Padstones are one of those things that barely get a mention during the planning stage, then suddenly become the reason a building control officer refuses to sign off a job. I’ve seen it happen more than once: a decent steel installation, everything looking solid, and the BCO picks up on an undersized or badly bedded padstone and the whole thing stalls. It’s a straightforward fix if you understand what you’re dealing with, but get it wrong and you’re either rebuilding masonry or waiting on a structural engineer’s report before you can close the job out.
This article covers how padstones work, what causes them to fail inspection, and how to calculate and specify the right one for typical domestic RSJ installations in the UK.
What a padstone actually does
When a steel beam (RSJ or universal beam) sits on a masonry wall, the load it carries gets transferred down into the wall through its bearing point. The problem is that point load. Brickwork and blockwork are strong under compression when that load is spread out, but concentrate it into a small area and you can exceed the crushing strength of the material beneath. That’s where the padstone comes in: it spreads the load over a wider area so the masonry underneath doesn’t fail.
Dense concrete padstones are the standard choice on domestic jobs. You’ll also see engineering bricks used, and on occasion precast concrete units. What you will not get away with is just sitting the steel on a couple of courses of standard facing bricks or medium-density blockwork and hoping for the best. The BCO will spot it, and structurally it’s a risk you don’t want to run.
Why undersized padstones fail BCO inspection
The most common failure I see is a padstone that’s too shallow or too narrow in plan. The result is a bearing pressure under the steel that exceeds what the masonry beneath can take. Building control will want to see that the padstone is properly sized for the load, properly bedded in mortar, and sitting on solid masonry rather than a cavity or a bit of freshly laid block that hasn’t had time to cure.
The second failure is positioning. A padstone must have full bearing on the wall, not hanging over a cavity. On cavity wall construction, you need the padstone to bear fully on the inner leaf (or both leaves if the beam is wide enough to span them). If the steel has insufficient bearing length, the load path is compromised, and that’s a structural issue, not just a cosmetic one.
Poor bedding is the third culprit. A padstone that’s just placed dry or on a thin scraping of mortar can rock under load, which is dangerous and will fail inspection. It needs to be properly bedded in a full bed of mortar, with the beam sitting on it level and square.
How to calculate padstone size on a domestic RSJ job
For most straightforward domestic beam installations, your structural engineer will specify the padstone size as part of the beam design. If you’re working from an engineer’s drawing, use the dimensions given. Don’t substitute a smaller padstone because it’s what you’ve got on the van.
Where you need to work it out yourself (and you should always have an engineer check the numbers before proceeding), the starting point is the reaction force at each end of the beam. This is the load the wall has to carry at that bearing point. Your engineer’s calculation will give you this figure in kilonewtons (kN).
From there, the required bearing area is calculated by dividing the reaction force by the allowable bearing stress of the masonry beneath. For standard medium-density blockwork (7.3 N/mm² compressive strength), the allowable bearing stress is typically around 2.0 to 3.5 N/mm² depending on mortar designation and whether the block is at the end of a wall. Dense concrete padstones themselves have a compressive strength of at least 25 N/mm², so they won’t crush, but the masonry below them might if the pad is too small.
As a rough guide for common domestic installations, here’s what I’d expect to see as a minimum on a typical ground floor or ground-to-first-floor RSJ removal:
For spans up to around 3 metres carrying a single floor, padstones of 215mm x 215mm x 100mm are common. For spans between 3 and 5 metres carrying floor and roof loads, 327mm x 215mm x 150mm is more typical. Anything bigger than that, or carrying unusual loads, needs engineer-specified sizes, full stop.
Bearing length is the other key figure. The beam must sit on the padstone with enough length to transfer the load. A minimum bearing length of 100mm is a reasonable working rule for domestic steel, but again your engineer’s drawing takes precedence. I’d always aim for 150mm on a loaded beam if the wall thickness allows it.
Specifying the right padstone material
Dense aggregate concrete padstones to BS EN 771-3 are the go-to for domestic jobs. You can buy them precast from most builders’ merchants or cast them in situ using a 1:2:4 concrete mix (by volume). The