What is the difference between a cut roof and a truss roof?

A cut roof is built on site by a carpenter, cutting and fixing each rafter individually, which suits irregular shapes, lean-to geometry, and vaulted ceilings. A truss roof uses factory-made frames craned into place, cheaper and faster but only for regular rectangular footprints with standard pitch.

What is the difference between a ridge board and a ridge beam?

A ridge board is a thin non-structural timber the rafters butt against, with ceiling joists resisting the outward thrust. A ridge beam is a load-bearing member, timber or steel, that carries the rafter weight and is needed where there are no ceiling joists, such as a vaulted ceiling.

Should extension roof timber be C16 or C24?

It depends on the span and load in your structural calculation. C16 is the standard domestic grade and adequate when correctly sized; C24 is stronger and stiffer for longer spans. If the specification says C24, do not accept C16 as a substitute.

What is the difference between a warm roof and a cold roof?

A warm roof keeps insulation between and below the rafters so the whole structure stays within the heated envelope, which is the standard for new extensions. A cold roof puts insulation at ceiling level and needs ventilation above it, carrying a higher condensation risk.

Why does a lean-to extension roof need a breathable membrane?

A lean-to butts against the existing house wall, so there is no ridge to ventilate through. A breathable membrane lets water vapour escape to the outside without high-level vents, which is why traditional bitumen felt is a poor choice on a lean-to.

When does building control inspect the roof structure?

Before the roofer starts battening and tiling, while the timber frame, straps, membrane, and insulation are still visible. If the covering goes on first, the officer can require it all to be stripped back to verify the structure.

Back to Single Storey Rear Extension

Phase 4 · Structure · Task 04 of 09

Roof Structure: Getting Your Extension Watertight

How the timber roof frame goes onto your extension: wall plate, rafters, ridge, and warm roof insulation. Covers cut roofs vs trusses, the ridge beam vs ridge board distinction that catches builders out, warm roof construction, and what building control inspects before you cover anything up.

17 min readUpdated 2026Premium

Technical cross-section of a pitched lean-to roof structure for a single-storey extension showing wall plate, rafters with birdsmouth cuts, ridge connection, breathable membrane, and insulation layers with annotation callouts

Prerequisites

Complete these first

Your roof structure is the single biggest milestone in the build. Once the timber frame is up, the breathable membrane is on, and the covering goes over it, your extension is watertight. Internal trades can start. The clock on weather damage stops ticking. Everything before this point is exposed. Everything after is protected.

Which makes the quality of this stage non-negotiable. A sagging rafter, a wall plate that's 15mm out of level, or the wrong timber grade might not be visible once the plasterboard goes on, but building control will catch it if you don't. And if they don't catch it before the roof is covered, they can require you to strip it back.

Do this first

Your walls must be at plate height with the wall plate bedded and strapped. Your structural steels must be in position, including any steel ridge beam. If you're having roof windows (Velux or similar), order them now if you haven't already. Lead times run 8-10 weeks and your carpenter needs the exact dimensions before cutting trimmer openings.

What this guide covers

01Which Roof Type Are You Getting?
02Cut Roof vs Truss Roof
03The Wall Plate: Where Most Problems Start
04Rafters, Ridge, and the Components That Matter
05Timber Grade: C16 vs C24
06Warm Roof vs Cold Roof
07The Breathable Membrane
08What Building Control Inspects
09What It Costs
10Timeline
11Common Problems
12Your Roof Structure Checklist
13What Comes Next

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