Tributary area
Tributary area is the plan area of floor or roof whose load one post, beam or footing carries. How to work it out, and why it drives member and footing sizing.
Ask Chalkline about this →Tributary area is the plan area of floor, roof, or wall whose load is carried by a single supporting element: one beam, post, stud, or footing. Picture the building load spread across the plan; the tributary area of any one support is the slice of that plan it carries down to the next level. It is the starting point for sizing posts, footings, and beams, and the two-dimensional parent of load width, the one-dimensional strip used in AS 1684 span tables.
How it is worked out
For a support in a regular grid, the tributary area runs halfway to each neighbouring support in both directions:
- A deck post carries the rectangle bounded by half the bay to the posts on each side and half the joist span front and back (see residential decks).
- A pad footing under that post carries the same area down to the ground; its size comes from that load divided by the soil’s allowable bearing pressure (see pad footings).
- A beam or bearer carries a strip: half the joist span on each side, along its full length.
Hips, valleys, and irregular bays move the dividing lines, so on a real roof the area is rarely a tidy rectangle (see hip roof).
Why it matters
Tributary area is where load takes off. Double the area and you double the load on that post and footing. Two common errors: ignoring that one support draws load from a wider area than it sits under (an internal post under a long bay), and forgetting that a load-bearing wall or beam between two supports takes tributary area away from each. Get the area right and the member sizing, span-table lookup, and footing design all follow from it.
Also known as: tributary load area, contributing area.
Category: Structure
Related
- Load width, the one-dimensional strip cut from a tributary area for span-table lookups
- Span tables, where load width and spacing turn into a maximum span
- AS 1684, the residential timber-framing standard the tables come from
See also
- Pad footings, sized from the load the tributary area sheds to the ground
- Residential decks, where post and footing tributary areas are set out
- Hip roof, a geometry that skews the tributary lines
Last updated: 2026-05-26. Verified: 2026-05-26. Quarterly review for currency.