What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Statesyours
Same word, different thingjoist
also rafter, on center (o.c.), 16 o.c.
The member names match, but spacing is stated in INCHES on centre — 16 or 24 — and sheet goods are made in feet to suit, so the whole module differs from the metric one.
Canada
Same word, different thingjoist
also rafter, on centre
American spacing conventions with metric documentation, so drawings commonly state both — and the two do not divide into each other evenly.
United Kingdom
Same word, different thingjoist
also rafter, centres, 400 centres
Spacing is stated in MILLIMETRES between centres — 400 or 600 — and boards are 1200 or 2400 mm to suit. A detail drawn for 16 inch centres does not land on 400 mm.
Australia
Same word, different thingjoist
also rafter, centres, batten spacing
Metric centres as the UK, though the standard spacings and the sheet sizes differ from British ones in places, so neither converts cleanly to the other.
The governing standard, by market
United States
IRC / NDS
Span tables and design values for repetitive framing members.
United Kingdom
BS EN 1995 (Eurocode 5)
Design of timber structures, including floor and roof members.
Australia
AS 1684
Residential timber-framed construction, including span tables.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Why does the spacing module matter more than the number?
- Because everything sheet-shaped on a building is manufactured to land on it. Boards, plasterboard, sheathing, insulation batts and plasterboard ceilings are all made in widths that divide evenly by the local spacing — four feet by sixteen or twenty-four inches in North America, 1200 mm by 400 or 600 mm in Britain and Australia. Change the spacing and every sheet has to be cut, every joint lands mid-member or off it, and insulation batts either compress or leave gaps. That is why converting a detail between markets is not a matter of finding the nearest spacing: the whole set of materials above it is dimensioned to the module you left behind.
- Are span tables interchangeable between markets?
- No, and the reasons compound. The member sizes differ, because a nominal 2x4 has no metric equivalent. The grading systems differ, so the design strength assigned to an apparently similar piece is not the same number. The load cases differ, because each market's code sets its own imposed loads for a domestic floor. And the deflection limits differ. A span table is the output of all four decisions, so reading a span off one market's table and building to another market's materials is not a conversion — it is using the wrong design. Where a member has to be sized across markets, it is sized from that market's table and materials, or by an engineer.
- What is the difference between a rafter and a truss?
- How the roof's forces are resolved, and therefore what may be done with the space. A rafter is a single member spanning from wall plate to ridge, so the roof's spread is resisted by ceiling ties, purlins and the walls — which leaves the loft as usable or convertible space. A truss is a prefabricated triangulated frame in which the webs carry the forces internally, so it needs no intermediate support and gives a clear span, but the webs occupy the roof space and cannot be cut or altered without a designed intervention. That distinction is what decides whether a loft conversion is a fit-out or a structural alteration, and it is visible at a glance once you know to look for the webs.