Case study · Llanrwst, Conwy
A rear extension roof, rebuilt warm
between lunch and teatime.
A rendered semi in Llanrwst, in the Conwy valley, with a felt roof over the rear extension that had reached the end of its life and very little insulation beneath it. Energy Giants laid a vapour control layer and 140 mm of foil faced PIR above the joists, decked it with new OSB and finished it in black EPDM rubber with new edge trims, all on a Tuesday in September.
The job in numbers
1day
On site
Vapour control layer down before one o’clock, membrane and trims finished by quarter to six. Every photograph on this page is from the same Tuesday.1
4
Layers
Vapour control layer, 140 mm PIR insulation, new OSB deck, EPDM rubber membrane. A full warm roof build up over the existing roof.
140mm
Foil faced PIR
Board thickness as installed. The insulation now sits above the structure, so the timber stays on the warm, dry side of it.2
1sheet
EPDM rubber
A single sheet of EPDM rubber bonded to the new deck, with no seams across the roof for water to find. The household stayed put.
Before and after
The same corner of the roof, at noon and at teatime.
Two photographs from the scaffold, looking across the extension roof towards the back wall of the house. In the first, the original felt: patched, stained and holding water in the low spots. In the second, the finished warm roof, EPDM turned down over new edge trims, the edge protection still up.
12:11, before
17:41, afterThe work
The extension roof that half the estate has.
Rendered and pebbledashed semis like this one were built in their thousands across North Wales and the North West, and most have gained a single storey extension at the back at some point since, with a flat felt roof over it. Those roofs were rarely insulated to anything like today’s standard, and by the time the felt is stained and holding water the room under it is usually the coldest in the house.
We built a warm roof over it. A vapour control layer went down first, lapped and sealed, so warm air from the kitchen cannot get into the insulation from below. On top of that, 140 mm foil faced PIR insulation boards, cut tight to the edges and the upstand where the extension meets the main wall, joints staggered and taped. Then a new OSB deck fixed through to the joists, and finally a single sheet of black EPDM rubber, bonded down and dressed over new edge trims at the eaves.2
That is a warm roof: the insulation sits above the joists rather than between them, so the whole structure stays on the warm side and condensation has nowhere to form. The Conwy valley gets its share of rain, so the edges and the abutment matter as much as the field of the roof, and they are formed properly rather than dressed over. This one went from old felt to finished in a single working day.
Certified and guaranteed
PAS 2030:2023 certified. Energy Giants holds PAS 2030:2023 certification for flat roof insulation (measure B5) and is TrustMark registered. It is the installation standard government backed retrofit requires for each measure, and the warm roof build up on this page is the one we deliver on housing programmes.
Insurance Backed Guarantee, issued by Energy Giants. Every flat roof we install is handed over with an Insurance Backed Guarantee issued by Energy Giants and insured through the Bluedrop facility, so our workmanship guarantee is protected by an insurer as well as by us.
Certificate SICI23052 P2030 1, measure B5 flat roof insulation, held by Efficient Energy Systems Ltd. See our accreditations or view the certificate.
How it goes together
A warm roof, built over the existing one.
The old roof stayed where it was, and the new layers went on top of it in this order. The photographs below show the same layers on the day.
Layer by layer
What went into it, in the order it went on.
We photograph every layer on every roof we do, so the customer can see what they paid for and so, on housing programmes, the record is ready for handover. Here is Tuesday, layer by layer.
BeforeThe original coveringOld felt, patched and ponding, photographed before we started. The new warm roof was built up over it.
Vapour controlThe blue layerA vapour control layer over the deck, lapped and sealed, so moisture from the house cannot reach the insulation from below.
Insulation140 mm of foil faced PIRBoards cut tight to the edges and the upstand, joints staggered and taped, so there are no gaps for heat to find. This is the layer that changes how the kitchen below feels.2
DeckA new deck on topOSB fixed through the insulation into the joists. It gives the membrane a flat, firm base and ties the build up together.
- MembraneOne sheet of EPDM
A single sheet of EPDM rubber bonded to the deck, with no seams across the roof and the abutment dressed up the wall.
FinishNew edge trimsMembrane dressed down over new trims at the eaves, so the water goes into the gutter and nowhere else.
What this means for you
One extension roof, two reasons to read about it.
If you own a house like this
Your extension roof can be warm, dry and done in a day.
If the flat roof over your kitchen or extension is the original felt, it is almost certainly uninsulated, and the room under it will tell you so every winter. The signs it is time to act:
- Stained, patched or pooling felt, or moss holding water on the surface
- Damp patches or staining on the ceiling below
- A kitchen or extension that is cold in winter and hot in summer
- A recover already done once, with the same problems back
If you manage housing stock or run a programme
The same build up, roof after roof, with the evidence to match.
This is a single roof, but the method is the one we deliver across streets and schemes for housing providers and their main contractors. What you get on every roof:
- Each roof surveyed and priced individually, not by the metre
- A warm roof build up with the insulation above the deck, so the structure is protected as well as the home
- A photographic record of every layer, ready for your handover and funding evidence packs
- Flat roof insulation delivered under our PAS 2030:2023 certification as a TrustMark registered installer, where the programme requires it
Flat roof insulation near you: East Lancashire · Greater Manchester · South Ribble and Leyland. Anywhere else in the North West or North Wales, start here.
What was delivered
The specification.
| Property | Rendered semi detached house, Llanrwst, Conwy. Flat roof over the single storey rear extension |
|---|---|
| Roof before | Original felt covering on a timber deck, stained and holding water, at the end of its life |
| Roof type | Warm roof: insulation above the structural deck |
| Build up | Vapour control layer over the existing roof; 140 mm foil faced PIR insulation boards; new OSB deck; EPDM rubber single ply membrane; new edge trims2 |
| Time on site | One working day, Tuesday 1 September 20261 |
| Access | Scaffold with edge protection to the rear elevation |
| Delivered by | Energy Giants (Efficient Energy Systems Ltd), Clitheroe, Lancashire |
| Guarantee | Insurance Backed Guarantee issued by Energy Giants on completion, insured through the Bluedrop facility |
| Certification | Energy Giants holds PAS 2030:2023 certification for flat roof insulation (certificate SICI23052 P2030 1, measure B5) and is TrustMark registered. See our accreditations |
- Step oneSurvey
The roof measured, the deck and the fall checked, the abutment and outlets looked at, before a figure is given.
- Step twoPrepare
Existing roof cleared and checked, edges, abutment and outlets set out for the new build up.
- Step threeInsulate and deck
Vapour control layer, 140 mm PIR boards laid tight and taped, new OSB deck fixed through to the joists.
- Step fourMembrane and finish
EPDM rubber bonded and dressed, new trims, site cleared and the roof handed back the same day.
Questions people ask
Flat roof insulation, answered.
How long does it take to replace a flat roof on an extension?
A single storey extension roof like this one is usually insulated, decked and finished in one working day, weather permitting. Larger or L shaped roofs can take two, and the roof is weathertight before we leave each day.
What is a warm roof, and why does it matter?
A warm roof puts the insulation above the structural deck, with a vapour control layer beneath it and the waterproof membrane on top. The joists stay on the warm side, so there is no cold surface inside the roof for condensation to form on. A cold roof, with insulation pushed between the joists, needs ventilation to stay dry and is much harder to get right on an existing extension.
Why not just put a new covering over the old felt?
A new covering on its own keeps the rain out for a while and does nothing for the insulation. A warm roof build up puts a vapour control layer, 140 mm of PIR and a new deck above the existing roof, with the EPDM rubber on top, so the roof is insulated and waterproofed in one go. That is what happened here.
Do you work in the Conwy valley?
Yes. We are based in Clitheroe and cover North Wales alongside Lancashire, Greater Manchester, Merseyside and Cheshire. This roof is in Llanrwst; we have also delivered flat roof insulation in Ruthin and a solar and battery install in Aberystwyth.
Got a flat roof like this?
Tell us where it is and what is under it, send a photo if you have one, and we will come and measure it. Homeowners get a fixed price for a warm roof, done in a day where the roof allows. Housing providers and contractors get a roof by roof survey and a build up that stands up to audit. Start on our flat roof insulation page or get straight in touch.
Notes on the figures
- Timings are from the timestamps on our own site photographs, taken on Tuesday 1 September 2026 between 12:10 and 17:42. The before photograph was taken at 12:11 and the finished roof at 17:41.
- Insulation is 140 mm foil faced PIR board, as installed. The board and thickness for any roof is set at survey and stated on the quotation. Certification: PAS 2030:2023, measure B5 flat roof insulation, certificate SICI23052 P2030 1, held by Efficient Energy Systems Ltd trading as Energy Giants; TrustMark registered. The customer’s address and details are withheld for their privacy.