The short answer
A padstone calculation works out the size of the solid block that sits under the end of a beam, so the beam's load is spread over enough masonry that it does not crush the wall below. When a steel beam lands on a brick or block wall, its end reaction is a concentrated force over a small bearing area. If that area is too small, the bearing stress exceeds what the masonry can take and the brickwork crushes. A padstone — usually a dense pre-cast concrete block or an engineering-brick build-up — spreads the load over a larger footprint. The calculation takes the beam's reaction, divides it by the proposed padstone area, and checks the resulting stress against the masonry's design strength to BS EN 1996. The output is a padstone of a stated length, width and depth, for example a 215 x 100 x 140mm concrete padstone under each beam end.
Padstones are small but they are where many a beam design actually gets decided, because a strong beam is useless if it crushes the wall it lands on. Here is what the calculation does.
At a glance
- What it isBlock spreading a beam's end load
- The checkBearing stress vs masonry strength
- Worked toBS EN 1996 (masonry)
- Typical materialDense concrete / engineering brick
- OutputStated padstone size per beam end
What a padstone is and why it is needed
When a beam spans an opening, the load it carries has to go somewhere at each end — down into the wall below. The trouble is that a beam end is narrow, so all of that force is concentrated over a small patch of brickwork. Masonry is good in compression but only up to a point; concentrate too much load on too small an area and the bricks below the bearing crush and spall.
- The reaction: the calculation first finds the beam's end reaction — the load each end transfers to the wall, in kilonewtons.
- The bearing area: a bare beam might bear over only its flange width by a short length — far too small for a heavily loaded beam.
- The padstone: a solid, strong block under the beam end spreads that reaction over a bigger area, dropping the stress to a level the masonry can safely take.
A couple of bricks laid under the beam are not a padstone — they may not be strong enough or large enough, which is exactly what the calculation checks.
How the size is worked out
The padstone calculation is essentially a bearing-stress check. The engineer divides the beam's end reaction by the padstone's contact area and compares the result with the design bearing strength of the masonry below, following BS EN 1996. If the stress is too high, the padstone is made larger or a stronger material is specified.
| Input | Example | Role in the check |
|---|---|---|
| Beam end reaction | e.g. 30 kN | The load to be spread |
| Padstone area | 215 x 100mm | Spreads the reaction |
| Masonry strength | Per BS EN 1996 | The allowable stress |
| Padstone depth | e.g. 140mm | Lets load spread through it |
| Result | Stress < allowable | Confirms the size works |
Illustrative figures only; actual values come from the beam design and masonry type. Sources: BS EN 1996 (Eurocode 6); IStructE guidance.
Material and depth matter as much as area
Padstone area is only half the picture. The block also needs enough depth for the load to spread through it at a sensible angle before it reaches the masonry, and it needs to be made of a material strong enough to take the concentrated load at the top without itself failing.
Where padstones fit in the wider beam design
A padstone calculation is rarely a job on its own — it is the last step of a beam design, and it can change the beam scheme. Once the beam size is settled and its end reactions are known, the engineer sizes the padstone at each end. On a lightly loaded domestic beam the padstone is often a standard size that barely registers on the drawing. On a heavily loaded beam — one carrying a wall and two floors, say — the reaction can be large enough that the padstone, or the wall beneath it, becomes the controlling problem.
Several real situations make the padstone the deciding detail:
- Weak or thin walls: a single-skin or lightweight block wall may not have the strength to take even a spread load, pushing the design towards a column or a strengthened pier instead of a simple padstone.
- Beam-on-beam: where one steel lands on another rather than on masonry, a bearing plate and connection replace the padstone, designed as part of the steelwork.
- Cavity walls: the beam must bear on the inner load-bearing leaf, and the padstone is sized and positioned so the load goes into the structural leaf, not the outer skin.
- Existing masonry of unknown strength: in older buildings the engineer takes a conservative masonry strength, which tends to mean a larger padstone.
For the builder, the padstone detail is a practical instruction: build a padstone of the stated size and material at each bearing, bedded level, before the beam is lifted into place. Building Control inspects the bearings as part of signing off the structural work, so a padstone that does not match the calculation — too small, wrong material, or omitted in favour of a couple of bricks — is a common reason a beam installation gets pulled up on site. Getting the padstone right is cheap; correcting a crushed bearing after the beam is loaded is not.
Frequently asked questions
Can I use bricks instead of a padstone?
Not unless the engineer specifies them. A heavily loaded beam needs a padstone of a calculated size and strength — often dense concrete or engineering brick. Ordinary bricks laid under a beam may crush, which is exactly the failure the padstone calculation prevents.
What size is a typical padstone?
It varies with the beam's end reaction and the wall strength. A common domestic padstone might be around 215mm long, 100mm wide and 140mm deep in dense concrete, but a heavily loaded beam can need a larger one or a build-up of engineering brick.
Does every beam need a padstone?
Most beams bearing on masonry need one to spread the load. Where a beam lands on another steel beam or a column, a bearing plate and connection are designed instead. The engineer's calculations state what is needed at each end.
Sources & further reading
- The Institution of Structural Engineers — design guidance
- Planning Portal — Approved Document A (structure)
- LABC — beam bearings and Building Control
Figures on this page are typical UK ranges drawn from published sources and depend on your specific project. They are guidance, not a quotation.