What is a padstone, and why does a beam need one?
A beam concentrates the whole load of a wall onto two small points. A padstone spreads that reaction over enough masonry that the wall beneath is not crushed — and it is one of the most commonly omitted parts of a beam installation.
- Difficulty
- intermediate
- Time
- 8 min
- Read
- 4 min
- Safety
- warning
Short answer
When a beam replaces a section of wall, everything the wall was carrying arrives at the beam's two ends. A padstone is a solid block — usually dense concrete, sometimes engineering brick or steel — placed under each end to spread that concentrated force over a larger area of masonry. Without one, the load is applied to a small area and the masonry under it can crush.
People who have had a wall opened up remember the beam and rarely remember the padstones, which is a shame because they are where a lot of beam installations go wrong. The idea is simple and it is the same idea as a foundation, just further up the building.
Safety
Step by step
- See where the load goes.A wall spreads its load continuously along its length into whatever is below. Replace part of it with a beam and that whole load funnels to two points. The pressure at those two points is enormously higher than it was.
- Understand what the padstone does about it.It is a hard, strong block placed between the beam end and the masonry, sized so that the pressure on the masonry beneath it is back down to something that masonry can carry.
- Note that it is a calculation, not a component.Its plan size, its depth and its material all come from the load the beam delivers and the strength of the wall it is landing on. Two identical beams on different walls can require different padstones.
- Recognise the materials.Precast dense concrete padstones are the common choice. Engineering brick built up in courses is used where a padstone will not fit. A steel spreader plate is used where the bearing is onto a steel or where depth is very limited.
- Understand what bedding it properly means.The padstone must bear fully and evenly on the masonry, and the beam must bear fully and evenly on the padstone. A padstone rocking on a high spot concentrates the load again, which defeats the whole point.
- Follow the load further down.A padstone does not stop the question. What the wall beneath the padstone lands on — another opening, a chimney breast that has been removed, a floor rather than a foundation — is part of the same calculation, and it is why an engineer looks at the whole route down.
- Expect it to be inspected.Forming a structural opening is notifiable work, and the bearings are exactly what a building control inspector wants to see before the beam is built in and covered.
Tips
- If a quote for opening up a wall does not mention padstones or bearings at all, that is a fair question to ask before accepting it.
- Photograph the padstones and bearings before anything is plastered over. That photograph is worth a great deal when the house is sold.
- The same idea explains things elsewhere in a building: the spreader under a prop, the plate under a jack post, and the wide base of a foundation are all the same principle at different scales.
Common mistakes
- Landing a beam straight onto the existing brickwork — The concentrated reaction can crush the masonry under the beam end. The damage develops slowly and shows as cracking near the bearing, or as the beam settling and the opening distorting.
- Using a lightweight block as a padstone — Aerated and lightweight blocks are not made for concentrated point loads. The padstone has to be stronger than what it is protecting, not the same or weaker.
- Assuming the wall below can take it because it looks solid — A wall with an opening, a flue, a removed chimney breast or a poor bond below the bearing may not be able to carry a concentrated load down to the foundation. That is why the engineer traces the load path rather than looking at the bearing alone.
If it doesn't work
Cracking appearing near the end of a beam some time after an opening was formed
Cause: Overstressed masonry at the bearing, or a padstone that was omitted or undersized — Fix: This is structural and it is progressive. Get an engineer to it, and find the original calculations and building control paperwork if they exist.
A beam that has visibly settled at one end
Cause: Crushing or movement at the bearing — Fix: Keep load off it, do not attempt to jack it, and get it inspected. Jacking a settled beam without understanding why it settled can make things considerably worse.
Building control will not sign off the opening
Cause: Bearings covered before inspection, or no calculations available — Fix: Expect to expose the bearings again. Inspections of structural bearings exist because they cannot be verified once they are hidden.
Questions people ask
Does every beam need a padstone?
Every beam needs an adequate bearing. Whether that requires a separate padstone depends on the load and the masonry — a modest lintel over a small opening in strong brickwork may bear directly. The calculation decides, not a rule of thumb.
Can I use engineering bricks instead?
Often yes, and it is common where a precast padstone will not fit the space. It has to be the specified brick, in the specified mortar, built to the specified size. It is a substitution the engineer makes, not the builder.
What is a spreader plate?
A steel plate doing the same job, used where there is not room for a concrete padstone or where the bearing is onto steel. Same principle: increase the area, reduce the pressure.