The designer should take the following steps to avoid building problems into a detail:
• Determine how much each component can be expected to vary in size and position. For some items there may be no published standards, but professional designers should allow for some tolerance, and specify what will be accepted. If nothing is said, it is reasonable for standard trade practice to prevail whatever it may be. For many items, applicable standards will state tolerances.
• Look at each step in the order of construction. Will brickwork at it’s extreme position reduce the cavity to less than the minimum needed for venting and drainage, exceed the span or adjustment of the ties, or have inadequate bearing? Can the shelf angle be allowed to wander freely within allowed tolerance without constraining the brickwork? Follow the order of construction of the detail through mentally, asking at each step what will happen subsequently if the component just constructed is at it’s allowable smallest, largest, furthest up, furthest in, or other extreme of size and position. Can the remaining components be properly added, with allowance for their extremes as well?
• If it seems unreasonable to accommodate the established tolerances, consider specifying narrower tolerances. However, make sure that the reduction is physically and economically possible, and communicated clearly to the builder.
• Revise the detail if necessary so that adjustments are available to compensate for extremes at each stage, to make completion of the detail possible without reducing tolerances for subsequent stages. It is important when thinking about tolerances, or laying out the next stage of work at the site, to refer all measurements back to a common datum, so that errors do not accumulate. Reviewed by Jan Luistermans.
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