Timber

Timber touches almost every part of an extension, from the roof joists to the stud walls to the skirting board. The grade on your engineer's calculations is the one building control expects to see on site, and using the right grade for each job affects how the structure performs for the life of the building.

These guides cover the timber products used in a domestic extension: structural grades for load-bearing work, treated timber for anything in contact with masonry or the ground, sheet materials like OSB, plywood and chipboard for roofs, floors and sheathing, and the joinery that finishes internal walls and doorways. Each page explains the grade, the typical specification, and what to check when timber arrives on site.

Where to start

Start with C16 structural timber, the standard grade for domestic joists and rafters, and C24, the stronger grade your engineer may specify instead. Substituting one for the other without approval can stop the build. Then read treated timber, because an untreated wall plate sitting on blockwork can rot and take the roof structure with it.

The guides, grouped by job

Structural timber. C16, C24 and treated timber, plus end grain preservative, which every cut end of treated timber needs.

Sheet materials. OSB for flat roof decks and sheathing, structural plywood where the job needs it, P5 chipboard for floors, and MDF for internal joinery.

Doorways and trim. Door lining kits, door stop bead, architrave and skirting board. Getting the lining width right for the finished wall thickness decides whether the rest fits.

Where these fit in the build

Joists, rafters, wall plates and roof decking go in at the roof structure stage, and building control looks at them during the structure inspection. Door linings go in at first fix carpentry, before plastering; the carpenter comes back afterwards for door stops, architrave and skirting, ready for decoration. The roof timber quantity calculator helps you sense-check a roof timber order.