Intermediate

Steel roof sheets: install, profiles, waterproofing

23/04/2026 15 min read

Long confined to agricultural and industrial buildings, steel roof sheeting has become a serious option in self-build thanks to three decisive advantages: it goes up two to three times faster than a traditional roof covering, it works at very shallow pitches (down to 5%), and its per-square-metre price is hard to beat. For a self-builder who wants to get weathertight quickly, it is often the most rational choice — provided you pick the right profile, nail the laps, and manage condensation properly. This guide covers installation to DTU 40.35 (French standard — equivalent UK: BS 5427 / EN 14782), from choosing between single-skin, insulated sandwich panels or tile-effect sheets, right through to screw-by-screw fixing and flashings at junctions.

The three families of steel roofing

The term “steel roof sheet” actually covers three very different products in terms of use, price, and installation complexity. Your choice between the three shapes everything else on the job.

Single-skin ribbed steel sheet

This is the original product: a galvanised steel sheet, factory-lacquered and ribbed for rigidity. Standard profiles are named by three numbers: rib height × rib centres × useful width (e.g. 40-250-1000 = 40 mm rib, 250 mm centres, 1000 mm useful width).

Profile	Rib height	Typical use	Min pitch
18-76-988	18 mm	Cladding, outbuildings	15%
27-207-1035	27 mm	General buildings	10%
39-333-1000	39 mm	Standard residential	7%
40-250-1000	40 mm	Long spans, low pitch	5%
60-183-915	60 mm	Very low pitch, industrial	3%

The deeper the rib, the more load (snow, wind) the sheet carries and the shallower the pitch it can handle. Steel thickness: 0.50 to 0.75 mm in standard use — 0.63 mm is recommended for a house.

Insulated sandwich panel (composite panel)

Two lacquered steel faces sandwich polyurethane (PU) foam or mineral wool 40 to 100 mm thick. The result: roofing + insulation + interior finish in one operation.

Thermal R-value: 1.8 (40 mm PU) to 4.5 (100 mm PU) — useful as supplementary insulation, not sufficient on its own to meet current energy standards
Common thicknesses: 40, 60, 80 or 100 mm
Useful width: 1000 mm (standard)
Lengths: made to measure, up to 13 m in one piece

Tip — For a new self-build home, a sandwich panel alone is never enough to meet current insulation requirements. You will need to add insulation below rafter level (blown wool, wood fibre, cellulose) to reach an R-value of at least 6 m².K/W. The sandwich panel remains a great option for a garage, workshop or barn, where it delivers weathertightness and basic thermal comfort in a single pass.

Tile-effect steel sheet

A corrugated profile shaped to mimic a traditional clay tile covering. Brick red, aged brown, slate grey — the sheet is factory-lacquered with a relief finish. The upside: a traditional roof appearance with all the technical benefits of steel sheeting (speed, light weight, price).

Minimum pitch: 15 to 18% (more demanding than plain ribbed sheet, because the tile effect creates waves that slow drainage)
Weight: 5 to 7 kg/m² (versus 25–30 kg for real clay tiles)
Price: £17–30/m² (supply only)
Use: renovation in areas where planning rules require a traditional appearance; extension matching an existing tile roof

The leading manufacturers are ArcelorMittal, Joris Ide, Bacacier, Isocab, Huta Sidelor.

Question

When can you use steel roofing?

Steel sheeting is more forgiving than slate or clay tile, but it still has its eligibility conditions.

Minimum roof pitch

The minimum pitch depends on the profile chosen and the rafter length (distance from eaves to ridge). The longer the slope, the steeper the pitch must be, because water must flow fast enough to avoid backing up at the laps.

Rafter length	Min pitch 40 mm profile	Min pitch 60 mm profile
< 10 m	7% (4°)	5% (3°)
10 to 30 m	10% (6°)	7% (4°)
30 to 50 m	15% (9°)	10% (6°)
> 50 m	Waterproof membrane required	15% (9°)

Below 5% pitch you are no longer in roofing territory but in flat-roof waterproofing (EPDM membrane, bitumen, PVC) — see our flat roof and waterproofing guide.

The substrate

Steel roofing is fixed to:

Steel or timber purlins — for long spans (agricultural buildings, barns)
Timber battens on rafters — the standard approach for houses
Close-boarded decking — reserved for tile-effect sheets or renovation over an existing substrate

Batten/purlin centres: 60 to 80 cm as a rule, but always check the manufacturer’s technical data sheet for your climate zone (snow/wind load) and sheet thickness. A 0.63 mm sheet on a 40 mm profile handles 1 m centres in a sheltered zone but only 60 cm in an upland zone 3.

The structure

Because steel sheeting is 5 to 6 times lighter than a tile covering (6–8 kg/m² versus 40–50 kg/m²), it suits trussed rafter roofs and even some steel frames. But its lightness also makes it more vulnerable to wind uplift. Fixing design (number of screws/m²) must account for the wind zone — it can double between a sheltered inland area and an exposed coastal site.

Warning — In coastal Scotland, the Hebrides or along an exposed Welsh coast, the wind zone can demand up to 9 screws/m² at the verge and 6 screws/m² in the field, compared with 3 to 4 in a sheltered zone. Under-designing the fixings means the roof could be stripped clean in the first serious storm. Always consult the manufacturer’s load tables for your wind and snow zone (Eurocode 1 / EN 1991).

Managing condensation: the critical weak point

Bare steel sheeting is a cold surface that comes into direct contact with warm, humid loft air. Without precautions, water vapour condenses on the underside of the sheet and drips inside the house — homeowners sometimes call it “indoor rain” in winter. This is the number one nasty surprise for self-builders who underestimate the issue.

Three strategies to prevent it

Strategy 1 — Vapour-permeable breather membrane + ventilated air gap

A vapour-permeable breather membrane (HPV, Sd < 0.2 m) is unrolled beneath the battens. Between the membrane and the sheet, a 2 to 4 cm air gap ventilated at the eaves and ridge carries away moisture. This is the reference solution in new build.

Strategy 2 — Anti-condensation steel sheet (felt-backed)

Some sheets are supplied with a non-woven felt bonded to the underside. The felt absorbs condensation (up to 1 L/m²) and releases it as vapour when the temperature rises. Effective in outbuildings or light renovation, but not sufficient on its own in a dwelling — it should always be combined with a breather membrane.

Strategy 3 — Insulated sandwich panel (PU or mineral wool)

The insulating foam in the sandwich acts as a thermal barrier: the inner face stays at loft temperature, so there is no dew point at the surface. The most reliable solution, but also the most expensive.

Best practice — For a new self-build house, the winning combination is: breather membrane unrolled over the rafters + 40 mm counter-batten + battens + 40 mm ribbed steel sheet. The 4 cm ventilated air gap at eaves and ridge absorbs all vapour that passes through the membrane. Add an anti-condensation felt-backed sheet if you want a double safety net — belt and braces for roughly £2.50/m² extra.

Technical vocabulary and dimensions

Before tackling installation, you need to be comfortable with the specific terminology.

Rib (or “wave” / “flute”) — raised longitudinal fold that gives the sheet its rigidity
Pan (or “valley”) — flat section between two ribs, where the screws go
Overall width — physical dimension of the sheet (e.g. 1070 mm)
Useful width — dimension after lapping (e.g. 1000 mm = one rib overlaps the adjacent sheet)
End lap — along the slope, between two sheets end to end (150–200 mm depending on pitch)
Side lap — across the slope, between two adjacent strips (1 full rib)
Flashing — folded metal strip to seal a penetration (wall, chimney) or verge
Ridge cap — folded metal piece covering the ridge between two slopes
Eaves/ridge filler (closure foam) — profiled foam that follows the rib shape to close the air and water gap at eaves and under the ridge cap

Fittings and fixings

A steel roofing job is not just about the big sheets. The accessories make the difference between a watertight roof and a leaking one.

Self-drilling screws with EPDM washer (TTPC type)

Self-drilling screws with a metal thread (for fixing to steel purlins) or timber thread (for fixing to softwood battens). Each screw has a vulcanised EPDM washer under the head that seals the drill point.

Length: 35 to 80 mm depending on sheet + substrate thickness
Position: in the pan between two ribs (never on the rib, which carries the wind load)
Density: 3 to 6 screws/m² in the field, doubled at the verge
Torque: the EPDM washer should be compressed but not crushed — drive until the clutch on a torque-limiting electric screwdriver clicks

Recommended tool: an 18 V impact driver with a magnetic Torx T25 bit and torque limiter. Budget £130–220.

Waterproofing accessories

Accessory	Role	Guide price
Cranked or flat ridge cap	Seals the ridge between two slopes	£7–13/m
Plain or shaped verge trim	Finishes the lateral edges	£5–10/m
Ventilated eaves/ridge filler foam	Seals eaves + ridge against air and vermin	£3–4/m
Chimney flashing	Waterproofing around chimney or roof outlet	£21–51/unit
Eaves drip flashing	Directs water into the gutter	£3–7/m
Valley gutter profile	Internal angle between two slopes	£13–21/m

Warning — All accessories should come from the same manufacturer as the sheets, or at least carry the same lacquer colour and warranty. A ridge cap that is even two shades different will be visible from 50 metres — and an accessory from another source may have an incompatible coating (galvanic corrosion). Order everything together, in one batch, from one supplier.

Step-by-step installation

Advice

Step 1 — Prepare the substrate

On a trussed rafter roof, nail or screw the battens or purlins first, at the prescribed centres (60–80 cm as standard). Snap a chalk line for each run and check flatness with a 3 m straight-edge — steel sheeting is rigid, it will not mask substrate defects and will show every wave.

Step 2 — Lay the breather membrane

Unroll the breather membrane (Sd < 0.2 m) from the bottom up, working from the downslope edge upward, with a 100 mm horizontal overlap between rolls. Staple to the rafters, then fix the counter-battens (40×40 mm timber battens) to create the ventilated air gap. The battens sit on top, perpendicular to the rafters.

Step 3 — Check squareness and set out

Before laying the first sheet, verify that the slope is square: equal diagonals to within 10 mm. If not, all the ribs will progressively drift out of alignment. If the roof is not square (renovation, aged structure), plan to cut the first and last sheet at an angle.

Step 4 — Lay the first sheet

Golden rule: start at the edge away from the prevailing wind. This way the side lap faces downwind, not into the wind. Lay the first sheet aligned with the gable, leaving a 50 mm overhang at the eaves (into the gutter). Fix it first diagonally (4 screws at the 4 corners) to hold position, then complete all the screws.

Step 5 — Progress across the roof

Each subsequent sheet overlaps the previous one by one full rib. Alignment is by eye on the rib — if you have to force it, your squareness is out; do not persist (a forced sheet will work loose in the wind). Move strip by strip, fitting the eaves filler foam at the eaves as you go.

Step 6 — End lap (if rafter length > 13 m)

If the slope is longer than 13 m (maximum sheet length), two rows of sheets are needed. The upper sheet overlaps the lower sheet by at least 150 mm (pitch > 15%) to 200 mm (pitch 7–15%). A bead of polyurethane mastic between the two improves waterproofing. The screws pass through both sheets and the batten.

Step 7 — Close the ridge

Fit the ridge filler foam on the last row of sheets (foam that conforms to the rib shape). Then screw the ridge cap with 100–150 mm overlap on each slope, using screws fixed over the ribs (not in the pans, where water could ingress). Ventilate the ridge by leaving the filler foam air-permeable, or by fitting a ventilated ridge cap.

Step 8 — Verge trims and junctions

Verge trim: L- or U-profile screwed to the lateral edge, turned under the sheet to prevent wind-driven capillary rise
Eaves: drip flashing projecting 50 mm into the gutter, behind the filler foam and under the sheet
Penetrations (chimney, roof window, extract vent, aerial): lead or zinc flashing dressed on site, sealed with PU mastic, integrated into the sheet ribs
Valley (internal angle between two slopes): metal valley gutter profile, sheets cut at an angle on both sides, with 50 mm overlap onto the profile

Decision tree: which steel sheet for which project?

flowchart TD A{Building use?} -->|New house| B{Roof pitch?} A -->|Garage / workshop / barn| C{Budget and comfort?} A -->|Agricultural store / shelter| D[SINGLE-SKIN SHEET
profile 40-250-1000] B -->|More than 15 percent| E{Planning constraints?} B -->|Between 7 and 15 percent| F[SINGLE-SKIN SHEET
profile 40 or 60 mm
+ breather membrane + air gap] B -->|Less than 7 percent| G[FLAT ROOF MEMBRANE
Steel sheet not suitable - see flat roof guide] C -->|Tight budget| H[SINGLE-SKIN SHEET
+ felt-backed underside] C -->|Thermal comfort needed| I[SANDWICH PANEL
40 to 80 mm PU] E -->|Tile appearance required| J[TILE-EFFECT SHEET
min pitch 18 percent] E -->|No appearance restriction| K[SINGLE-SKIN SHEET
profile 39-333-1000] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#0F4C81,stroke:#0F4C81,color:#fff style C fill:#0F4C81,stroke:#0F4C81,color:#fff style E fill:#0F4C81,stroke:#0F4C81,color:#fff style D fill:#56C6A9,stroke:#56C6A9,color:#fff style F fill:#56C6A9,stroke:#56C6A9,color:#fff style G fill:#CD212A,stroke:#CD212A,color:#fff style H fill:#F58220,stroke:#F58220,color:#fff style I fill:#56C6A9,stroke:#56C6A9,color:#fff style J fill:#6B5876,stroke:#6B5876,color:#fff style K fill:#56C6A9,stroke:#56C6A9,color:#fff

Price per m² installed (2026 ranges)

Single-skin steel sheet

Item	Price
0.63 mm lacquered ribbed sheet, 40 mm profile	£12–19/m²
Accessories (ridge cap, verge trims, filler foams)	£4–7/m²
Self-drilling screws (4–6/m² at approx. £0.17 each)	£1–1.50/m²
Breather membrane + counter-battens + battens	£9–13/m²
Labour (contractor)	£21–34/m²
Total installed by contractor	£47–72/m²
Self-build total (materials only)	£26–38/m²

Insulated sandwich panel

Item	Price
PU sandwich panel 60 mm, 0.5/0.4 mm lacquered	£30–47/m²
Accessories + sealants + special fixings	£7–10/m²
Labour (contractor)	£21–34/m²
Total installed by contractor	£60–94/m²
Self-build total (materials only)	£38–55/m²

Tile-effect steel sheet

Item	Price
0.5 mm lacquered tile-effect sheet	£17–30/m²
Dedicated accessories (tile ridge cap, etc.)	£7–10/m²
Membrane + battens + screws	£9–14/m²
Labour	£21–34/m²
Total installed by contractor	£55–85/m²
Self-build total	£32–47/m²

For a typical 120 m² roof (8×15 m, two slopes)

Contractor price, single-skin sheet: £5,600–8,640
Contractor price, 60 mm sandwich panel: £7,200–11,280
Self-build, single-skin sheet: £3,120–4,560
Self-build saving: £2,550–6,800 depending on the solution chosen

Lifespan and maintenance

Lifespan

The lifespan of steel roofing depends above all on the surface coating:

Coating	Lifespan	Factory warranty
Galv Z275 + 25 µm polyester paint	25–35 years	10 years
Galv Z275 + HDX (35 µm reinforced paint)	35–50 years	15–20 years
Galvalume (Al+Zn) + polyester	40–60 years	20–25 years
Lacquered aluminium	50–70 years	20–30 years

The Achilles heel of steel roofing is not the metal but the paint: a knock that breaks through the lacquer exposes the steel to oxidation, and rust spreads under the film. An immediate touch-up with a matching aerosol (supplied by the manufacturer) stops the damage in its tracks.

Routine maintenance

Every 2 years: visual check from a ladder — look for loose screws, hardened EPDM washers, dents or scratches
Every 5 years: clean with plain water at low pressure (never a high-pressure washer aimed directly: it strips the lacquer)
Every 10–15 years: treat any moss if in a shaded area; touch up damaged paint spots
At 25–30 years: full inspection; replace screws if EPDM washers have deteriorated

Tip — Always keep a few spare self-drilling screws and a matching aerosol paint (RAL reference or manufacturer code) in a dry store. Steel sheet colours are updated regularly, and finding the exact same shade ten years later can be impossible. A £25 stock now can save £400 on a custom paint job later.

Common mistakes to avoid

Insufficient pitch → water backs up at the laps, leaks follow
Breather membrane missing or fitted upside down → condensation on the underside, indoor rain
Screws driven into the rib instead of the pan → watertight seal compromised, rib deformed
Screws over-tightened → EPDM washer crushed and cracked within 2–3 years
Screws under-tightened → washer not compressed, no longer seals
Side lap facing the prevailing wind → roof stripped in the first storm
Insufficient end lap (< 150 mm) → water backs up, leak
No eaves filler foam → birds and insects nesting under the sheet
Walking in the pan instead of on the rib → permanent deformation
Mixing accessories from different manufacturers → galvanic corrosion, mismatched colours

Standards and references

DTU 40.35 — French standard for profiled metal sheet roofing: reference document for pitches, laps and fixings (equivalent UK: BS 5427 / EN 14782)
DTU 40.36 — Profiled pre-lacquered aluminium sheet roofing
NF EN 14782 — Self-supporting metal sheet characteristics
CSTB — Technical approvals for insulated steel panels and waterproofing
SNPPA — French Steel Profiling Association: technical resources
ArcelorMittal Construction — Technical data sheets and load tables for major profiles

Checklist before ordering

Useful links

Before roofing: install trussed rafters or choose between cut and trussed roofs
Steel sheets need a vapour-permeable breather membrane to manage condensation
Alternatives: clay tiles, natural or synthetic slate
Your choice of roof type determines the pitch and therefore whether steel sheeting is suitable
For very shallow pitches: flat roof and waterproofing
Roof windows compatible with steel sheeting: fit a Velux