Summary
A pitched roof is a triangulated structure: the slope (pitch) lets water and snow run off, and the geometry turns the roof covering's weight and the external loads into forces the supporting walls can carry. Two broad forms exist. A cut roof is built on site from individual rafters, purlins, ridge boards, ceiling joists and struts — flexible for complex or traditional shapes and for loft conversions. A trussed roof uses factory-made, gang-nailed truss rafters at close centres, which is faster and cheaper but fills the roof space with webs and is not designed to be cut without an engineer's approval.
The covering dictates the minimum pitch. Plain clay or concrete tiles need a steep pitch (around 35–40 degrees) because each small unit overlaps the ones below; interlocking concrete tiles can go much lower (some down to 15 degrees), and natural slate sits in between depending on slate size and lap. Get the pitch wrong for the covering and the roof will leak by wind-driven rain tracking back under the units, regardless of workmanship.
Two things sink otherwise sound pitched roofs: inadequate fixing and inadequate ventilation. BS 5534 sets out how every tile and slate must be mechanically fixed to resist wind uplift — modern practice is to fix far more units than the old "every fifth course" rule, and to clip and nail perimeters and verges. BS 5250 deals with moisture: a traditional cold roof (insulation at ceiling level, cold void above) needs cross-ventilation so warm moist air from the house does not condense on the cold underside of the roof and rot the timbers.
Key Facts
- Approved Document A (Part A) — the structure of the roof (loads, stability, member sizing) must satisfy Building Regulations Part A.
- BS 5534 — code of practice for slating and tiling; mandates mechanical fixing of tiles/slates against wind uplift, plus mortar/dry-fix detailing.
- BS 5250 — management of moisture in buildings; sets ventilation requirements for cold roofs.
- BS EN 1991 (Eurocode 1) — actions on structures: dead, imposed (snow), and wind loads used to design the roof.
- Eaves ventilation — a cold pitched roof typically needs a continuous 25mm equivalent free air gap at the eaves (10mm for some warm/low-pitch cases).
- Ridge ventilation — high-level ventilation typically equivalent to a continuous 5mm gap at the ridge for roofs over ~35 degrees or over a given span.
- Wall plate — the horizontal timber bedded on the wall head that distributes rafter loads and is strapped down against wind uplift.
- Rafter centres — commonly 400mm or 600mm centres; truss rafters often at 600mm centres.
- Dead load — the permanent weight: tiles/slates, battens, felt/membrane, timber (plain tiles are far heavier per m² than slate or interlocking tiles).
- Imposed load — snow and maintenance access loads to Eurocode 1; snow load varies with site altitude and location.
- Wind load — uplift and pressure; the dominant design case for the covering's fixing, worst at eaves, verges and ridge.
- Lateral restraint — gable walls and roofs are tied with restraint straps (e.g. at ~2m centres) to stop the wall and roof racking.
- Underlay — a breathable membrane or traditional bitumen felt sits below the battens as a secondary barrier; choice affects ventilation strategy.
- Trusses are engineered — never cut, notch or remove a truss member without designer/engineer sign-off; it voids the design.
- Minimum pitch — always check the manufacturer's stated minimum pitch for the specific tile/slate and lap before quoting.
Quick Reference Table
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Try squote free →| Covering | Typical minimum pitch | Approx. dead load (kg/m²) | Notes |
|---|---|---|---|
| Plain clay/concrete tile | 35–40 degrees | 65–80 | Small units, double lap, heavy |
| Interlocking concrete tile | 15–22.5 degrees | 45–55 | Low-pitch capable, single lap |
| Natural slate (large) | 20–25 degrees | 25–35 | Depends on size and lap |
| Natural slate (small) | 30–35 degrees | 25–40 | Smaller slates need steeper pitch |
| Fibre-cement slate | 15–20 degrees | 19–21 | Light, check manufacturer min pitch |
| Clay pantile | 30–35 degrees | 45–55 | Single lap, large format |
| Metal sheet/standing seam | 5–15 degrees | 5–15 | Different fixing rules, not BS 5534 |
Pitches and loads are indicative — always use the manufacturer's data sheet for the specific product.
Detailed Guidance
Anatomy of a pitched roof
The principal members, working from the wall up:
- Wall plate — timber bedded along the top of the wall, strapped down; rafters bear onto it via a birdsmouth notch.
- Rafters (common rafters) — the sloping members carrying the battens and covering down to the wall plate.
- Ridge board / ridge beam — the apex member the rafters meet at the top.
- Purlins — horizontal members partway up the slope that support the rafters mid-span, reducing rafter size (cut roofs).
- Ceiling joists / binders — tie the feet of opposing rafters together, resisting the outward spread that the roof's geometry generates.
- Struts, hangers, collars — internal bracing transferring purlin loads down to load-bearing walls and stopping rafter sag/spread.
- Battens — fixed across the rafters to the correct gauge (spacing) for the covering's lap.
RIDGE
/\
purlin-> / \ <-purlin
/ \
rafter->/ \<-rafter
/ \
======/==========\====== <- wall plate (strapped down)
| ceiling joist/tie |
WALL WALL
Cut roof vs trussed roof
A cut roof is assembled from individual timbers on site. It suits complex geometries, hipped and valley roofs, vaulted ceilings, and loft conversions where you want a usable space. It needs more skilled labour and the members must be sized for the spans and loads. A trussed rafter roof is delivered as prefabricated triangulated frames at close centres, craned or lifted into place and braced — far quicker and well-suited to standard rectangular roofs. The trade-off: the truss webs occupy the loft, and a trussed roof is a complete engineered system. Cutting or removing any member to "make room" destroys the load path and can cause collapse — alterations need a structural engineer.
Pitch and the covering
Minimum pitch is set by the roof covering, not by taste. Small double-lapped units (plain tiles, small slates) need steep pitches so water sheds before it can be driven back under the lap. Large single-lap interlocking tiles can go much lower because the interlock and underlay handle wind-driven rain. Below a covering's stated minimum pitch the manufacturer's warranty is void and the roof will leak. Always confirm the minimum pitch from the specific product's data sheet — a "concrete tile" is not one number; it depends on profile and lap.
Loads: dead, imposed and wind
Roof design under BS EN 1991 (Eurocode 1) considers three load families:
- Dead load — the permanent self-weight of covering, battens, underlay and timber. Re-covering a slate roof in heavy concrete plain tiles can roughly double the dead load and may overload original rafters — check before re-roofing.
- Imposed load — snow and maintenance access. Snow load depends on site location and altitude; valleys and abutments can see drifting that increases local load.
- Wind load — pressure and, critically, uplift. Wind tries to peel the covering off, worst at the eaves, verges, hips and ridge. This is why BS 5534 fixing matters and why perimeters get extra clips and nails.
A re-roof that changes the covering weight or pitch is a Part A structural matter — the rafters, wall plate fixings and restraint must still work for the new loads.
Ventilation and condensation (cold roof)
In a traditional cold roof the insulation sits at ceiling (joist) level and the loft void above is cold. Warm, moist air leaks up from the house; if the void is not ventilated, that moisture condenses on the cold underside of the roof and the timbers, leading to rot and mould. BS 5250 requires cross-ventilation:
- Eaves: a continuous low-level air path equivalent to a 25mm gap (often via over-fascia vents, eaves vent trays or soffit vents).
- Ridge / high level: an outlet equivalent to a 5mm continuous gap (ridge vents) for steeper or larger roofs to drive cross-flow.
A warm roof (insulation following the slope, at rafter level) changes the strategy — it may be unventilated if a vapour-control layer and the right membrane are used, or have a ventilated air gap above the insulation. The membrane choice (breathable vs non-breathable) interacts with the ventilation design, so the two must be specified together, not in isolation.
Fixing and weatherproofing details
BS 5534 governs how the covering is held down. Modern practice fixes far more tiles/slates than the old rule of thumb, with the fixing specification driven by a wind-uplift calculation for the building's location, height and exposure. Dry-fix systems (dry ridge, dry verge, dry valley) using mechanical clips and unions have largely replaced sand/cement mortar bedding for ridges and verges because mortar cracks and fails. Eaves, verges, ridges and abutments are the detail-critical zones — that is where wind gets under the covering and where most leaks start.
Frequently Asked Questions
Can I cut a trussed rafter to make loft space?
No — not without a structural engineer designing a replacement load path. Trussed rafters are an engineered system where every web carries load; cutting one transfers force to members never designed for it and can cause progressive failure. Loft conversions on a trussed roof usually need new structural steel or a rafter/floor redesign.
What's the minimum pitch I can lay tiles to?
It depends entirely on the tile. Plain tiles need around 35–40 degrees, large interlocking concrete tiles can go down to about 15 degrees, and slate sits in between depending on size and lap. Always use the manufacturer's data sheet for the exact product — laying below the stated minimum voids the warranty and the roof will leak.
Why does my cold loft need vents at both eaves and ridge?
Ventilation works by cross-flow: cool dry air enters low at the eaves and warm moist air leaves high at the ridge. With only one opening there is no through-path, so moisture stagnates and condenses on the cold roof timbers. BS 5250 sets the eaves (about 25mm) and high-level (about 5mm) provision for this reason.
Does re-roofing with heavier tiles need building control?
Often yes. Changing the covering weight or pitch is a structural alteration under Part A — heavier tiles increase dead load and may overload the existing rafters, wall plate and fixings. A significant re-roof (commonly more than 25% of the roof area) is notifiable to building control, who will want the structure and thermal/ventilation details checked.
What stops the roof pushing the walls outwards?
The ceiling joists or collar ties act as the bottom chord of the triangle, holding the feet of the rafters together so the roof's geometry doesn't spread and shove the walls out. Restraint straps tie the roof and gable walls together against wind racking. Remove or weaken those ties and the walls can bow.
Regulations & Standards
Building Regulations Approved Document A (Part A) — structure: roof loads, member sizing, stability and lateral restraint.
BS 5534 — code of practice for slating and tiling (including shingles); mechanical fixing against wind uplift and dry-fix detailing.
BS 5250 — code of practice for management of moisture in buildings; roof ventilation requirements for condensation control.
BS EN 1991 (Eurocode 1) — actions on structures: dead, imposed (snow) and wind loads.
Building Regulations Approved Document L (Part L) — thermal performance, relevant when insulating or re-roofing.
Approved Document A: Structure (gov.uk) — Building Regulations structural requirements.
BS 5534 Slating and tiling (BSI) — code of practice for fixing roof coverings against wind uplift.
BS 5250 Moisture management / roof ventilation (BSI) — condensation control and ventilation guidance.
NHBC Standards: roofs — practical roofing detailing accepted by warranty providers.
pitched roof structure — detailed look at the structural members and spans.
roof ventilation — eaves, ridge and tile-vent ventilation in depth.
roof tile types — choosing tiles and slates and their minimum pitches.
roof trusses — trussed rafter design, handling and bracing.