Foundation Load from a Wall: kN/m Worked Example

Work out a wall's foundation load step by step: roof, floor and wall self-weight to 29.8 kN/m, then footing weight and a 70 kN/m² bearing pressure.

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The foundation load from a wall is the total load per metre run (kN/m) that reaches its strip footing. Start at the roof and work down. At each level add the floor or roof line load (area load × loaded width) and one storey of the wall's own weight. Then add the weight of the wall below ground, the footing and its backfill, and divide by the footing width to get the bearing pressure.

For the external wall of a typical two-storey UK house worked below, that comes to 29.8 kN/m at ground level, 42.1 kN/m at the underside of the footing, and a bearing pressure of about 70 kN/m² on a 600 mm strip.

What makes up the load on a wall's foundation?

Four things, all kept in kN per metre run of wall:

• Roof and floor loads. Each one is the area load (kN/m²) times the width of roof or floor that bears on the wall. The tributary regions article shows how to set those widths out on a plan.
• Wall self-weight. The wall's weight per m² of face times its height. See brick and block wall self-weight.
• Anything else that bears on the wall. Beam reactions, an upper wall that doesn't stack, a chimney. These are often point loads and need their own check (see beam to foundation and offset walls).
• The substructure. The wall below ground, the footing itself, and the backfill sitting on the footing.

Keep permanent loads (Gk) and variable loads (Qk) in separate columns all the way down. You need both, and they take different factors.

The example wall

Front wall W1 of a two-storey house, 9.0 m wide and 8.0 m deep. It is the same house used in the six quick checks article.

• Roof: tiled, on trusses spanning 8.0 m front to back. 0.9 kN/m² permanent + 0.6 kN/m² imposed (the full 0.6 is conservative above 30° pitch). Loaded width to W1 = 8.0 / 2 = 4.0 m.
• First floor: timber joists spanning 4.0 m from W1 to a central spine wall. 0.6 kN/m² permanent + 1.5 kN/m² imposed + 0.8 kN/m² for movable partitions (the cautious band for partitions up to 2.0 kN/m; light timber studs fall in the 0.5 band). Loaded width = 4.0 / 2 = 2.0 m.
• Wall: brick and 100 mm block cavity wall, plastered inside, 3.6 kN/m². 2.6 m from ground to first floor, 2.4 m from first floor to eaves.
• Ground floor: ground-bearing slab, so it does not load the wall.
• Footing: concrete strip 600 mm wide × 300 mm thick, base 1.0 m below ground.

These are typical values; check them with a competent engineer and against the actual construction. Typical UK floor and roof loads has fuller tables.

Step 1: How much load reaches the wall at each level?

Multiply each area load by its loaded width, keeping Gk and Qk apart:

Roof         Gk = 0.9 × 4.0 = 3.6 kN/m    Qk = 0.6 × 4.0 = 2.4 kN/m
First floor  Gk = 0.6 × 2.0 = 1.2 kN/m    Qk = (1.5 + 0.8) × 2.0 = 4.6 kN/m

So the roof brings 6.0 kN/m and the first floor 5.8 kN/m.

Step 2: How do you add the wall's own weight?

Self-weight per m² × storey height, one storey at a time:

Upper storey  3.6 × 2.4 = 8.6 kN/m
Lower storey  3.6 × 2.6 = 9.4 kN/m

On a two-storey house the walls are often more than half the load at the bottom, as they are here. Leaving them out is the most common way to under-size a footing.

Step 3: Keep a running total down the wall

Add each item in the order the load meets it, from the top:

Check that the load only grows going down. If a running total ever drops, a level has been missed.

Step 4: Add the substructure and find the bearing pressure

Below ground the cavity is normally filled with concrete, so treat the wall as solid. The footing and the backfill on its projections (the 150 mm each side of the wall) also bear on the ground.

Load at ground level                         = 29.8 kN/m
Filled wall below ground  0.3 × 0.7 × 20     =  4.2 kN/m
Footing                   0.6 × 0.3 × 24     =  4.3 kN/m
Backfill on projections   0.3 × 0.7 × 18     =  3.8 kN/m
Total at underside of footing                = 42.1 kN/m

Bearing pressure = 42.1 / 0.6 = 70 kN/m²

The densities are about 20 kN/m³ for the filled wall, 24 kN/m³ for concrete and 18 kN/m³ for soil. With an allowable bearing pressure of 100 kN/m², a common presumed value for firm clay, 70 kN/m² is comfortable. The real allowable pressure must come from a site investigation or a qualified ground assessment, and that is the number that decides the footing width.

This is the gross pressure, which is the simple and conservative check. Some engineers compare the net pressure instead, taking off the weight of soil dug out of the trench (0.6 × 1.0 × 18 = 10.8 kN/m). That gives (42.1 − 10.8) / 0.6 = 52 kN/m². Only use the net figure if the allowable pressure you have been given is a net value.

Do you use factored or unfactored loads for the foundation?

Both, for different checks:

• Bearing pressure against a presumed allowable value: characteristic (unfactored) loads, as above. The allowable value already has its safety margin built in.
• Masonry, padstones and the footing's own strength: design loads, using 1.35G + 1.5Q from BS EN 1990.

At ground level on W1:

Design load = 1.35 × 22.8 + 1.5 × 7.0 = 30.8 + 10.5 = 41.3 kN/m

Applying 1.5 to both the roof and the floor imposed load is slightly conservative. Strictly, only the leading variable action takes the full 1.5 and the others are reduced by a combination factor. Many engineers accept the small overestimate in a takedown.

If the ground is designed to Eurocode 7 instead of against a presumed allowable pressure, the method changes: factored loads go against factored soil resistance. Don't mix the two methods on one sheet. Characteristic vs design loads explains the difference.

Summary of the method

1. Find every roof and floor that bears on the wall and its loaded width.
2. Turn each area load into a line load, Gk and Qk separately.
3. Add one storey of wall self-weight at a time, top to bottom.
4. Add the wall below ground, the footing and the backfill.
5. Divide the characteristic total by the footing width and compare with the allowable bearing pressure.
6. Use design loads (1.35G + 1.5Q) for the structural checks.

For W1 that gives 29.8 kN/m at ground level, 42.1 kN/m on the soil and 70 kN/m² under a 600 mm strip.

FAQ

How do you calculate the load on a strip foundation?

Add up everything that bears on the wall per metre run: roof and floor line loads (area load × loaded width), the wall's own weight over its full height, then the wall below ground, the footing and the backfill. Divide that characteristic total by the footing width to get the bearing pressure in kN/m².

What is a typical foundation line load for a two-storey house?

For an external cavity wall carrying a timber floor and a trussed roof, often about 25 to 35 kN/m at ground level. This example gives 29.8 kN/m. Internal walls carrying floor from both sides, or walls with concrete floors, can be well above that. Always work it out for the actual building.

Should the footing's own weight be included?

Yes, when you compare against an allowable bearing pressure given as a gross value, which is the usual case. Here the footing, filled wall and backfill add 12.3 kN/m, which is about 40% on top of the 29.8 kN/m from above.

How wide should a strip footing be?

At least wide enough that the characteristic load divided by the width stays below the allowable bearing pressure. Practical minimums for building the wall and digging the trench often govern on a house, which is why 600 mm is common. The width must be confirmed against the ground conditions on site.

LoadTakedown imports each page of the architect's PDF as a level and carries the loads down level by level, with running totals on each wall and characteristic and design loads kept apart. Try it on your own drawings at LoadTakedown.