One-Way Spanning Floors: How the Load Splits to the Walls

A one-way floor loads only the two walls it spans between. See the 50/50 split, the 5/8 vs 3/8 split over a middle wall, and a worked 4 m x 6 m room.

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A one-way spanning floor puts its load on the two walls or beams it spans between, and nowhere else. The walls running parallel to the span carry only their own weight and anything built on them. For a simply supported floor, each loaded wall takes half the span width of floor.

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How do I tell which way a floor spans?

Look at what the floor is made of. The span direction is almost always the direction of the main structural elements.

• Timber joists: they span along their length. The joists run from wall to wall in the span direction.
• Beam and block: the prestressed beams span between the walls. The infill blocks do not change the direction.
• Precast hollow-core planks: they span along their length, in the direction of the cores.
• One-way reinforced concrete slab: the main bars run in the span direction, with lighter distribution bars across them.
• Steel beams with decking: the deck spans between the secondary beams, the beams span onto walls or columns.

On a drawing, a floor arrow or a joist note ("JOISTS 47 x 200 @ 400") tells you the direction. If a drawing does not show it, ask. Guessing the direction is the most common way to get a takedown wrong.

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Which walls carry the floor load and which do not?

Take a room 4 m wide by 6 m long, with joists spanning the 4 m direction. They bear on the two 6 m long walls.

• The two 6 m walls carry all of the floor load, each with a loaded width of 2 m.
• The two 4 m walls are parallel to the span. They carry no floor load, only their own weight and whatever sits on them (see wall self-weight).

This is why the four walls of a room rarely carry the same load. Rotate the joists through 90 degrees and the load moves to the other pair of walls entirely.

One practical point: the joists still need to sit on or beside the end walls for lateral restraint (straps, noggins), but that is a stability detail. It does not make the end wall a vertical load path.

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When does a floor stop being one-way?

A floor spans one way when it is only supported on two opposite sides, or when it is supported on all four sides but is long and thin. Once the long side is less than about twice the short side and the slab is supported on all four edges, it spans both ways and the load spreads to all four walls. That case is covered separately in two-way floor loads and the 45 degree rule.

Timber joists and most precast units are one-way regardless of the room shape.

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What changes when the floor runs over an internal wall?

A simply supported span splits its load evenly: half to each wall. A floor that runs continuously over an internal support does not.

For two equal spans of length L under a uniform load w (kN/m²), the standard elastic reactions are:

Another way to say it: each span sends 5/8 of its load to the middle wall and 3/8 to its end wall, instead of 1/2 and 1/2. The middle wall takes 25% more than the simply supported assumption. The two end walls take 25% less.

This comes from standard continuous beam theory (three-moment equation for two equal spans, uniform load, constant stiffness). It is not a code table, so check the basis for your own floor: unequal spans, partial loads or discontinuous joists change the numbers.

In practice, two things apply:

• Joists lapped or butted over a wall behave as continuous only if the joint carries the moment. Joists cut and butted at the wall act as two simply supported spans, so the middle wall takes 1.0 × w × L and the end walls 0.5 × w × L.
• Taking 1.25 for the middle wall and 0.5 for the ends is a common conservative hand shortcut. Each wall is covered, but the total is 12.5% over the real load, so it does not balance.

Whichever you pick, write the choice into your assumptions so a reviewer can follow it.

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What about cantilevers and back spans?

A floor that projects past a wall (a balcony, a jettied upper floor, a bay) is a cantilever. For a uniform load w, a back span L and an overhang c, the reactions per metre are:

• Inner support (the wall under the overhang): w × (L + c)² / (2 L)
• Far support: w × (L² − c²) / (2 L)

Check: the two add up to w × (L + c), the full load on the floor. Take L = 4 m and c = 1 m, with w = 3.0 kN/m². The inner wall carries 3.0 × 25 / 8 = 9.4 kN/m and the far wall carries 3.0 × 15 / 8 = 5.6 kN/m. Total 15.0 kN/m, which is 3.0 × 5 m.

Two points to keep in mind. First, the inner wall carries more than a simple "half of each side" would suggest. Second, the far support gets lighter as the overhang grows, and it can go into uplift if the cantilever is long relative to the back span or if the imposed load is only on the overhang. Check the pattern loading and the holding-down detail. Timber back spans are typically kept to at least twice the cantilever, but follow the joist designer's advice.

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What do trimmers do around a stair opening?

An opening in a one-way floor cuts the joists that would have crossed it. The cut joists (trimmed joists) hang off a header, and the header is carried by trimmer joists at each side of the opening.

The result is that load which would have been spread along the wall is now collected and delivered at fewer points. The trimmer joists carry their own share of floor, plus the reaction from the header. The walls under the trimmer bearings see higher load over a short length.

For a takedown:

• Treat the area of floor around the opening as part of the tributary area, then remove the opening itself.
• Add the header reaction as a point load onto each trimmer, then work out the trimmer reaction onto the wall.
• Where the trimmer bearing is on a short length of wall, check the bearing and consider a spreader or padstone.

For a small opening, a conservative short cut is to ignore the opening and keep the full uniform load on the wall. For a large opening, the concentrated reaction needs checking.

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Worked example: 4 m x 6 m room

A single room, 4 m by 6 m internal. Timber joists span the 4 m direction onto the two 6 m walls. Assume a deliberately conservative floor dead load of 1.5 kN/m² (joists, boards, ceiling, finishes and services) and an imposed load of 1.5 kN/m² (UK domestic floor, BS EN 1991-1-1 with UK NA). See typical UK floor and roof loads for how to build up your own dead load.

Case A: simply supported, one room

Case B: two rooms side by side, joists continuous over the middle wall

Now suppose a second 4 m span continues past the middle wall, still 6 m long, same loads.

Compare with the simple assumption: 6.0 kN/m on each outer wall and 12.0 kN/m on the middle wall. The continuous floor moves load from the outer walls onto the middle wall. If you were sizing a strip foundation under the middle wall using the simple figure, you would be 20% light (12.0 against 15.0 kN/m).

From here, add wall self-weight and carry the loads down through each storey, as in the worked example or the manual load takedown guide. If you only want a quick tributary check, the free tributary area calculator does the geometry for you.

These values are typical and the method is a simplification. Check them against your actual floor build-up and have a competent engineer confirm them before they are used for design.

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What mistakes do people make with one-way floors?

1. Sharing the load between all four walls. A one-way floor only loads two. Spreading it over four under-loads the real supports and over-loads walls that carry nothing.
2. Missing a change of span direction. Joists in the next room, or on the next floor, may run the other way. Check the framing plan for each room and each level, not once per building.
3. Using 50/50 over a continuous support. The middle wall takes about 25% more, which matters for foundation sizing.
4. Forgetting the end walls still carry their own weight. No floor load does not mean no load. Walls above still stack down.
5. Treating a two-way slab as one-way (or the reverse). A slab on four walls with an aspect ratio under about 2 is not one-way. See two-way floor loads.
6. Ignoring cantilevers and openings. They concentrate load, so the uniform line load can understate what a wall sees locally.

When you have a lot of rooms and levels, the bookkeeping is the hard part, not the arithmetic. LoadTakedown keeps track of span direction and tributary regions for you. For the geometry behind irregular rooms, see tributary regions.

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FAQ

Which walls carry the load from a one-way spanning floor?

Only the two walls (or beams) the floor spans between. For joists, that is the walls the joist ends bear on. Walls running parallel to the joists carry no floor load, only their own weight and anything above them. Always confirm the span direction from the framing plan before assigning load.

How is the load shared between the two supports of a one-way floor?

For a simply supported span under a uniform load, it is shared equally: each support takes w × L / 2 per metre of wall, where w is the area load and L is the span. Over an internal wall with two equal continuous spans, the split changes to 3/8, 5/8 and 3/8 of the span load.

Does the end wall carry any load from a one-way slab?

An end wall parallel to the span carries no vertical floor load. It carries its own weight, and loads from above. It may restrain the floor sideways, but in a takedown the floor load on it is normally taken as zero.

When does a floor stop being one-way?

When it is supported on all four sides and its long side is less than about twice its short side. It then spans both ways and the load reaches all four supports. Joists and planks stay one-way whatever the room shape. See two-way floor loads.