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Make your own construction tolerances

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13 September 2017

I want to talk to you today about tolerance. I am not talking about a workmate who has halitosis, or a relative with questionable personal habits – I am talking about construction tolerances.

I want to talk to you today about tolerance. I am not talking about a workmate who has halitosis, or a relative with questionable personal habits – I am talking about construction tolerances.

Anyone who has talked to a builder on site will have heard the phrase – “But mate! You have to allow some sort of tolerance. This IS a construction site”.

Before BCA2016 version you will not find any reference to this concept. It does appear in AS1428.1:2009 in very limited cases, mostly positional dimensions for sanitary fixtures and grabrails. It certainly does not appear in relation to doorway widths, circulation spaces, ramp gradients etc.

If the building industry is so convinced tolerance exists, yet the regulators do not acknowledge except for a few special circumstances – where is the common ground?

Quite simple – you have to make your own allowance. If the minimum height of a handrail is to be 1000mm; and if you know your building skills are such that you can only achieve a 10mm tolerance, then you have to build the handrail to a design dimension of 1010mm. That way it will comply even in the worst case of tolerances working against you.

For designers there are a few classic tricks to assist the construction industry. Don’t design ramps or walkways to minimum grades: use 1:15 for ramps and 1:21 for walkways. Don’t dimension rooms to frame size, or if you do add enough to ensure that wall finishes are covered. And don’t run handrails into balustrades. Handrails have a maximum height of 1000mm. Balustrades have a minimum height of 1000mm. No middle ground there for a tolerance, best to separate them by some other design device.

Garry Leis 

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