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Decking Joist Span Tables - Softwood Joists

The span of joists in the information below should only be taken as a guide. Deck joist spacing will be determined by experience and knowledge, as well as the minimum spacings listed.

They have been prepared from experience and also reference to Government Building Controls Data, and are based upon load bearings for floors to houses.

The dimensions quoted are absolute maximum for decking joist spans and should not be exceeded. Where possible, and certainly on raised decks, the span allowance should be shortened - if only to avoid annoying 'rattle' and bounce.

Decking Joist Span Dimensions for Joist 6x2in. 147 x 47mm C20 Grade.

The distance between the joists will determine how long to supported joist span can be. The distance between the joist will be determined by the type of decking used - dimensions and quality. These spans relate to the use of softwood decking only.

Maximum Spans for Deck Joist Spacing

Experience has shown that in the event of of such long spans, then there is considerable benefit to be gained from cross bracing at right angles, a further joist in the centers of the span. Not as a bearer per se, but as a further aid to prevent isolated dipping or bulging points, and to solidify the overall structure.

**** These dimensions are NOT to be used for any form of plastic decking. In this case adhere to the manufacturer's specification sheets.

****Where decking is used with dimensions smaller than 147 x 28mm, then the distance between joists should be reduced to the minimum size.

***For raised decking projects, then the 500mm deck joist spacing, is recommended - together with a shortening of the span allowed to 2meters. This can either be achieved by intermediate bearers, or by shortened anchorage stations.

These decking joist spans have been extracted from Building Control Guidance Notes for domestic flooring. Unless there is to be provision for abnormal loads such as with a hot tub sat on the deck, they should be sufficient for most normal non-commercial decking projects.

Where the joist spans are more than 2.5metres - in all of the above examples for instance, there should be struts (noggins) positioned between the joists at the halfway position to prevent any warping or other distortion of the joist. It is not sufficient to think that the affixed deckboard will do this job. The depth of the strut should ideally be that of the joists.

Decking Joist Spans Dimansions for 5x2in (122x47mm) C20 Joists.

Our recommendation is that this type of joist should NOT be used for raised decking - other than garden decks with a height of up to 600mm.

Building Control Guidance Note...

Our recommendation is that these spans should be decreased to.....

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