First Fix Timber Calculator

First fix carpentry covers all the structural timber work done before plastering: wall plates, stud walls, floor joists, roof timbers, door linings, and window boards. It also includes noggings for plasterboard support and herringbone strutting between joists. First fix is done after the building is weather-tight but before internal finishes. For stud wall details, use our Stud Wall Calculator. For joist rules, see our Joist Notching and Drilling Calculator.

One row of noggings at mid-height is standard for 2.4m walls (providing support for plasterboard and lateral bracing). For walls over 2.7m, use two rows. Noggings are cut to fit between studs at the specified centres. Stagger them slightly for easier nailing — offset by 25mm alternately each side of the centre line.

Herringbone strutting consists of short diagonal timber pieces (minimum 38×38mm) fitted in a cross pattern between floor joists. It prevents the joists from twisting and distributes loads between them. Per BS 8103-3:2009 (as set out in the LABC Warranty Technical Manual): spans up to 2.5m need no strutting; spans over 2.5m up to 4.5m need one row at mid-span; spans over 4.5m need two rows at the third points (one-third and two-thirds of the span). Alternatives include solid timber blocking (at least 38mm thick and three-quarters of the joist depth) or proprietary metal struts.

Joist hangers are used where a joist end meets a wall or beam without bearing on a ledge or wall plate. They are needed for timber-to-masonry connections where there is no bearing. This calculator counts one hanger per joist — the common case where each joist bears on a wall plate at one end and is carried on a hanger at the other. If both ends of every joist are hung (for example between two steel or timber beams), double the hanger count. Use the correct size — hangers are matched to joist dimensions (e.g., 44×200mm for a 47×200mm joist).

Common first fix timber sizes: wall plates 100×50mm, studs 89×38mm or 100×50mm at 600mm centres, noggings same section as studs, floor joists 47×175mm to 47×250mm at 400mm centres, and herringbone strutting 47×47mm or 50×50mm. All structural timber must be C16 or C24 graded.