What Is a Loaded Width? How Floor Load Reaches a Wall
21 March 2026 · 6 min read
Loaded width explained: why it is half of each span for simple floors, how continuity and cantilevers change it, and how it turns kN/m² into kN/m.
The loaded width is the width of floor whose load is carried by a wall or beam. For a simply supported one-way floor it is half the span on each side, so a wall under a 6.0 m span has a loaded width of 3.0 m. Multiply it by the area load (kN/m²) and you get the line load on the wall (kN/m). Continuous joists and cantilevers change the split, so they need their own figures.
It is the number behind almost every line in a load takedown, so it is worth getting right.
What is a loaded width?
The loaded width is measured at right angles to the wall, from the wall to the edge of its tributary region. The tributary region is the patch of floor that loads that wall.
- Tributary region: the shape, an area on the floor plan.
- Loaded width: its width, one dimension of that shape.
For a one-way floor spanning between two walls, the edge of each wall's region is the line where the floor's load stops going to one wall and starts going to the other. For a simply supported span under a uniform load, that line is at midspan.
A wall with floor on both sides adds the two halves together.
How do you calculate a loaded width?
It depends on how the floor is supported. For a uniform load:
| Support condition | Loaded width |
|---|---|
| Simple span L, one wall each end | L / 2 to each wall |
| Two equal spans L, joists continuous over the middle wall | 3/8 × L to each end wall, 1.25 × L to the middle wall |
| Back span L plus cantilever c | (L + c)² / 2L to the wall at the overhang, (L² − c²) / 2L to the far wall |
| Cantilever c only, built in at the wall | c, the full overhang |
Simple span. The usual hand assumption, and the right one for joists cut and butted over a wall:
Loaded width = 6.0 / 2 = 3.0 m to each wall
Continuous joists. When joists or a slab run unbroken over a middle wall, that wall attracts more load and the end walls get less. For two equal 4.0 m spans:
End walls: 3/8 × 4.0 = 1.5 m each
Middle wall: 1.25 × 4.0 = 5.0 m
Check: 1.5 + 5.0 + 1.5 = 8.0 m, the full floor width
With the simple split the middle wall would get 2.0 + 2.0 = 4.0 m, so continuity adds 25%. Unequal spans give different factors. One-way spans has more on when to treat a floor as continuous.
Cantilever with a back span. A floor that runs past a wall to a free edge (a balcony, a jettied upper floor) loads the wall under the overhang with more than the overhang plus half the back span. For L = 4.0 m and c = 1.0 m:
Wall at overhang: (4.0 + 1.0)² / (2 × 4.0) = 25 / 8 = 3.13 m
Far wall: (4.0² − 1.0²) / (2 × 4.0) = 15 / 8 = 1.88 m
Check: 3.125 + 1.875 = 5.0 m, back span plus overhang
The simple "overhang plus half the back span" would give 3.0 m and 2.0 m. As the overhang grows, the far wall's share drops, and it can lift if the imposed load sits only on the cantilever. Check that case and the holding-down detail.
Cantilever only. If nothing runs back past the wall, as with a slab or steel built into it, the wall carries the whole overhang, so the loaded width is c. The wall must also resist the cantilever's moment, which is a separate design check.
How does loaded width turn an area load into a line load?
Multiply:
Line load (kN/m) = area load (kN/m²) × loaded width (m)
Take a first floor spanning 6.0 m between two walls, simply supported, carrying 5.0 kN/m² permanent load and 2.0 kN/m² imposed load (1.5 for domestic use plus 0.5 for light partitions). These are typical values; check them with a competent engineer for your own build-up.
Loaded width = 6.0 / 2 = 3.0 m
Gk = 5.0 × 3.0 = 15.0 kN/m
Qk = 2.0 × 3.0 = 6.0 kN/m
Characteristic total = 21.0 kN/m
ULS = 1.35 × 15.0 + 1.5 × 6.0 = 20.3 + 9.0 = 29.3 kN/m
Keep the permanent and imposed parts separate all the way down. The characteristic total is what you compare with an allowable bearing pressure. The ULS figure is for member design. Characteristic vs design loads explains the difference.
What about a wall with floor on both sides?
Add each side's loaded width. In the plan above, internal wall W2 has floor A spanning 6.0 m on one side and floor B spanning 4.0 m on the other, both at 7.0 kN/m² characteristic:
Loaded width to W2 = 6.0 / 2 + 4.0 / 2 = 3.0 + 2.0 = 5.0 m
Line load on W2 = 7.0 × 5.0 = 35 kN/m
W1 = 7.0 × 3.0 = 21 kN/m
W3 = 7.0 × 2.0 = 14 kN/m
Check: 21 + 35 + 14 = 70 kN/m = 7.0 × 10.0 m
This is common in houses, where rooms of different sizes sit either side of an internal spine wall. If the joists run continuously over W2, it takes more than 5.0 m, as shown above.
The check line is worth doing every time. The loaded widths across a plan must add up to the total floor width, or something has been missed or counted twice.
What if the loaded width varies along the wall?
The examples above have a constant loaded width because the floors are rectangular. In real buildings it can vary along the wall:
- Non-rectangular rooms. The distance to midspan changes along the wall.
- Openings in the floor. A stairwell or void removes part of the tributary region, and the trimmers around it concentrate load at their bearings.
- Two-way slabs. The tributary regions become triangles and trapezoids. See two-way floor loads.
When the loaded width varies, so does the line load. In a hand calculation the usual approach is to take the largest loaded width along the wall, which is conservative. LoadTakedown works out the tributary regions from the plan, and its wall report lists the minimum and maximum loaded width for each wall.
What comes after the loaded width?
The loaded width gives you the floor and roof loads on each wall. Add the wall's own weight from the wall self-weight table and carry the total down level by level to the foundation load. The load takedown worked example does this for a whole house, and the hand takedown guide covers internal walls and beams.
Free load takedown spreadsheet
SLS and ULS line loads for every wall, a UK load library and a footing width guide. Free Excel file, no account.
FAQ
What is the loaded width of a wall?
It is the width of floor whose load is carried by that wall, measured at right angles to it. For a simply supported one-way floor it is half the span. A wall with floor on both sides takes half of each span, so 6.0 m and 4.0 m spans give 3.0 + 2.0 = 5.0 m.
Is loaded width the same as tributary width?
Yes, the two terms mean the same thing. Tributary area is the loaded width multiplied by the length of wall or beam, giving an area in m².
Does a floor parallel to a wall load it?
Not in a one-way floor. Joists running parallel to a wall do not bear on it, so the wall carries little or no floor load. Some engineers allow a nominal strip, such as 0.5 m, for the edge joist. A two-way slab does load all its edges.
How do continuous joists change the loaded width?
Joists running unbroken over a middle wall put more load on it. For two equal spans the middle wall takes 1.25 × the span instead of 1.0 × the span, and each end wall takes 3/8 of the span instead of half. Joists cut and butted over the wall act as two simple spans.
What is the loaded width for a cantilever?
With a back span L and overhang c, the wall under the overhang takes (L + c)² / 2L and the far wall (L² − c²) / 2L. For a 4.0 m back span and a 1.0 m overhang that is 3.13 m and 1.88 m. A cantilever built in with no back span puts the full overhang on its wall.
If you want loaded widths worked out from your own drawings, LoadTakedown lets you trace walls and spans over each PDF page and carries the loads down level by level.
Sketch your structure straight onto your drawings and get tributary loads for every wall and level, instantly.
Try the example in the app