What span can I-joists achieve?

I-joist spans depend on depth and spacing. At 400mm centres: 200mm depth spans 3.5m, 240mm spans 4.5m, 300mm spans 5.5m, 360mm spans 6.5m, 400mm spans 7.2m. These are approximate for domestic loading (1.5 kN/m² imposed). At wider 600mm centres, maximum spans reduce by about 20%. Always get manufacturer's span tables for your specific product.

What is the difference between I-joists and Posi-joists?

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.

Can I cut holes in engineered joists?

I-joists: holes can only be cut in the web (never the flanges) and must follow the manufacturer's guidelines — typically maximum 70% of joist depth, centred on the neutral axis, and not in the outer quarter of the span. Posi-joists: services can pass freely through the open metal web without cutting. Never cut or notch engineered joist flanges.

What joist hangers do I need?

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. Two hangers per joist (one each end).

Do I need noggings between engineered joists?

Yes, noggings (blocking) are required to prevent joists twisting and to support board edges. Install noggings at maximum 2.4m centres and at all board joints. 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.

TradeCalculator

Engineered Joist Calculator — Span Table, Count & Hangers

Calculate I-joist or Posi-joist count from span table, with hanger selection and web hole guidance.

Floor Span (Joist Length)

Clear span between supports

Floor Width

Direction across joists

Joist Type

Joist Depth

Max span at 400mm: 4.5m

Joist Spacing

Joist Price

Hanger Price

How We Calculate This

This calculator determines engineered joist count, checks span capacity from span tables, and calculates hanger and nogging quantities.

Joist count

Joists = ceil(Floor Width / Spacing) + 1
Total Metres = Joists × Floor Length
Hangers: 2 per joist (one each end).

Span check

Maximum span is looked up from manufacturer span tables based on joist depth and spacing. If the floor length exceeds the max span, a deeper joist or intermediate support is needed.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.