts, and an airless silicone coating, applied as one continuous bonded system over the sheets, the rooflights and the valley gutter.

1575m²
Roof under one bonded system
25mm
Closed-cell roofing foam, in lifts
4 + 1
Pitched roofs and an office roof
30m
To the nearest homes and businesses

01The building

Four old roofs, one valley gutter, and neighbours thirty metres away.

A business park under four pitched asbestos-cement roofs with an integral valley gutter, plus an office roof. The sheets showed age: staining, surface wear, and the microfractures that let water into a fibre-cement roof. Taking them off was the obvious answer and the wrong one.

Why not remove. Stripping 1575 m² of asbestos-cement is licensed work with scaffold, enclosure, a programme measured in weeks and a great deal of regulated waste, next to occupied homes and businesses within 30 m. The disruption and the risk were out of proportion to a roof that was worn, not failed.

Why not patch. Patching a fibre-cement roof chases cracks. The microfractures that were suspected would carry on opening, and the staining and moss would carry on holding water in the troughs.

The brief. Keep the sheets, make the whole roof one sealed surface from above, including the rooflights and the valley gutter, and do it without disturbing the people next door.

Building
Business park: four pitched roofs and an office roof
Client
Global Property Investments Ltd
Location
Surbiton, Surrey
Roofs
Asbestos-cement sheet with an integral valley gutterKept in place; nothing cut, lifted or broken
Area
1575 m²
System
Low-pressure biocidal wash, 25 mm closed-cell roofing foam, GacoFlex S42 silicone
Neighbours
Homes and businesses within 30 m, undisturbed

02What we specified, and why

Wash, foam, coat: one bonded system over everything, applied from above.

Three layers, each with a job. The wash takes the biology off without pressure-washing a fragile sheet. The foam bonds to the sheet, fills the profile and re-forms the gutter. The silicone makes the whole thing one weatherproof skin.

  • The wash: low-pressure biocidal clean. Moss and lichen in the troughs hold water against the sheet and drive freeze-thaw damage. They were killed and washed off at low pressure, because a pressure washer on asbestos-cement releases exactly what encapsulation is meant to seal in.
  • The foam: 25 mm of closed-cell polyurethane roofing foam, in lifts. Sprayed over the washed sheets, it bonds to the surface, fills the corrugations and the laps and encloses the fixings. In the valley gutter it was built up to re-profile the gutter so water runs the way it should. Roof coatings and waterproofing, the application.
  • The coat: GacoFlex S42 silicone, airless-applied. A continuous silicone membrane over the cured foam, applied by airless spray with wet-film thickness checked every 20 m², so the coverage is proved as it goes rather than assumed at the end.
  • Everything under it. The system ran continuously across all four pitched roofs including the rooflights, through the valley gutter and over the office roof. The client chose a single sealed surface over daylight through the old rooflights; that was their call to make and they made it.
Preparation
Low-pressure biocidal wash
Foam
Closed-cell polyurethane roofing foam, 25 mm, in lifts
Coating
GacoFlex S42 silicone, airless, wet-film checked every 20 m²
Extent
Four pitched roofs, rooflights, valley gutter, office roof
During the work at Global House: closed-cell roofing foam being sprayed over the washed asbestos-cement sheets, the corrugations filling under a continuous layer.

The foam going on. Over the washed sheet, filling the profile and enclosing the laps, before the silicone coat. Project record.

Photographs · Project record

The roofs before, the foam going on, the four bays from above.

The worn sheets, the roofing foam over them, and the completed roofs seen from the air and at dusk.

Before: the worn, stained asbestos-cement roofs of Global House, moss in the troughs.
Project recordBefore: worn sheet
During: roofing foam sprayed over the washed sheets, the corrugations filling.
Project recordDuring: foam
After, from above: the four pitched bays and the valley gutter under one continuous coated surface.
Project recordAfter: the four bays

Photographs from the job, as it ran. Labelled Project record because they are.All project records

03How the work ran

From the roof, quietly, with the neighbours in mind.

External work on a fragile roof next to occupied buildings: access and edge protection first, then the wash, the foam in lifts and the airless coat, with the coverage checked as it went.

  1. Survey

    Four roofs and a gutter read.

    Sheet condition, the suspected microfractures, the valley gutter's fall and the rooflights; the decision to keep the sheets and seal from above confirmed with the client.

  2. Wash

    Biology off, at low pressure.

    A biocidal wash applied and rinsed at low pressure across every slope and the gutter, so the sheets were clean and dry for the foam without being disturbed.

  3. Foam

    25 mm in lifts, gutter re-profiled.

    Closed-cell roofing foam sprayed over the sheets in lifts to 25 mm, laps and fixings enclosed, the valley gutter built up to run water the way it should.

  4. Coat and check

    Silicone, gauged every 20 m².

    GacoFlex S42 applied by airless spray over the cured foam, wet-film thickness checked every 20 m², and the roofs handed back as one surface.

04The outcome

One bonded system over 1575 m², no asbestos waste, and the neighbours none the wiser.

The record states what was done and what was avoided. No thermal or energy claim was published for this roof and none is made here; it is a weathering and encapsulation record.

Roof sealed

1575m²one bonded system

Four pitched roofs, the rooflights, the valley gutter and the office roof under a single continuous surface.

Sheets disturbed

0nothing cut or lifted

The asbestos-cement stayed exactly where it was, washed, foamed and coated in place.

Asbestos waste

Noneoff site

No licensed removal, no enclosure, no regulated waste leaving the park.

Neighbours

Undisturbedwithin 30 m

Homes and businesses next door carried on as normal through the work.

Where this record comes from

A weathering record, not a thermal one. No U-value or energy figure was published for these roofs and we do not attach one. 25 mm of roofing foam is a sealing layer.

Kept in place is the method. Where sheets may contain asbestos, encapsulation leaves them undisturbed; a condition survey decides whether a roof is a candidate. Fibre-cement and asbestos-cement roofs.

05Where this record leads

If this looks like your roof.

The application this roof was treated under, the problem it started from, and the same roof type sealed the other way.

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.

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