Roof Joists vs Rafters vs Trusses 2026

Roof joists, rafters and trusses — what's the difference

Rafters are the sloping timbers that run from ridge to eaves and carry the roof covering. Ceiling or roof joists run horizontally, tie the feet of the rafters together to stop the roof spreading, and support the ceiling below. A truss is a factory-made triangular frame that combines rafters, joists and internal webs in one engineered unit, which is why modern homes are built with them.

Typical roof joist sizes and spans

Joist size depends on the span and the load it carries, so there is no single answer. As a rough guide, ceiling joists are commonly 47mm wide and 100–220mm deep, getting deeper as the span grows. For habitable rooms and loft conversions the timbers are larger again. The exact size must be calculated by a structural engineer and signed off under Building Regulations — never guessed from a table alone.

What it means for a loft conversion

A traditional “cut” roof with rafters and joists is usually straightforward to convert. A trussed roof is trickier: the internal webs of standard ‘W’ trusses cannot simply be cut out, so the roof must be re-supported with steel beams and new floor joists and rafters designed by an engineer. That extra structural work is why trussed-roof conversions cost more.

How to plan and compare quotes

1. Get a structural survey

Have a structural engineer inspect the roof and calculate the members before any work is priced.

2. Identify joists vs trusses

Look in the loft: a web of angled timbers means trusses; clear rafters with a boarded floor means a cut roof.

3. Plan upgrades for the load

For a loft conversion, budget for new floor joists, steels and any rafter strengthening the engineer specifies.

4. Get three like-for-like quotes

Compare builders on the same engineer's drawings so every quote covers the same structural work.

FAQs

Related guides

• Roof trusses explained
• Roof ventilation explained
• Comparison guides
• Is it worth it? Insights