Lintel loaded dimension calculator
A lintel span table asks for the loaded dimension, not the span of the roof. This works out the roof and floor loaded dimensions from your framing, identifies the load case, and lists the other inputs you need before the table will give you a member size.
Loaded dimension is the number people get wrong
Loaded dimension is the tributary width the lintel carries. For a roof it is half the roof span plus the eaves overhang, because the lintel picks up everything from the ridge to the eave on its side of the building. For a floor it is half the joist span.
The error that keeps recurring is entering the roof span itself. On an eight metre span with a 600mm eave that is 8.0m going into a table expecting 4.6m, which oversizes the member and the price with it. In the other direction, forgetting the eave understates it, and that is the dangerous one.
Roof mass changes the answer more than people expect
Light roof means steel or pressed metal tile. Heavy roof means concrete or clay tile. The dead load difference is large, and a lintel sized for a light roof does not simply carry a heavy one with a margin to spare. If the roof specification changes late, the lintel schedule has to be rechecked.
What else the table needs
Loaded dimension alone will not get you a member size. You also need the wind zone, the snow zone if the site is in one, and the timber grade and member type.
Wind zones run Low, Medium, High, Very High and Extra High, and above Extra High the building needs Specific Engineering Design. Your territorial authority publishes a wind zone map, and the zone is site specific rather than a regional guess. A sheltered suburban section and an exposed ridge a kilometre away are not in the same zone.
Where to take these numbers
BRANZ publishes a free Lintels and Beams calculator that takes exactly these inputs and returns a member size. It is the right next step, it is free, and it is authoritative in a way a copied table on a forum is not. Beyond the scope of NZS 3604 you need Specific Engineering Design from a chartered professional engineer regardless.
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Start free with Built SimpleFrequently Asked Questions
What is loaded dimension for a lintel? It is the tributary width the lintel carries. For a roof it is half the roof span plus the eaves overhang. For a floor it is half the joist span. Span tables ask for loaded dimension, not the span of the roof itself.
Why can you not just give me the lintel size? Because the span tables are in NZS 3604, which is a paid Standard and copyright under the Copyright Act 1994. We can calculate the inputs, which is ordinary geometry, but not reproduce the table. Use the free BRANZ Lintels and Beams calculator for the member size.
What wind zone am I in? Your territorial authority publishes a wind zone map. Zones run Low, Medium, High, Very High and Extra High, and above that the building needs Specific Engineering Design. The zone is site specific, so do not assume it from the region.
Does roof mass change the lintel size? Yes, substantially. Light means steel or pressed metal tile, heavy means concrete or clay tile. If the roof spec changes after the lintels are scheduled, recheck them.
When do I need an engineer instead of a span table? Any time the building falls outside the scope of NZS 3604, including above the Extra High wind zone. Then you need Specific Engineering Design from a chartered professional engineer.
Related calculators and guides
Sources
- BRANZ Lintels and Beams calculator, free sizing tool.
- Copyright Act 1994, sections 27(1)(c) and 27(1C).
- Standards New Zealand, for NZS 3604 itself.
