Padstones Explained: What They Are, Why Your Steel Beam Needs One, and What Building Control Checks

Your structural drawing says "440 x 215 x 100 padstone to each beam end" and your builder turns up with a single dense concrete block off the merchant's shelf. It looks close enough. It isn't. Get the padstone wrong and one of two things happens: the block crushes under the steel and the wall above cracks, or building control spots it at the structure inspection and makes you strip back finished work to put it right. The padstone is a small, cheap, easily-ignored part of a steel installation, and it is the part that goes wrong most often.

What a padstone is and why your beam needs one

A padstone is a dense block that sits between the end of a steel beam and the masonry it rests on. Its job is to take the concentrated load coming down through the narrow end of the beam and spread it out into a wider area of the wall below, so the masonry isn't asked to carry more than it can bear.

To understand why that matters, you need one number. The aerated and medium-density concrete blocks that form the inner leaf of most modern cavity walls are not strong in compression. A typical lightweight inner-leaf block has a compressive strength of around 3.6 N/mm² (newtons per square millimetre, the standard measure of how much squeezing force a material takes before it crushes). Once you allow for mortar and safety factors, the wall can only carry about 1 N/mm² of bearing stress in practice.

Now picture a steel beam carrying the floor and roof above a 3-metre opening. The whole of that load funnels down to the two beam ends. The contact area where a steel flange meets the wall is small, maybe 100mm wide. Push a heavy point load through a small area onto a weak block and the block crushes, exactly like a table leg punching through soft ground. The padstone is the snowshoe. It takes the point load from the steel, which is a strong material, and hands it to the wall across a larger footprint, so the stress that actually reaches the block stays under what the block can survive.

The load doesn't spread sideways magically. It fans out through the body of the padstone at roughly 45 degrees, then carries on down through the pier of masonry below. That is why a padstone has to be solid, dense, and properly bedded onto a sound pier. A padstone sitting on a weak or hollow section of wall just moves the crushing problem one course lower.

Not every steel needs a padstone. Where the masonry below is strong (engineering brick, or a high-strength block) and the load is light, an engineer may confirm the bearing works without one. But that is a design decision your structural engineer makes and writes down, not a judgement call for the builder on the day. If a padstone is on the drawing, it is on the drawing for a reason that has been calculated.

A bearing plate is not a padstone

This trips up homeowners and some builders, and the confusion is made worse by the fact that "bearing plate" is even listed as an informal alias for padstone in places. They are two different things doing two different jobs.

A padstone is masonry: a block of dense concrete or engineering brick built into the wall, underneath the steel, spreading the load downward into the blockwork. It is part of the wall.

A steel bearing plate (sometimes called a spreader plate) is a flat steel plate that sits on top of the padstone, under the beam, or is welded to the beam end. It does a different job: it levels the beam, distributes the load evenly across the top face of the padstone if the beam web sits proud, and on wide openings with very high point loads it spreads the load over a larger pad area than the steel flange alone would. On a heavy beam, an engineer might specify both a padstone and a steel spreader plate on top of it.

So the rule is simple. A steel plate on its own, bolted or welded to the beam, is not a substitute for a padstone. If your beam arrives with a welded end plate and no padstone is built into the wall below it, the load still has nowhere safe to go. Forum threads are full of builders who fitted a steel plate and called the job done, then had building control fail it. If the drawing calls for a padstone, you need the masonry padstone, regardless of any plate on the steel.

What padstones are made of

Three materials cover almost every domestic job, and your engineer will name which one on the beam schedule.

The strength floor for any of them is set by NHBC Standards Chapter 6.5: a padstone must achieve a minimum compressive strength of 10 N/mm². Every material in the table above clears that floor with room to spare. The point of the minimum isn't to make you choose a special product, it's to rule out the thing builders reach for when they're short: a standard or lightweight block, which would crush just as the unprotected wall would.

Precast dense concrete is what you'll buy nine times out of ten. The Supreme Concrete PAD range stocked by Travis Perkins, Jewson and most merchants is C40/50, meaning a characteristic strength of 40 to 50 N/mm². For a small bearing, Class B engineering bricks bedded up in a strong mortar do the job and cost less, since the bricks themselves run well above the strength a padstone needs. Where the engineer wants something the merchant doesn't stock, the answer is an in-situ pour, cast in a shutter against the wall with the beam temporarily propped, at a minimum of 25 N/mm² and usually RC30/37. On-site pours are the last resort because curing quality on site is variable and the bearing can't be loaded until the concrete has gone off.

Who specifies the size, and what the sizes mean

You do not pick the padstone size. Your structural engineer does, on the beam schedule, and the number comes straight out of the beam's end reaction (the load arriving at each end) divided by the bearing strength of the wall below. Bigger load or weaker wall means bigger padstone. The calculation is theirs, and deviating from it transfers the liability onto you.

Typical domestic sizes, written as length x width x height in millimetres:

The 440 x 215 x 100 is the workhorse: it suits most single-storey rear extension wall removals, and it's the size the merchant Price below is keyed to. Wider openings carrying a roof, or beams landing on weak walls, push you up to the 215mm-deep sizes or to a bespoke pad with a steel spreader plate on top.

One firm rule from NHBC 6.5.5: a padstone must be formed in one unit. You cannot stack two cut sections of block on top of each other to make up the height. The padstone depth should match the coursing of the adjacent masonry so it builds in cleanly. If the specified padstone won't physically fit the space available either side of the opening, that is a question for the engineer, not a cue to grind one down or improvise.

How much bearing the beam needs

Bearing length is how far the end of the steel sits onto the padstone. Too little, and the load is concentrated at the very edge of the pad and the masonry, exactly where it's most likely to fail.

The figure to know is 150mm at each end. That is what the Planning Portal sets out and what most building control officers enforce. NHBC's absolute floor is 100mm onto the padstone, but treat 150mm as the practical target and don't let a builder cut it finer without the engineer signing it off. On a narrow pier where there genuinely isn't 150mm of wall, that is, again, an engineer's problem to solve, possibly with a deeper steel that needs less bearing or a wider pad.

Where the steel runs in line with the wall and acts as a lintel rather than spanning across an opening, NHBC asks for a minimum padstone length of 200mm with the beam web centred over the wall.

Installing and anchoring a padstone

The padstone is fully bedded, dead level, in a strong mortar, onto a sound section of wall or pier. "Fully bedded" means a complete mortar bed under the whole base of the padstone, not three dabs with voids in between. The mortar is given 24 to 48 hours to cure before the beam is loaded onto the padstone, so it isn't being squeezed out while it's still soft.

Level matters more than homeowners expect. If the padstone rocks or sits even a few millimetres out of true, the beam bears on one corner and the load concentrates exactly where you didn't want it. Where a beam lands fractionally high or low, steel shims or a structural grout (the kind sold for bedding machine bases) take up the gap. Dry-packing the void with a rammed stiff mortar mix is standard practice and is sometimes written into the spec.

In a cavity wall, the padstone goes on the inner leaf only, which carries the floor and roof load. The outer brick skin carries its own much lighter load and is tied back across the cavity. A padstone that bridges the cavity is wrong: it creates a cold bridge and ignores how the two leaves actually work. Where the engineer wants both leaves supported, you get a separate padstone on each leaf, never one block spanning the gap.

If the beam is fixed down to a bearing plate that is itself bolted into the padstone, those anchors are structural and fall under BS 8539, the code of practice for post-installed anchors in concrete and masonry. BS 8539:2012+A1:2021 requires ETA-approved anchors, installed by a competent person, with the type and size documented. For the common M12 expansion (shield) anchor, the bolt has to sit at least 90mm from any free edge of the padstone, with a minimum 60mm between bolts. For resin-fixed studs the equivalent rule is 8 × anchor diameter (≥80 mm for M10). Run the maths and a four-bolt plate needs a pad face of roughly 240 x 240mm just to clear the edge distances, which is one more reason the engineer, not the merchant's stock list, sets the pad size.

Most domestic beams are not bolted down at all. UK practice is to build the beam end into the masonry and let the dead load hold it, unless there are uplift or rotational forces in play, in which case the engineer specifies the fixings.

What building control checks, and how to be ready

The padstones get inspected at the structure stage, while the steel is in and before anything covers it up. This is a hold point. Once the floor deck goes down or the ceiling is boarded, the bearing is hidden, and putting a defect right afterwards means dismantling finished work.

Call your building control officer to inspect the steelwork before it's concealed. That single phone call is the most useful thing a homeowner does on this part of the build, because the inspector is checking the things that go wrong:

• Is there a padstone under every steel end? Per the beam schedule. Not most ends. Every end.
• Does the size match the drawing? Measure it. The most common defect is a padstone smaller than specified because the builder used what was available.
• Is the material right? Dense concrete or engineering brick, not a standard or lightweight block.
• Is the bearing length met? Around 150mm onto the pad at each end.
• Is it fully bedded and level? No rocking, no gaps, no dry packing standing in for a proper mortar bed, and a sound pier underneath.

The inspector may also ask for confirmation that the masonry below the padstone is strong enough, and some authorities want the engineer's calculations or a stated block grade for the pier. Keep the beam schedule and the engineer's calcs on site for the visit.

Cost and where to buy

Padstones are a trivial line in a steel installation that runs into thousands. A standard precast 440 x 215 x 100mm dense concrete padstone is £8 – £20. In practice, single-unit retail prices at builders' merchants in mid-2026 sit toward the upper end of that range once VAT is in, with William Hercock around seventeen pounds, JT Atkinson around sixteen, and Howarth Timber and Lawsons in the low-to-mid twenties; a trade account shaves a few pounds off. The deeper 440 x 215 x 140mm and 440 x 215 x 215mm sizes cost more, into the high twenties and thirties each.

A typical extension needs two to four padstones, so you're looking at a materials cost of well under a hundred pounds against a steel package of several thousand. Engineering bricks built up as a small padstone are cheaper still. The lesson buried in those numbers: nobody saves meaningful money by undersizing a padstone, so the only reason it ever happens is that the right size wasn't on the shelf and the wrong one was. Order the size on the drawing, even if it means waiting a day.

Padstones come from any builders' merchant: Travis Perkins, Jewson, Howarth Timber, Selco, Huws Gray and the rest stock the Supreme Concrete, Stressline and Naylor ranges. They're heavy (a 440 x 215 x 102mm unit is around 23kg) but small, so collection or merchant delivery with the rest of your masonry is straightforward. Buy them with the steel order so they're on site when the beam goes in.

External resource

Travis Perkins Padstones

Supreme Concrete padstone range including PAD12 (440 x 215 x 102mm) and the common domestic sizes. Branch pricing requires login.

travisperkins.co.uk

Common mistakes

No padstone at all. The steel sits straight on the blockwork, the block crushes, and the wall above cracks. This is the worst defect and it's depressingly common on loft conversions where old beams are rested on existing Victorian brickwork "because it was strong enough." If the brick isn't engineering brick and the engineer hasn't checked it, it isn't.

A padstone smaller than specified. The single most frequent defect. The builder fits what the merchant had rather than what the schedule says. A smaller pad means a smaller spread area, which means higher stress in the wall below, which means the calculation the engineer did no longer holds.

A cut or snapped block used as a padstone. A fragment of block has no bond into the surrounding masonry and concentrates the load onto a broken edge. NHBC requires the padstone to be one formed unit for exactly this reason.

Padstone on the wrong leaf, or bridging the cavity. In a cavity wall the load-bearing inner leaf needs the padstone. A pad on the outer skin, or one block spanning the cavity, gets the structure and the thermal detail both wrong.

Not level, not bedded. A padstone that rocks transfers the whole load through one corner. A padstone sitting on three mortar dabs has voids under it that close up under load and let the beam drop. Full bed, dead level, sound pier.

Where you'll need this

Padstones sit at the junction of structural design and the structure phase of any extension or renovation that removes load-bearing wall:

• Steels and lintels - padstones bear every steel beam end; the beam schedule names the size and material for each one
• Structural engineer - the engineer calculates and specifies the padstone size from the beam's end reaction and the wall's bearing strength
• Building control inspection: structure - the inspector checks every padstone, its size, material, bearing and bedding before the steelwork is concealed

You'll meet padstones on any project where a steel beam lands on masonry: rear extensions, side returns, loft conversions, knock-throughs and internal load-bearing wall removals all rely on them.