Wood is the only truly sustainable material on this planet. We can grow as much timber as we need, in well-managed forests, and its a great way to lock away carbon for long periods.
Softwoods are the most frequently used timbers for load bearing situations and must always be strength graded. Timbers can be graded by a machine to achieve its required class and the timber is then stamped. Structural timber can be marked C16 or C24 and marked with the company responsible for grading. It is important to minimise storage time, and if they have to be stored they should be sitting on racks and be protected.
The floor joist system acts as a horizontal plane that transfers weight load to beams, load bearing walls and foundations. When one or more joists fail you can get floor bounce, sagging or heaving in the floor. Loud squeaks may also signify joist troubles. As well as getting the correct strength of joist and ensuring the joists are stamped and treated getting the correct joist spans is pivotal to the integrity and longevity of a suspended floor. Too great a joist span is the major cause of joist failure followed by notching a joist during the installation process.
Note* Structural softwood for internal use should be graded to BS 4978 and marked DRY or KD & joists should not be spaced out more than 600mm but this can 450mm where the floor board is 15mm as opposed to 18mm.
LABC Guidance joists
- Joists should have a minimum end bearing of 90mm unless joist hangers are being used where 35mm bearing is acceptable.
- Double joists should be bolted together at 600mm centres with 10mm diameter bolts, and large washers.
- The bolting of double joists should be along the centre line of the joists.
- Trimmer joists will be needed to form openings for chimneys and stairs. They can be supported using hangers or a structurally designed connection.
- There should be at least 2 members of timber trimmers around openings.