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A flat roof over a UK commercial building seen from above at first light, the pale membrane, a parapet upstand and rooflight kerbs casting long soft shadows.

Application

A flat roof, insulated from beneath: sealed to the deck, no voids, no dew point in the wrong place.

Timber joists and deck, profiled steel deck or a concrete slab: closed-cell foam is applied to the underside, fills the voids against the deck and acts as its own vapour control layer.

Start a project brief What it costs: the cost guide
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Is this your roof?

01The build-up

The deck stays. The layer seals it from inside.

A flat roof fails from the inside more often than from the top: warm, wet air rises into the joist voids, meets the cold deck and condenses there, unseen, until the ceiling stains. Switch the drawing between the bare roof and the sprayed roof.

Two states. Switch between them; the marks and the reading follow.

Swipe across to see the whole section.

BARE DECK · WARM AIR IN THE VOIDSSection, schematic, not to scale. Three marks in each state; the cards beneath read them.

Cold deck · below the dew pointThe dew point · water formsAbove the dew point · dry

1Membrane and deck

Waterproof on top, cold underneath.

The membrane keeps the rain out. It also keeps moisture in: a flat roof cannot dry upwards, so anything that condenses on the underside of the deck stays there.

2Condensation on the deck

The dew point sits inside the roof.

In winter the deck is the coldest surface the internal air can reach. Vapour passes through gaps in the ceiling and any poor vapour control, and condenses on the timber or steel.

3Warm air in the void

Every gap in the ceiling feeds it.

Downlights, service penetrations and board joints let warm, wet room air into the void. Insulation that leaves an air path to the deck leaves the problem where it was.

The short version: on a flat roof the dew point tends to sit inside the construction, so where the insulation goes and how it controls vapour matter more than how thick it is. A sprayed closed-cell layer against the deck does both jobs at once.

02The order of work

Five steps, from the underside.

Access is from the room or the void below, so the roof stays weathertight throughout and the building stays open. Each step is checked before the next begins.

  1. Substrate and build-up

    The deck, joists and existing vapour control are inspected, and the whole build-up is recorded: membrane, deck, void, ceiling. The roof must be weathertight; a leaking roof is repaired first, by someone else.

    HOLVEM checksSound, dry deck; the real build-up recorded for the analysis.

  2. Preparation

    Old insulation that would trap moisture between itself and the foam is removed where the analysis says so. Loose material, dust and contamination come off the deck. Services are protected or re-routed.

    HOLVEM checksClean, dry surface; services and ceiling protected.

  3. Passes and depth

    Closed-cell foam is sprayed onto the underside of the deck between the joists, in passes, to the specified depth. It bonds to timber, steel and concrete alike and fills the awkward corners a board cannot.

    HOLVEM checksApplied thickness measured and recorded.

  4. Coating, where specified

    Where the foam will remain exposed in an occupied space and a reaction-to-fire classification is required, an intumescent coating is applied. Behind a ceiling, the foam is normally left as applied.

    HOLVEM checksCoverage logged where a coating is specified.

  5. Terminations

    Parapet upstands, rooflight kerbs, outlets and penetrations are detailed per project, the layer stopping at a closed edge. Where the roof edge meets a wall, the junction is closed too.

    HOLVEM checksEvery stop closed and detailed.

A close detail of the underside of a bare flat roof deck between timber joists, plywood grain and nail heads visible, dark staining at one joist.
BeforeThe bare deck, between joists
The substrate as it is inspected: the deck underside, the joists and the void the room air has been reaching.

Every step is checked before the next. Thickness measured, coverage logged, compiled into a QA record at completion.

03Preparation

Before the first pass: three things a flat roof has to be.

A flat roof is the one place where the wrong insulation in the wrong position causes the problem it was meant to cure. Three things HOLVEM establishes first.

Weathertight

The membrane is sound, or repaired first.

Foam beneath a leaking roof traps the water in the deck. If the membrane has failed, it is repaired or the roof is coated from above first, and the layer goes on afterwards.

Analysed

The build-up passes the condensation risk analysis.

Existing quilt, vapour barriers and ceiling linings are modelled with the proposed foam. Where a layer would trap moisture, it comes out or the specification changes. Nothing is sprayed on a guess.

Accessible and protected

The void can be reached and the room kept working.

Ceilings are opened where needed, services protected, the space beneath sheeted. On an occupied building the work is sequenced room by room.

Survey, or photographs?How HOLVEM works: we understand your project before we price it

04Thickness and objective

The depth is set by what the roof has to do.

Three objectives, three depths, each confirmed by the analysis for your roof rather than assumed. What it costs is on the cost guide, not here.

25 mmCondensation control

Keep the deck out of the dew point.

Enough to move the dew point out of the deck on many roofs and to close the air paths into the void. The objective where a roof stains but is otherwise adequate.

50 mmHeat retention

Hold useful heat overnight.

Controls condensation and keeps warmth in the room below. The depth HOLVEM used between the timbers on a real metal standing-seam flat roof in South East England, combined with mineral wool in the deeper void, to reach a calculated 0.14 W/m²K for the whole roof.

75 mm and aboveA calculated U-value

A figure to meet, and to prove.

Set by the U-value the project has to reach, calculated for this roof as built, joists included. Often achieved as a hybrid: closed-cell against the deck for vapour control, a second insulation in the rest of the void.

The underside of a flat roof deck between timber joists with a continuous cream closed-cell layer against the deck, the joist faces left visible, even soft light.
AfterThe layer, against the deck
The finished face: foam against the deck in every bay, the joists still visible for fixings and services.

Thickness and objectiveWhat it costs: the cost guide

05Where it suits

The buildings that carry a flat roof.

Flat roofs sit over offices, extensions, plant rooms, retail and public buildings across the UK. Three building pages where this application is common.

Building

Offices and commercial

Flat-roofed office floors, plant rooms and extensions where the ceiling stains every winter and the top floor is cold.

Read the building page

Building

Retail units

Flat-roofed retail and storage with suspended ceilings hiding a cold, damp void above the sales floor.

Read the building page

Building

Schools

Flat-roofed classroom blocks and halls, worked around term time, where condensation and heat loss meet a tight budget.

Read the building page

Every building type HOLVEM works on

06What HOLVEM checks

A flat roof is analysed before it is sprayed. Here is a real one.

On a metal standing-seam flat roof in South East England, HOLVEM calculated the U-value, ran the simple condensation check and, when that was inconclusive, ran a ten-year dynamic simulation. The roof dried year on year. That is the standard every flat roof gets.

Condensation risk analysis

The simple method flagged it. The dynamic model cleared it.

The Glaser method to BS EN ISO 13788 showed condensate at the deck interface not fully clearing within the year. A WUFI simulation to BS EN 15026, with real hourly weather over ten years, showed the total water content falling from 2.19 to 1.34 kg/m². The roof dries; nothing accumulates.

Fire

Is a reaction-to-fire classification required?

Where the foam remains exposed in an occupied space and a classification is required, the coated system is specified, classified B-s2,d0 in the build-up tested. Behind a ceiling lining, the requirement is usually met by the lining. Established before the depth.

U-value

0.14 W/m²K for the whole roof, not the foam alone.

Calculated to BS EN ISO 6946 with the timber fraction and the air-gap correction. The clear path alone would have been 0.12; the timber pulled the roof to 0.14. That is the honest figure, and it is yours to keep.

Condensation risk analysisU-values and Approved Document LFire performance and coatings

07Straight answers

Ask us the hard ones.

The questions people with a flat roof actually ask on the call, answered the way the specialist answers them.

Do you spray on top of the roof or underneath it?
Underneath, to the deck, from inside the building or the void. The roof stays weathertight throughout and the membrane is not touched. Where the top of the roof has failed, that is a coating or roofing job first; see the roof coatings page.
My flat roof has old quilt in the void. Does it come out?
Sometimes. The condensation risk analysis models the existing build-up with the proposed foam. Where the quilt would sit between the foam and a cold surface and trap moisture, it comes out. Where it works as part of a hybrid, it stays.
Does spray foam need a separate vapour barrier?
Closed-cell foam acts as a vapour control layer in its own right at the depths specified here. Whether an additional membrane is needed on the room side is a question the analysis answers for your roof, not a default.
Can you insulate a flat roof without closing the building?
Usually. The work is from beneath, room by room or bay by bay, with the space sheeted and protected. Occupied buildings are sequenced around the working day.
How do you prove it was done properly?
Applied thickness is measured and recorded, coverage is logged, and the U-value calculation and condensation risk analysis are handed over with the QA record. The building file answers the surveyor's question before it is asked.

Project records

Where we have done this before.

All project records
Project record29 schools across Hampshire and Berkshire150 mm of open-cell to the flat roofs, cavity walls filled at several.Read the record

The next step

Your roof.
Your brief.
A specialist.

If you already know the thickness or U-value you need, we price to it and confirm it is right; if not, we work it out with you. Tell us what the roof is built from, what happens in winter and what is above and below it. Photographs of the void are the most useful thing you can send.

Start a project brief Send photographs first

Or call . Enquiries received before 3pm on a working day are contacted the same day. After 3pm, the next working day. We understand your project before we price it.