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Stepped Foundation Calculator (Steps, Bays and Overlaps on a Slope)

How a sloping site steps a wall's foundation: the fall along the wall, the steps at a chosen height, each level bay, and Approved Document A's overlap.

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Imperial · sales tax
The length of the wall whose foundation steps, measured on plan.

The run between the two ends of the stepping part; a wall that turns a corner is two runs with their own angles to the slope.

The gradient down the line of maximum fall, as 1 in N — enter 10 for 1 in 10.

The gradient on the topographical survey is measured down the line of maximum fall. Where the bearing stratum falls more steeply than the surface, use its gradient instead: the steps follow the worse of the two.

0 for a wall running straight down the slope, 90 for one along the contours.

A wall at an angle to the fall sees the gradient times the cosine of that angle: at 30 degrees it keeps 87 per cent of the gradient, at 60 half, and at 90 it is level and takes no steps.

A strip of concrete under a masonry substructure, or a trench filled with concrete to near ground level.

Approved Document A laps them by different rules: a strip by the greatest of twice the step, its thickness or 300 mm; trench fill by the greater of twice the step or one metre.

The depth of concrete: the strip's thickness, or the depth of the trench fill.

Approved Document A sets a minimum of 150 mm (6 in) for a strip, and no step may be taller than the foundation is thick.

The height of each step — pick a multiple of the masonry course the wall is built in.

British brickwork gauges at 75 mm (3 in) and blockwork at 225 mm (9 in), so a 225 mm step is one block or three bricks. A step off the coursing means cutting units below ground or thickening a bed joint.

Whole steps at the chosen height

5 steps

High confidence

The fall does not divide by the step. Carry the height left over in one step rather than spreading it across every bay, and never at the shallow end, where the minimum founding depth has to be met.

Gradient along the wall
10 %
Fall along the wall
4 ft
Height left after the whole steps
4.25 in
Level bay between steps
7.29 ft
Overlap at each step (Approved Document A)
1.46 ft
Tallest step the thickness allows
8.75 in
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • Approved Document A (2004 edition incorporating 2004, 2010 and 2013 amendments), paragraph 2E2 d and e with Diagram 21: foundations stepped on elevation should overlap by twice the height of the step, by the thickness of the foundation, or 300 mm, whichever is greater; for trench fill foundations the overlap should be twice the height of the step or 1 m, whichever is greater; steps should not be of greater height than the thickness of the foundation
  • The fall a line sees across a plane sloping at gradient g is g × cos θ, where θ is the angle between the line and the direction of maximum fall; a level bay between steps of height s is s ÷ (g × cos θ) long

Inputs used

Wall Length on Plan
40 ft
Site Gradient, 1 in
10
Angle Between the Wall and the Line of Maximum Fall (degrees)
0
Foundation Type
Strip foundation
Foundation Thickness
8.75 in
Step Height
8.75 in

Intermediate steps

Gradient along the wall
10 %
Fall along the wall
4 ft
Height left after the whole steps
4.25 in
Level bay between steps
7.29 ft
Overlap at each step (Approved Document A)
1.46 ft
Tallest step the thickness allows
8.75 in
Final result5 steps

Confidence note: The fall does not divide by the step. Carry the height left over in one step rather than spreading it across every bay, and never at the shallow end, where the minimum founding depth has to be met.

What this calculation does not cover

  • It steps a foundation on a plane slope. Ground that changes gradient along the wall, or a bearing stratum that dips differently from the surface, needs the levels taken along the wall line and the steps placed where the stratum sets them.
  • Approved Document A's provisions are for plain concrete strip and trench fill foundations to low-rise buildings in England. Anything outside them — a reinforced foundation, poor ground, a building beyond the document's scope — is designed by an engineer.
  • It gives the geometry, not the founding depth. The depth below finished ground at every point along the run comes from the ground, frost and any nearby trees, and the shallowest point governs.
40 ft8.75 in
Schematic, drawn to the proportions you entered — not to scale on screen.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-22 · v1.0.0

Regulatory standards & verification citations2
  1. Approved Document A (2004 edition incorporating 2004, 2010 and 2013 amendments), paragraph 2E2 d and e with Diagram 21: foundations stepped on elevation should overlap by twice the height of the step, by the thickness of the foundation, or 300 mm, whichever is greater; for trench fill foundations the overlap should be twice the height of the step or 1 m, whichever is greater; steps should not be of greater height than the thickness of the foundation
  2. The fall a line sees across a plane sloping at gradient g is g × cos θ, where θ is the angle between the line and the direction of maximum fall; a level bay between steps of height s is s ÷ (g × cos θ) long

Which documents these citations point at

  • Approved Documents to the Building Regulations (England) — A (United Kingdom)Practical guidance on meeting the Building Regulations, one lettered part per subject — Part A structure, Part B fire, Part K protection from falling, Part L conservation of fuel and power, and the rest.

A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Tools and safety for this job

To place and finish concrete. Generic types, no brands, no prices.

Protection this work requires

  • Wet cement and lime burn skin and eyes painlessly until the damage is done: waterproof gloves to EN 374, safety glasses whenever the mix can splash, and never kneel in wet mix in permeable trousers.
  • Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
Show the 7 tools this job needs

Essential

  • Bull float and edger

  • Screed bar or rail

  • Shovel and spade

  • Spirit level

  • Wheelbarrow

Recommended

  • Plate compactor

Optional

  • Power float

what each concrete tool is for, and the spec that decides which to buy where one does.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

  • Stepping Foundations Down a Sloping Siteuses this calculator

    A metre of fall turns one strip footing into a staircase of separate pours. Where the step height, the lap and the made-up ground come from.

Worked example: Brick garden wall — step 1 of 8

How to calculate stepped foundation (steps, bays and overlaps on a slope) in 7 steps

  1. Wall Length on PlanThe length of the wall whose foundation steps, measured on plan.
  2. Site Gradient, 1 inThe gradient down the line of maximum fall, as 1 in N — enter 10 for 1 in 10.
  3. Angle Between the Wall and the Line of Maximum Fall (degrees)0 for a wall running straight down the slope, 90 for one along the contours.
  4. Foundation TypeA strip of concrete under a masonry substructure, or a trench filled with concrete to near ground level.
  5. Foundation ThicknessThe depth of concrete: the strip's thickness, or the depth of the trench fill.
  6. Step HeightThe height of each step — pick a multiple of the masonry course the wall is built in.
  7. Whole steps at the chosen heightThe tool computes the whole steps at the chosen height from those figures and shows the formula, its sources, and a confidence rating alongside it.

Whole steps at the chosen height by wall length on plan

Page defaults, not your figures above.

Wall Length on PlanWhole steps at the chosen height (steps)
20 ft2
30 ft4
40 ft5
50 ft6
60 ft8
70 ft9

Frequently asked questions

How long must the overlap be at each step?
Approved Document A asks a strip foundation to overlap by the greatest of twice the step height, the foundation's thickness or 300 mm (12 in), and trench fill by the greater of twice the step or 1 m (3 ft 3 in). On trench fill a step up to 500 mm (20 in) therefore costs the same one-metre lap as a small one.
Why does the wall's angle to the slope matter?
A wall only sees the part of the slope that runs along it: the site gradient times the cosine of the angle between the wall and the line of maximum fall. At 30 degrees the wall keeps 87 per cent of the gradient and at 60 degrees half. On a rectangle set square to the contours, two walls do all the stepping and the other two are level at different heights.
What happens to the height left over?
Carry it in one step rather than spreading it across every bay. The slope guide's 1.204 m (3 ft 11 in) of fall in 225 mm (9 in) steps leaves 79 mm (3.1 in), which makes one step 300 mm (11.8 in) — and not at the shallow end, which has a minimum founding depth to meet.
When do the bays become shorter than the overlap?
When the step is small against the gradient, because a bay is the step height divided by the gradient. So 225 mm (9 in) steps give 2.25 m (7 ft 5 in) bays on a 1 in 10 fall but only 900 mm (35 in) on a 1 in 4. Against trench fill's one-metre overlap every bay then laps into the next, and only a taller step lengthens them again.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.