What is the difference between C16 and C24 timber?

C16 is the standard construction-grade timber in the UK, suitable for most general projects. C24 is a higher-quality timber with fewer knots and higher density, making it stronger and stiffer. C24 allows for longer spans with smaller timber depths.

How do I measure the 'clear span'?

The clear span is the distance between the internal faces of the supports (e.g. wall to wall, or post to post). It is NOT the total length of the timber joist. Our calculator uses the clear span to determine the load-bearing requirements.

Should I use 400mm or 600mm joist centres?

400mm centres are the industry standard for domestic floors and decks. They provide a much stiffer surface with less 'bounce'. 600mm centres are sometimes used for light-duty roofs or small sheds but are generally avoided for high-traffic areas.

Do I need noggins between my joists?

Yes. For spans over 2.5 metres, you must use noggins (bridging) in the centre of the span to prevent the joists from twisting or buckling under load. For spans over 4.5m, two rows of noggins are required.

What is the difference between C16 and C24 timber?

C16 is the standard construction-grade timber in the UK, suitable for most general projects. C24 is a higher-quality timber with fewer knots and higher density, making it stronger and stiffer. C24 allows for longer spans with smaller timber depths.

How do I measure the 'clear span'?

The clear span is the distance between the internal faces of the supports (e.g. wall to wall, or post to post). It is NOT the total length of the timber joist. Our calculator uses the clear span to determine the load-bearing requirements.

Should I use 400mm or 600mm joist centres?

400mm centres are the industry standard for domestic floors and decks. They provide a much stiffer surface with less 'bounce'. 600mm centres are sometimes used for light-duty roofs or small sheds but are generally avoided for high-traffic areas.

Do I need noggins between my joists?

Yes. For spans over 2.5 metres, you must use noggins (bridging) in the centre of the span to prevent the joists from twisting or buckling under load. For spans over 4.5m, two rows of noggins are required.

Timber Span Calculator

Structural guidance based on UK TRADA standards.

Clear Span Distance (m)

The distance between supports (not total joist length)

Timber Grade

Joist Spacing (mm)

Structural Result

Span exceeds standard timber limits. Consider adding intermediate supports (posts) or consulting a structural engineer.

Structural Integrity: Measuring the Load

C16 vs C24 Grade

In the UK, timber is strength-graded for structural use. Selecting the right grade and depth is the difference between a solid floor and one that feels bouncy or, worse, fails under load.

Our **timber joist span calculator** provides guidance based on standard domestic load assumptions (1.5kN/m² uniformly distributed load).

Span Comparison

C16 (47x150mm)~2.8m Max

C24 (47x150mm)~3.2m Max

C24 (47x225mm)~4.9m Max

Measurement Accuracy

When using a span table, the **Clear Span** is the critical measurement. This is the horizontal distance between the faces of the supports.

If you are building a deck, this would be the distance from the ledger board on the house to the internal face of the support beam. If the joist overhangs the beam, that overhang (cantilever) should not exceed 1/4 of the total span.

Joist Hangers

Always use galvanised steel joist hangers for all structural connections. Ensure that every hole in the hanger is filled with the correct size galvanised square twist nails (usually 30mm x 3.75mm).

Doubling Up

If your span is borderline for a specific timber size, consider 'doubling up' joists at critical points, such as under heavy hot tubs or large planters. Bolt them together at 600mm centres using timber connectors.

Pro Tip: Buying C24

In many UK timber yards, C24 is now the standard stock. Even if your project only requires C16, buying C24 provides a much stiffer and higher-quality result for a very small increase in cost (usually less than 10%).

Timber Span & Grade FAQ

Structural Disclaimer

This calculator is for guidance only and is based on typical domestic floor loads. It should not be used for industrial projects, roofs, or cases with unusual heavy loads. For any project involving structural building work, always verify your designs with a qualified structural engineer or local building control officer.