With 20 years’ of experience working in latent defects insurance, common causes for timber floor construction failures become apparent and whilst you might think the cause of most floor defects arise from an inadequate joist size, we’ve found it is most likely due to the use of the joist.
• Excessive notching – when cut outs are made too big and put in the wrong place this can weaken the integrity of the joist (see drawing reproduced from Chapter 8.2 of our Technical Manual)
• Incorrectly positioned service holes – again these can weaken the joist resulting in joists unable to support the load they were designed for
• Incorrect or lack of sufficient bearings
• Twisting due to poor protection to the joist from moisture prior to installation
There are, however, still a number of instances where the joist failure was due to the incorrect size of span.
Joist sizes must be checked to ensure they are correct for the span and spacing as found on site. We recommend using TRADA span tables to assist you when checking on site. These span tables are referenced in Approved Document A which supports the Building Regulations (England and Wales) and can be purchased online.
Typical claims associated with ‘out of level’ floors are either due to poor workmanship of the floor installation or because the joists excessively deflect. In our Technical Manual, Chapter 1.4, Tolerances - Floors, we give guidance on the acceptable deflection for upper floors. Designers and Engineers must observe our tolerances requirements in this chapter for levelness of floors, which are: A maximum 4mm out of level per metre, up to 6m across, maximum 25 mm overall in any other case.
For further technical guidance, access our Technical Manual.