Timber Joist Floor Installation

Timber is much lighter than concrete and easy to work with. Installation is a dry process, so there is no waiting for anything to cure. In older houses this often decides the matter, because the original masonry frequently cannot carry a reinforced concrete slab. In a smaller house, carpenters can usually fit the joists and boarding within a few days.

Before you book a carpenter, decide what the floor will be used for. An unheated loft, a habitable attic room with furniture and an exposed-beam ceiling have very different needs for load capacity, insulation and finish. These needs drive the joist size, the spacing and the whole build-up.

A structural engineer should set the joist size, spacing, bearing and fixings. A tradesperson's estimate is not enough. Replacing or adding a floor changes the load-bearing structure of the building, so check with your local building authority whether you need approval under the building regulations and what kind. If you live in a flat, also speak to the building management or owners' association before any work starts.

The process differs between a new build and an old house, where the existing floor has to be opened up first. The basic steps are similar, though:

1. Measure the space, check the load-bearing walls and compare what is on site with the drawings and the structural design.
2. In a renovation, expose the existing joists and assess their ends, the pockets in the masonry and the moisture levels.
3. Prepare the timber: cut it to length, shape it and treat it against fungal decay and wood-boring insects. For exposed joists, also plane and sand them.
4. Prepare the bearings: level the bearing surfaces and separate the timber from the masonry with a damp-proof layer.
5. Set the joists into wall pockets, onto a wall plate or onto a supporting beam at the specified spacing and fix them in place.
6. Keep checking heights, level and centre-to-centre spacing as the work goes on.
7. Fit the boarding and the other layers from the design: vapour control layer, insulation, impact sound layer, floor finish or ceiling.

Why so much attention to level? A small difference in joist height later shows up as an uneven floor or a cracked ceiling, and putting it right means taking finished layers apart again.

Floor joists are usually made from structural softwood, typically spruce or pine, graded to a recognised strength class. For longer spans, or where the joists must stay straight and stable, carpenters often use glulam beams, which twist and crack less. Whatever is used, the timber must be properly dried, of the right strength class and free from rot or signs of insects.

The build-up depends on what is above and below the floor:

• Floor under an unheated loft: mainly needs thermal insulation between the joists and a vapour barrier or vapour control layer on the warm side, so that indoor moisture cannot get into the build-up
• Floor under a habitable attic: also needs impact sound insulation, a resilient layer and a heavier load-spreading board or a dry floor system, so that footsteps do not carry into the room below
• Exposed-beam ceiling: needs better quality timber, visible boarding and careful sanding, staining and a protective finish
• Concealed floor hidden behind a ceiling: can use OSB or structural boards for the boarding, because function matters more than looks

Mineral wool or wood fibre insulation goes between the joists. Fill the space evenly, with no gaps and no squashed areas. Otherwise you get cold bridges and a risk of condensation.

The simplest case is a new build with straight walls, known dimensions and good access for materials. The job gets more demanding when:

• The room has a long span and needs heavier joists, closer spacing, supporting beams or steel elements
• It is an old house where opening up reveals rotten joist ends, damp masonry or damaged pockets
• The joists will stay exposed and need precise shaping and finishing
• A habitable attic above needs acoustic and flooring layers
• Materials have to come in through a narrow access or be lifted to height

When you compare quotes, check exactly what each one includes. Some cover only fitting the joists. Others include timber, treatment, fixings, boarding, insulation and delivery. A fair comparison is only possible when the scope of work is the same.

The same problems come up again and again in practice:

• Damp or badly stored timber that twists and cracks once installed, leaving a creaky floor above
• Underestimated structural design, where excessive deflection only shows after the floor is laid or partition walls go up
• Joist ends sitting directly in damp masonry without separation, which leads to rot exactly where the joist carries the most load
• Covering old joists with new boarding without checking them first, so rot or insects carry on under the new layer
• Missing or broken vapour control layer, which causes condensation in the insulation
• Floor finish fixed rigidly to the joists with no resilient layer, so every step can be heard downstairs

Most of these faults are invisible once the job is finished. That is why it pays to check the work before the joists are covered.

Ideally visit the site twice: once after the joists are in and before they are covered, and again at handover. At handover, check that:

• Joist size and spacing match the structural design
• Joist ends are separated from the masonry and sit in dry conditions
• Fixings follow the design and the joists do not move
• The timber is treated and free from cracks or signs of pests
• The boarding is level and does not creak when you walk on it
• Insulation fills the space without gaps and the vapour control layer is continuous, including at walls and penetrations
• Under a habitable attic, there is a resilient impact sound layer below the floor finish
• Exposed joists have an even surface and finish
• You have received documents for the timber and treatment, plus a record of the work carried out