Engineered Joist Calculator — Span Table, Count & Hangers

I-joist spans depend on depth and spacing. As a guide for domestic floors at 400mm centres (dead load ~0.75 kN/m², imposed 1.5 kN/m², deflection limited to span/333 or 12mm): a 200mm joist spans about 4.1m, 240mm about 4.6m, 300mm about 5.3m, 360mm about 5.9m, and a 400mm joist about 6.3m (deeper grades reach further). These figures are SJ45-equivalent values from the STEICO span table to BS 5268. At 600mm centres maximum spans reduce by roughly 12–15%. Always confirm against the manufacturer’s span table for your specific product. See our Timber Span Calculator for solid timber joist alternatives.

I-joists have solid timber flanges (top and bottom) with an OSB or plywood web. Posi-joists (metal web joists) have timber flanges connected by a galvanised steel lattice web. Posi-joists allow services to pass through the web without drilling, while I-joists have strict rules about web hole sizes and positions. Both offer longer spans than solid timber.

I-joists: holes are only ever cut in the web, never the flanges, and the permissible size depends on where the hole sits along the span. Manufacturer hole charts (TJI, JJI, STEICO) allow large holes — up to roughly 70–80% of the joist depth — only well away from the bearings; the maximum size shrinks as you move toward a support, and holes are prohibited close to the bearing. A safe no-calc rule of thumb is to keep holes to about one third of the depth in the centre third of the web; for anything larger, work to the manufacturer's hole chart. Posi-joists: services pass freely through the open metal web without cutting. Never cut or notch engineered joist flanges.

Engineered joists require specific hangers designed for their profile — standard timber joist hangers don't fit. I-joist hangers support the bottom flange and clamp the top flange. Posi-joist hangers are similar but accommodate the metal web. Always use the hanger specified by the joist manufacturer. The calculator assumes both ends are hung (two hangers per joist), which suits joists carried off steel or timber beams and trimmers. Where joists instead bear on a timber wall plate or build into masonry with restraint straps, that end needs no hanger — so reduce the count accordingly.

Yes, noggings (blocking) are required to prevent joists twisting and to support board edges. Install rows of blocking at maximum 2.4m centres and at all board joints (per the JJI-Joist Site Guide and MiTek Posi-Joist Technical Handbook). For I-joists, use proprietary blocking panels or solid timber blocking cut to fit. For Posi-joists, use the manufacturer's strongback and blocking details.