How beam sizing works
A beam has to pass two tests:
- Strength: it mustn’t break or crush under the heaviest load it will carry. We check bending and shear using the timber design standard (AS 1720.1), with the usual safety factors.
- Stiffness: it mustn’t sag too much. Timber keeps sagging slowly under permanent load (“creep”), so we check the long-term sag against span/300 for floors and span/250 for roofs.
For longer spans, stiffness usually decides the size. That’s why a deeper beam (for example 240 × 45) often works where a wider one (140 × 90) doesn’t.
What is the load width?
It’s the strip of floor or roof the beam holds up. For a beam with joists landing on it from both sides, add half the joist span on each side. For example, joists spanning 3.6 m on one side and nothing on the other give a load width of 1.8 m.
Pine, hardwood or LVL?
- MGP10 pine is the everyday framing timber: cheap, light and easy to work.
- MGP12 is stronger and stiffer for a small extra cost.
- F17 hardwood is strong and stiff, but heavy and harder to get in long straight lengths.
- LVL (laminated veneer lumber) is straight, consistent and comes long. It’s the usual choice for long spans and lintels over wide openings.
Builders in Australia also use the span tables in AS 1684 and the timber suppliers’ own span tables, which already include the right loads and limits for houses. Use this calculator for a quick answer or a sense check, and use those tables or an engineer for the final size.
Common questions
How far can a 190 × 45 beam span?
As a floor bearer in MGP10 pine with a 1.8 m load width, a little over 2 m. As a roof beam under a sheet roof it can go further. Enter your span and load above to check your case.
What size beam do I need to span 4 metres?
For a floor with a 1.8 m load width, about 290 × 45 in MGP12 pine or LVL, depending on the load. For a roof it’s smaller. A 4 m opening in a load-bearing wall is worth getting an engineer to check.
Can I double up two 45 mm beams?
Yes. Two pieces properly nailed together act roughly like one 90 mm beam, which is how the 90 mm sizes in the table are made.
Do I need an engineer to remove a wall?
If the wall is load-bearing, yes in most cases. The new beam, the posts holding it up and the footings under those posts all need to be checked, and a building permit is usually required.