A warm roof is simply a roof with its insulation on the outside of the structure — above the rafters or above the deck, with the weatherproof layer on top. That one change puts the whole timber structure inside the insulated envelope, and it’s why most new pitched and flat roof build-ups you’ll see on site now are “warm” rather than “cold”.
But warm roofs changed the fixing problem too. You’re no longer screwing into 100mm of rafter you can see — you’re driving through battens, through insulation you can’t see through, and still expecting the screw to land solidly in the timber below. That’s exactly where long structural screws earn their keep.
Here’s how the two build-ups work, and what the fixing looks like in each.
Warm roof vs cold roof — 30-second version
| Warm roof | Cold roof | |
| Insulation position | Above the rafters/deck | Between the rafters/joists, inside |
| Structure temperature | Warm — inside the envelope | Cold — outside the insulation |
| Ventilation | Not needed above the insulation | Ventilated air gap needed above the insulation |
| Thermal bridging | Low — continuous insulation layer | Higher — every rafter bridges the insulation |
| Loft space | Stays warm | Stays cold |
Cold roof — the traditional way
A cold roof puts insulation between the rafters or joists, with the roof structure outside the insulation. The structure sits in the cold zone, so the space above the insulation must be ventilated — a crossed-ventilated air gap between the insulation and the roof covering, to carry away moist air before it condenses on cold timbers.
It works, and it’s still the right answer on retrofit jobs where the roof height can’t grow. But every rafter is a thermal bridge (insulation only fills the gaps between them), the ventilation gap eats head height, and it depends on the ventilation never being blocked.
Warm roof — insulation above the structure
A warm roof flips it: rigid insulation sits above the rafters (pitched) or above the deck (flat), with a vapour control layer below it and the weatherproof layer above. Now the rafters are on the warm side of the insulation. There’s no cold structure to condense on, so there’s no ventilated gap to build in, and the insulation runs as one continuous layer with no timber bridging it.
Two build-ups to know:
Flat warm roof: deck → vapour control layer → rigid insulation (PIR or mineral wool) → membrane. The insulation is commonly held with mechanical tube fixings — a long fastener through a thermally broken tube washer, the pattern and count set by a wind-load calculation. (Those specialist tube systems are a roofing-supplier item — talk to us on 01270 252 326 and we’ll point you right.)
Pitched warm roof: insulation boards over the rafters, then counter-battens fixed down through the insulation into the rafters, then tile battens and tiles into the counter-battens. Every fixing here has to pass through the insulation and still grip structural timber — and the fixing itself is carrying roof weight and wind suction.
Where the long screws come in
The counter-batten fixing is the heart of a pitched warm roof, and it sets your screw length:
screw length = counter-batten + insulation thickness + bite into the rafter
Worked examples with a 50mm counter-batten:
| Insulation above rafters | Counter-batten | Minimum bite into rafter | Screw length needed |
| 100mm | 50mm | 50mm+ | 200mm+ |
| 150mm | 50mm | 50mm+ | 250mm+ |
| 200mm | 50mm | 50mm+ | 300mm |
That’s why ordinary 100mm screws can’t do this job — and why long structural screws exist. Our Spectre long woodscrews are the trade’s staple here — the Spectre 6.0×240mm is the one that’s proven particularly popular with warm roof installation — with the 8.0×240mm, 8.0×250mm and 8.0×280mm covering thicker build-ups, and the SiDAX 8.0×300mm wafer head at the top of the range. Every one is CE approved for load-bearing timber construction, and they’re built for exactly this fixing:
- CE approved for load-bearing timber construction — this is structural work, not jobbing-site guesswork
- Countersunk Pozi head pulls the batten down tight and sits flush under the tile batten above
- Sawtooth thread and type 17 slash point — quick start, no pre-drilling, and no splitting even near the batten edge
- Reamer shank (50mm and up) stops the head jacking as the screw goes deep
- Anti-corrosion coating and surface hardening — driven through insulation into timber that’s carrying roof weight and wind suction
Shop the range: Multi-Purpose Woodscrews · Spectre 6.0 x 240mm · Spectre 8.0 x 280mm
Insulation that’s fixed, not just laid
Where boards are fixed to a timber surface rather than sat over rafters — boarded-out roofs, soffits, lining jobs — the same long-screw principle applies with an insulation washer or disc under the head to stop the screw pulling through the board face. We stock plastic insulation washers (60mm, pack of 100) and galvanised metal insulation discs (35mm, box of 100) that pair with our screws for exactly this.
Browse: Insulation Washers & Discs
Five warm roof fixing mistakes
- Screws that only bite the batten. If the screw stops inside the counter-batten without reaching the rafter, the batten is held by thread friction alone. Count insulation thickness honestly — a squashed or stepped board run changes the depth.
- Assuming board thickness. 100mm of PIR on the label can sit proud on battens or dips in rafters. Check the actual depth over the run before ordering one screw length.
- Under-specifying for wind suction. The number and pattern of fixings comes from the insulation manufacturer’s fixing spec and wind-load calculations — our screws cover the fixing itself; follow the pattern on the spec sheet.
- Fixing too close to board edges with screws that need pre-drilling — the sawtooth thread and type 17 point on the Spectre range removes the splitting risk when you’re near the edge.
- Mixing up warm and cold on a retrofit. If you can’t add roof height, a warm build-up may not fit; forcing it means an unventilated cold roof — the classic condensation trap. Measure the build-up before you commit.
Quick answers
What is a warm roof? A roof with the insulation above the structure (rafters or deck), keeping the whole structure on the warm side of the insulation — no ventilation gap needed, minimal thermal bridging.
Warm roof vs cold roof — which is better? Warm roofs insulate continuously and don’t need ventilation, which is why they dominate new builds. Cold roofs still suit retrofit jobs where roof height can’t increase, but need a ventilated air gap above the insulation.
What screws are used for warm roofs? Long structural screws — for pitched warm roofs, counter-battens are fixed through the insulation into the rafters, so screw length = batten + insulation + a solid bite into the rafter. Our Spectre 6.0×240mm, 8.0×240/250/280mm and SiDAX 8.0×300mm are the usual picks — all CE approved for load-bearing timber.
How long should warm roof screws be? Add the counter-batten, the actual insulation thickness, and at least 50mm of penetration into the rafter — then round up to the available length.
Founded in 1999, Fastco (Fasteners & Fixings) Ltd is a leading supplier of fasteners and fixings to trades, rated 4.9/5 on Trustpilot. Order online for fast UK delivery — and if you’re not sure which screw length your build-up needs, speak to an expert on 01270 252326 or email [email protected].
