This Part of BS 8103 gives recommendations for the sizes of timber elements, fixings, and their connections with masonry walls etc. It applies to detached, semi-detached, terraced houses and flats (with not more than four self-contained dwelling units per floor, accessible from one staircase), of not more than three storeys above ground, intended for domestic occupation. The recommendations also apply to single-storey non-residential buildings, e.g. domestic garages and annexes, not exceeding 36 m2 in floor area. The design of proprietary housing systems, i.e. cross wall constructions, timber framed houses, steel or concrete framed constructions, concrete panel constructions, is not covered by this standard.
The recommendations of this Part of BS 8103 are applicable only to housing where:
a) the construction comprises masonry walls, timber roofs, suspended timber or precast concrete intermediate floors, suspended timber or concrete ground floors (either suspended or ground bearing);
b) the roof profile is symmetrical duopitch, monopitch or flat;
NOTE Pitched roofs of trussed rafter or traditional construction are covered, but not constructions using principal trusses. Hip ends and “L” shaped plans are not specifically covered although values are given for jack rafters.
c) the maximum clear span of a pitched roof does not exceed 12.0 m and imposes a total load on the supporting perimeter walls not greater than 12 kN/m run;
d) gable and separating walls are of masonry construction extending up to the roof line in the case of pitched roofs;
e) the roof area is not larger than 200 m2 on plan;
or, in the case of pitched roofs where no parapets exist, one of the roof dimensions does not exceed 10 m;
f) there are no other buildings within 1.5 m of the perimeter of the building;
g) the roof has no abrupt changes of height greater than 1 m on which snow drifting could occur;
NOTE This limitation does not apply to the change of height between the main roof and the roof of a lower annexe, if the roof area of the annexe does not exceed 36 m2. Chimneys and dormers whose vertical elevation areas, against which a drift could form, are less than 1 m2, need not be considered as being an abrupt change of height.
h) the maximum clear span of any floor joist or flat roof joist does not exceed 6.0 m;
i) the design wind speed does not exceed 44 m/s;
NOTE Guidance is given in BS 8103-1 and 8103-2 on how to determine whether a building is on a site with a design wind speed of more than 44 m/s. See also Figure H.3, related to
standard trussed rafter bracing, in this Part of BS 8103. Guidance is given in Annex P regarding the determination of snow load for a particular site.
j) no part of any wall or roof is higher than 15m above the lowest adjacent ground level;
k) floor joists, ceiling joists, flat roof joists, common or jack rafters or trussed rafters are spaced at centres which do not exceed 600 mm;
l) the stability requirements and dimensional limitations accord with clause 5 of BS 8103-1.
The sizes of timber shown in the span tables are limited to those normally readily available but do not preclude the use of larger or different sizes if justified by specialist calculation. The design requirements and those for durability and timber preservation given in this Part of BS 8103 are related only to structural considerations. Reference should be made to other appropriate British Standards or statutory regulations to satisfy other considerations such as resistance to damp penetration, durability and timber preservation, fire resistance and surface spread of flame, thermal and sound insulation, and ventilation.
For trussed rafter roofs, this standard contains only as much information as is necessary for builders’ staff to prepare specifications for trussed rafters and instructions on how to construct a simple trussed rafter roof.
