This design is in accordance with BS 5268-2:2002 Structural use of timber - Part 2: Code of practice for permissible stress design, materials and workmanship.
Timber to be covered, this calculation is not to be used for timber which is fully exposed to the elements.
The depth to width ratio of the timber does not exceed 6 and as per the requirements of BS 5268-2 Table 19 there is no risk of buckling under design load provided; The ends are held in position and compression edge held in line, as by direct connection of sheathing, deck or joists.
Notches are not to exceed 0.125 of the depth of the joist and are to be located between 0.07 and 0.25 of the span from the support.
Holes drilled at the neutral axis (centre line) are not to exceed 0.25 of the depth of a joist and are not to be less than than three diameters (centre to centre) apart and are to be located between 0.25 and 0.4 of the span from the support.
Strutting or blocking between joists is required as follows;
- Joist spans up to 2.5m - no strutting required
- Joist spans 2.5m to 4.5m - provide strutting at mid-span
- Joist spans over 4.5m - provide 2 rows of strutting at third span positions
Solid timber strutting to be at least 38mm thick and be a minimum of three quarters of the joist depth.
Proprietary strutting to be used in accordance with manufacturers recommendations.
The joists are to have a minimum end bearing of 40mm.
Wane as allowed in BS 4978:2007 + A2:2017 is permitted.
These calculations are only applicable for flat roofs consisting of four or more joists.