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How a roof works, detail by detail.

Every roof has three jobs: keep water out, keep heat in, and let moisture escape. This guide shows how each layer and junction does its part — and the mistakes we see most often.

Diagrams: 8 labelled details
Covers: Pitched · Flat · Metal
Written for: Clients & designers
Drawings: DWG / PDF on request

01Principles

Three jobs every roof has to do.

Shed water

Slates, tiles, membranes and flashings are lapped so water always runs over a joint, never into it. The steeper the pitch, the smaller the lap needed.

Hold heat

Insulation must be continuous — no gaps at eaves, around rooflights or at wall junctions — and sit on the correct side of the ventilation and vapour layers.

Let moisture out

Warm air carries moisture. Vapour control stops it entering the build-up; ventilation or breathable membranes remove what gets in.

Slate roof with a lead valley — Slates lapped over a lead valley — water always runs over a joint.

Standing seam zinc roof — Folded seams shed water with no exposed fixings.

Carpenter framing a cut roof — The timber structure every covering depends on.

02Details

The roof, layer by layer and junction by junction.

Numbers on each drawing match the notes alongside it. Drawings are schematic, not to scale.

Pitched roof cross-section

The layers from outside to inside on a typical slate or tile roof.

1
Slates / tiles
Lapped so each unit covers the joint below.
2
Battens
Treated timber the slates are fixed to.
3
Breathable membrane
Secondary barrier — catches wind-driven rain, lets vapour out.
4
Rafters
The structural slope of the roof.
5
Insulation
Between/over rafters (warm) or on the ceiling (cold).
6
Ventilation path
Air enters at the eaves and leaves at the ridge.
7
Ridge
Caps the top where the two slopes meet.

Common failure points

Slipped or cracked slates
Torn underlay at eaves
Blocked ventilation causing condensation

Warm flat roof

Insulation above the deck — the recommended approach.

1
Membrane
Waterproof layer exposed to the weather.
2
Insulation
Warm roof: above deck. Cold roof: between joists.
3
Vapour control layer
Always on the warm side of the insulation.
4
Deck
Structural board carrying the membrane.

Common failure points

Insufficient fall leading to ponding

Cold flat roof

Insulation between joists with a ventilated void above.

1
Membrane
Waterproof layer exposed to the weather.
2
Insulation
Warm roof: above deck. Cold roof: between joists.
3
Vapour control layer
Always on the warm side of the insulation.
4
Deck
Structural board carrying the membrane.
6
Ventilated void
Must be cross-ventilated or condensation forms under the deck.

Common failure points

Unventilated void → condensation & rot
Hard to achieve modern U-values

Eaves detail

Where water leaves the roof and air enters it.

1
Gutter
Sized to the roof area it drains.
2
Fascia
Board that carries the gutter.
3
Soffit vent
Air inlet into the roof void.
4
Rafter & wall plate
Load path down into the wall.
5
Insulation
Stops short to keep the air path open.
6
Eaves support tray
Prevents underlay sagging and ponding.

Ridge & hip

Closing the top of the roof while letting air out.

1
Ridge tile
Mechanically fixed in a dry-ridge system.
2
Ridge roll / vent strip
Seals against weather, allows air out.
3
Ridge board
Timber the rafters meet at.
4
Top course
Cut slates/tiles finished tight to the ridge.

Common failure points

Mortar-bedded ridges cracking and loosening

Valley

Where two slopes meet and water concentrates.

1
Valley liner
Lead, GRP or metal channel carrying water down.
2
Cut slates / tiles
Trimmed to leave an open channel.
3
Water flow
Highest volume on the roof — detail must be generous.

Common failure points

Debris blocking the channel
Liner too narrow or poorly lapped

Abutment flashing

Sealing the joint where a roof meets a wall.

1
Step / soaker flashing
Turned up the wall under the cover flashing.
2
Cover flashing
Laps down over the soakers.
3
Chase into mortar joint
Wedged and sealed 25mm into the wall.
4
Roof covering
Runs under the flashing.

Common failure points

Sealant-only joints with no chase
Missing cavity tray above

Standing seam metal

Concealed-fix metal roofing for low pitches and façades.

1
Double-lock seam
Watertight folded joint.
2
Metal tray
Continuous panel.
3
Clips
Hidden fixings allowing movement.
4
Underlay
Separation and drainage.
5
Deck
Ply or OSB substrate.
6
Insulation
Warm roof build-up.

03Glossary

Roofing terms, in plain English.

Abutment: Where a roof meets a wall that rises above it.
Batten: Treated timber strip that slates or tiles are fixed to.
Counter-batten: Batten fixed down the slope to create a drainage and ventilation gap.
Eaves: The lower edge of a pitched roof, where the gutter sits.
Fall: The slope given to a flat roof so water drains.
Flashing: Metal or membrane strip that seals a junction against water.
Soaker: Small lead or metal piece interleaved with slates at an abutment.
Upstand: Where a flat roof membrane turns up a wall or kerb.
Valley: The internal angle where two roof slopes meet.
Verge: The sloping edge of a roof at a gable end.
Vapour control layer: Membrane on the warm side of insulation that limits moisture passing through.
Warm deck: Flat roof with insulation above the deck, keeping the structure warm.

04Specification

Notes for architects & QS

Energy: Building Regulations Part L — U-values calculated for each build-up.
Fire: Part B — external fire performance of roof coverings by boundary distance.
Ventilation: Part F and BS 5250 — condensation control in roof voids.
Slating & tiling: IS ICP 2 and BS 5534 — fixing, laps and batten specifications.
Flat roofs: BS 6229 — falls, drainage and membrane detailing.
Leadwork: Lead Sheet Training Academy guidance — codes, lengths and fixings.

Guidance figures only — final details are confirmed against manufacturer literature and the project engineer's design.

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