Figures 17.1 and 17.2 show the general construction of timber upper floors. Except where built over partially ventilated spaces such as garages, timber upper floors do not require insulation. Unlike suspended timber ground floors, the joists of upper floors have to be of a much larger section because the spans cannot be broken by means of such things as sleeper walls. Joist sizes can be obtained from Approved Document A. The normal centres for joists used on upper floors is 450 mm. The normal procedure that I adopt when designing a timber upper floor is to find the longest span and make all the joists the same size. The reason for this overdesign is cosmetic. If you do not keep all the sections the same, the floor to ceiling heights in each room will vary considerably and this can look ridiculous. Where stud partitions are
Fig. 17.2 Cross-section of timber floor over partially ventilated space (e.g. garage). built off upper floors, the floor beneath each partition running with the joists by doubling up the joists. Doubling up also is necessary under baths. Upper floors
can be covered with tongued and grooved boarding or more commonly nowadays, flooring grade chipboard. The NHBC recommend 16 mm minimum thick floor boards or 18/20 mm minimum chipboard type C4.