19. Notching & Drilling of Timber Joists

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Technical Guidance Note 19: Notching & Drilling of Timber Joists

The installation of central heating pipes, electrical wiring and other service pipes often results in timber floor joists being notched or drilled.  Incorrect notching or drilling could result in the joist being weakened.  The diagram below gives guidance on the permitted locations for notching and drilling for normal domestic floors.  Any notching or drilling proposed outside of these limits should be justified by structural calculation.

EXAMPLE 1
For a joist 150mm deep and 50mm wide spanning 3.0m, notching may occur between 210mm and 750mm from the support wall, and anywhere up to 18.75mm from the top of the joist.

EXAMPLE 2
For a joist 150mm deep and 50mm wide spanning 3.0m, drilling may occur between 750mm and 1200mm from the support wall, and anywhere within 18.75mm either side of the centre line of the joist.

FLOOR JOISTS
The design of floor joists is dependent upon the loading imposed, ie. domestic or commercial, and the clear span between supports.  This guidance note deals solely with domestic floor joists with a maximum loading of 1.5kN/m2.  For commercial loading, structural calculations should be prepared.

This table can be used when a bath is to be installed provided joists supporting the bath are doubled up.  T&G floorboards of 16mm thick to be used for joists spacings up to 500mm, 19mm thick for spacings up to 600mm

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