6
Existing structure - supporting elements
It is important that your structural investigation covers the existing elements that will be supporting the new
structure, for example internal walls, beams etc. These elements need to be load bearing and capable of
carrying additional loads. To ensure these elements are adequate they may need to be exposed for inspection
prior to the work being carried out. Any previous structural work to the house, for example the removal of a
wall and introduction of a steel beam between a dining room and sitting room, will require checking to ensure
the additional loading it may take is acceptable.
Roof covering
If converting the loft involves the replacement of the roof covering, the following structural issues should be
considered:
1. If the new roof material is heavier, the additional loading on the structure must be calculated and the
structure upgraded accordingly.
2. Conversely, if the new roof material is lighter, the possibility of increased wind uplift must also be
considered.
Notches and holes in joists
Notches and holes in simply supported floor and roof joists should be within the following limits:
·
Notches should be not deeper than 0.125 times the depth of a joist and should not be cut closer to the
support than 0.07 of the span, nor further away than 0.25 times the span.
·
Holes should have a diameter not greater than 0.25 times the depth of a joist and should be drilled at
the joist centre line. They should be not less than 3 diameters (centre to centre) apart and should be
located between 0.25 and 0.4 times the span from the support.
·
Notches or holes should not be cut in rafters, purlins or binders unless approved by the building
designer.
·
Rafters restrained by ceiling ties at eaves level may be birdsmouthed at supports to a depth not exceeding
one third of the rafter depth.
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