What centres should joists be at?

In UK timber construction, floor joists are typically installed at 400mm or 600mm centres. The correct centres depend on the joist size, span, and the load it carries - span tables in Approved Document A give the allowable sizes for different spans and spacings.

Can you notch or drill a joist?

Yes, but only within strict limits. Notches are permitted in the top of the joist within the outer quarter of the span, to a maximum depth of 0.125 times the joist depth. Holes may be drilled in the middle half of the span, keeping to the neutral axis area. Notching or drilling outside these zones significantly weakens the joist.

When is herringbone strutting required?

Herringbone strutting or solid blocking is required between joists when the span exceeds 2.5m, to prevent lateral torsional buckling and stiffen the floor. A single row of strutting at mid-span is used for spans up to 4.5m; two rows are needed for longer spans.

Joist - Construction Glossary

Q: What is a joist?

A joist is a horizontal structural member - usually timber - that spans between walls, beams, or other supports to carry the load of a floor, ceiling, or flat roof. Joists are installed in parallel at regular centres and support the decking, boarding, or plasterboard fixed to them.

Definition

A joist is a horizontal structural member - typically sawn or engineered timber - that spans between load-bearing walls, beams, or other supports to carry the load of a floor, ceiling, or flat roof. Joists are installed in parallel rows at regular centres and support the decking, floorboards, or plasterboard fixed to them.

In practice

Floor joists in UK timber construction are typically spaced at 400mm or 600mm centres. Joist size (depth and width) is determined by span, spacing, and load using span tables from Approved Document A. Ends should achieve a minimum 90mm bearing on masonry walls, or be fixed using metal joist hangers where they frame into the face of a beam.

For spans over 2.5m, herringbone strutting or solid blocking must be installed between joists at mid-span to prevent lateral movement and stiffen the floor. Notching and drilling for services is permitted only within strict zones - cutting outside these limits significantly weakens the joist and can cause structural failure or excessive deflection.

Building Regulations

Relevant PartPart A - Structure

Minimum end bearing90 mm onto masonry

Approved DocumentAD A (2010, amended 2013) - Annex A span tables

Strutting requiredSpans over 2.5 m

Approved Document A Annex A provides span tables for floor, ceiling, and flat roof joists covering common timber species and grades. Any new floor structure or significant alteration to an existing floor is notifiable work requiring Building Control approval.

Full Building Regulations guidance

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Just so you know...