Estimate preliminary floor load width
Use this as a starting point for understanding load width. Complex layouts, cantilevers, stairs, roof loads, point loads and concentrated loads need separate review.
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Floor Load Width Calculator & FLW Guide
Estimate a preliminary floor load width for deck bearer checks, then use the guide below to understand how FLW is used in bearer span tables.
FLWWork out a starting floor load width for edge and internal bearers before comparing deck bearer span-table options.
Estimated floor load width
1200 mm
For an edge bearer, preliminary FLW is half the joist span loading from one side.
What is floor load width?
Floor load width, usually shortened to FLW, is the tributary width of deck or floor area that is assumed to load a bearer or beam. It helps turn a floor area into a line load for span-table checks, so it is not simply the full deck width.
Edge bearer
An edge bearer usually receives joist load from one side only. As a simple starting point, the FLW is commonly related to half of the joist span feeding into that bearer.
Internal bearer
An internal bearer may receive joist load from both sides. For preliminary estimating, add the tributary width from each side before checking the bearer span table.
Use the calculator above for a quick estimate, then check the matching FLW column in the relevant bearer span table.
How to use FLW in bearer span tables
- Start by identifying whether the bearer is an edge bearer, internal bearer or a custom load case.
- For simple edge bearers, the tributary width commonly comes from one side of the joists only.
- For simple internal bearers, add the contributing width from both sides of the bearer.
- Bearer span tables often use FLW columns such as 900, 1200, 1800, 2400 or 3000 mm.
- When choosing between published columns, using the next higher FLW is usually more conservative for preliminary comparisons.
Floor load width FAQs
Plain-English answers for reading FLW columns in deck bearer span tables and using the calculator as a preliminary guide.
What does FLW mean in a deck bearer span table?
FLW means floor load width. It is the tributary width of deck or floor area assumed to load a bearer or beam when using a span table.
How do I calculate FLW for an edge bearer?
For a simple edge bearer carrying joists from one side, a common preliminary starting point is half of the joist span feeding into that bearer. Always check the source span table rules before using the result for design.
How do I calculate FLW for an internal bearer?
For a simple internal bearer, add the tributary width from each side. This is often half the joist span from one side plus half the joist span from the other side.
Is floor load width the same as deck width or joist span?
No. Floor load width is the tributary width carried by the bearer or beam. It can be different from the full deck width and different from the full joist span, depending on the support layout.
Why does a larger FLW usually reduce bearer span?
A larger FLW means the bearer is carrying load from a wider strip of deck or floor area. With more load on the bearer, the allowable span in a bearer span table will generally reduce.
Should I round FLW up to the next span-table column?
For preliminary comparisons, using the next higher published FLW column is usually the conservative approach, unless the manufacturer span table gives different instructions.
Can this FLW calculator be used for final design?
No. This calculator is a preliminary estimating guide only. Final selection should be checked against the original manufacturer span tables, project conditions, NCC requirements and engineering advice where required.
SpanFinder toolkit
Popular calculators and span guides
Move between the live finder, load-width calculators and plain-English guide pages without losing the flow of the job.
Deck span calculatorCheck deck joist and bearer options using span, spacing, dead load and support assumptions.View deck calculator
Floor load width calculatorEstimate the tributary floor width carried by a bearer before checking a span table.Calculate FLW
Joist spacing calculatorWork out joist count and spacing before comparing member options.Check joist spacing
Bearer post spacingReview how posts and supports affect bearer span selection.Review post spacing
Deck span tablesUnderstand the terms used in span tables, span charts and product data.Read span-table guide
Calculation pathway
A simple pathway for checking deck spans
Choose the member
Decide whether you are checking a joist, bearer, beam or related support member.
Confirm the load width
Use FLW or spacing inputs to describe how much deck or floor area the member is carrying.
Compare suitable spans
Use the calculator and guide pages to compare preliminary member options.
Verify the final design
Confirm the original product data, site conditions, NCC requirements and engineering advice where required.