The short answer
How to insulate a flat roof from the inside depends on the deck. Metal decks and concrete soffits can take closed-cell spray foam bonded directly to the underside, with a fire coating where it stays exposed. Timber decks need a condensation risk analysis first, because a sealed layer below can trap moisture. We confirm the depth at survey.
The attraction is keeping your roof on while we work beneath it. But first we need to know what is above the ceiling: the deck, old insulation, membranes and weather covering. That is what makes an inside route a sound choice.
HOLVEM insulates commercial buildings nationwide across England and Wales and specifies the insulation, depth and finish for each construction. See our flat roof spray foam insulation service.
01What the deck is
First: what is the deck made of?
Start with the roof layers, from the weather covering down to the lining. Send drawings, reports and photographs of water marks or known repairs. Tell us which areas you can see and which are concealed.
Three decks, from outside to inside
Metal deckClosed-cell bonded below
- Weather coveringRetained; leaks go to a roofer first.
- Old insulation and membranesRetained, and part of the moisture check.
- Metal deckSound and prepared on the underside.
- Closed-cell, bonded belowDirectly to the underside; depth set at survey.
- CoatingWhere it stays exposed, as your fire strategy requires.
Concrete soffitBonded insulation below
- Weather coveringRetained; leaks go to a roofer first.
- Construction aboveWhat the slab separates affects the calculation.
- Concrete slabSlab, supports and junctions checked.
- Closed-cell, bonded belowTo the whole-element target; depth set at survey.
- FinishSet by the fire strategy and the tested assembly.
Timber deckMoisture path first
- Weather membraneIncluded in the condensation risk analysis.
- Timber deckIts state checked before any inside build.
- Internal layer, undecidedOnly if the analysis supports it; no standard foam depth.
- Joists, old insulation and cavitiesDrying route and ventilation in the design.
- LiningWith future inspection access.
| Deck or soffit | Internal route to assess | Moisture check | Fire and finish | Depth basis |
|---|---|---|---|---|
| Metal deck | Suitable closed-cell layer bonded below, with any other specified layers | Retained build-up, operating conditions and interfaces | Applicable tested assembly and required coating or lining | Calculation and objective; food-sector example used 140 mm |
| Concrete soffit | Bonded insulation below a suitable slab | Construction above, slab details and conditions below | Applicable tested assembly and finish | Whole-element target; The Box example used 120 mm |
| Timber deck | A designed internal or hybrid assembly, if the assessment supports it | Condensation risk analysis before material choice; drying and vapour control | Applicable assembly, lining and needed inspection details | Complete proposed build-up; no standard foam depth |
These depths come from real jobs. We set the depth for your roof at survey.
If the roof leaks, get a roofer in first. If the deck or supports need checking, bring in the roofing or structural professional before adding insulation.
02Metal deck from below
Metal deck from below
On a sound metal deck, we can bond closed-cell foam directly to the underside. Your proposal shows the preparation, depth, coverage, edges and finish, including what stays clear.
Next, look at the space below. Services, old linings and tight voids change how we inspect and install. We'll agree what moves, what is protected and how we reach the deck.
At the food-sector units in London, we insulated 324 m² of metal-deck soffit with 140 mm closed-cell, gauged at 29 points and coated.
At Derriford District Centre, the design U-value was 0.18 W/m²K, using closed-cell bonded to a metal-deck soffit plus open-cell. Read Derriford.
03Concrete soffit from below
Concrete soffit from below
With a concrete soffit, tell us what the slab separates. The space above and use below affect the heat-loss calculation and the installation plan.
We need the slab, supports, services and junctions as well as the target and finish. A soffit beneath fixed equipment needs a different protection plan from an empty floor.
At The Box, Plymouth, we applied 120 mm closed-cell beneath 787 m² of concrete soffit to a specified U-value of 0.18 W/m²K.
At Pondswick substation in Luton, we insulated and coated 127 m² of concrete soffit beneath live switchgear, so the access and protection plan mattered as much as the insulation.
Read soffit insulation and The Box. Other matched soffit records are available through HOLVEM projects.
04Timber decks
Timber decks: when to insulate from inside and when not
With timber, the moisture path comes first. A sealed layer below the deck changes how the roof dries, so we check the old layers and timber before choosing an inside build.
Your condensation risk analysis includes the weather membrane, deck, existing insulation, cavities and lining. Ventilation and future inspection access are part of that design.
The result may support insulation from inside, a hybrid build or a different route. We'll explain the choice. Access convenience alone can't decide the moisture design.
Access convenience alone can't decide the moisture design.
Across more than 29 schools in Hampshire and Berkshire, we applied open-cell foam within flat-roof constructions. Read the schools programme.
If we can't identify the old layers or state of the deck, we arrange further inspection. Where the roof already needs renewal, it makes sense to compare the external insulation route at the same time.
05Condensation risk analysis
Condensation risk analysis
Our condensation risk analysis checks your roof against its operating conditions. We use BS EN ISO 13788, with dynamic analysis to BS EN 15026 where needed. The result comes with the layers and assumptions that produced it.
So you can see where air and vapour control sit and how moisture behaves through the roof. The foam name is only one part of that answer.
Correctly diagnosed and specified, closed-cell spray foam stops roof condensation. Zero call-backs for condensation. The depth is typically 25 mm or 50 mm closed-cell, depending on the roof make-up, confirmed before we spray, with a condensation risk analysis where needed. Spray foam is not waterproofing, so existing leaks go to a roofer first.
TJW Grab Hire in Bournemouth has a pitched metal roof, not a flat deck, but it shows the result: about 505 m² at 50 mm closed-cell under a sound box-profile roof, and the customer reported the condensation was eliminated. Read TJW Grab Hire.
Backed by our 25 year HOLVEM workmanship warranty, plus the manufacturer's 25 year product warranty issued directly to you.
06Fire and coatings
Fire and coatings
Your fire strategy sets the required finish. We include the coating or lining in the design and quote, with the test evidence that applies to that build-up.
The manufacturer's coated B-s2,d0 test used 35 to 40 mm on 9 mm plasterboard, with intumescent coating at 0.7 mm wet film thickness. The separate uncoated Class 1 surface-spread-of-flame test to BS 476-7 used 45 to 50 mm on 9 mm plasterboard.
Each result applies to its tested build-up. We check it against the metal deck, concrete or timber construction proposed for your building, then include the finish evidence in your handover.
Read fire performance and coatings for the test details and how we use them.
07U-values
U-values: calculate the whole roof
The U-value calculation includes the old layers, new insulation and thermal bridges. Send your designer's target and the roof drawings; we'll show how the complete element performs.
For new or replacement elements under England's 2021 Approved Document L Volume 2, incorporating 2023 amendments, Table 4.1 gives a flat-roof limit of 0.18 W/m²K. Renovations use their own provisions, and Wales has separate guidance. Your designer confirms the target for your work.
A new English edition is published for the coming changes. Its main start date is 24 March 2027, with separate higher-risk-building and transitional provisions. Your designer confirms which edition applies to the work.
08The commissioning checklist
Step by step: the commissioning checklist
- Identify the deck and all known layers from covering to internal finish.
- Establish water entry, visible condition and any roofing or structural work needed first.
- State the building use and objective, including any written thermal target.
- Assess the proposed moisture arrangement, especially for timber and hybrid assemblies.
- Calculate the complete element with the thermal bridges.
- Establish applicable fire evidence and the coating or lining required.
- Agree access, protection, working zones, ventilation and re-entry plan.
- Write the surface, area, depth, interfaces, exclusions and finish into the work.
- Check applied thickness and coverage against that work and compile the evidence.
- Keep calculations, specification and installation records with the building file.
Use this checklist to commission the work and check the quote. The installation itself needs a planned working zone, access and trained installers.
Compare the installed methods
| Method | Typical installed cost per m² | What you are buying |
|---|---|---|
| Closed-cell spray foam, 25 mm | £16 to £25 + VAT | Foam only on a sound, prepared surface; the roof stays in place |
| Closed-cell spray foam, 50 mm | £22 to £35 + VAT | Foam only, with depth confirmed for your roof at survey |
| PIR or rigid boards | £30 to £50 installed | A fitted board system; compare supports, joints and finish |
| Mineral wool and liner | £20 to £30 installed | Insulation with its supporting liner and vapour-control details |
| Roof overcladding | £60 to £90 | A new weather-facing roof system over the retained roof |
| Insulated panels or replacement | £100 to £160 + VAT | A new insulated roof, with removal and access checked in the quote |
These are the method figures in our cost guide, not a quote for your building. The guide does not state a VAT basis for the board, mineral-wool and overcladding rows, so ask the contractor to confirm it. Added coatings and external foam systems are priced per project. Compare what is included before you compare the totals.
09What the projects prove
Projects: what each example proves
The London food-sector job shows a coated metal deck checked at 29 points. The Box shows a concrete-soffit calculation; Derriford uses different layers; Pondswick shows how live switchgear changes the plan below. Those are the details to compare with your building.
Your own roof gets its own depth, finish and access plan.
The school roofs add an open-cell route to that picture. So begin with the construction you have, then choose the material.
