Padstones Explained: What Builders Need to Know

A padstone is a block of dense material placed beneath the end of a steel beam where it bears onto a wall or pier. Its job is to spread the load from the beam across a wider area of masonry so the bearing stress does not exceed what the wall can handle.

Without a padstone, the concentrated point load from a steel beam can crush or crack the masonry below it. A standard brick or block has nowhere near the compressive strength needed to take that kind of load at a single contact point. The padstone effectively acts as a bridge between the steel and the wall, distributing the force more evenly.

The three most common options are:

Engineering brick: Dense blue engineering bricks are widely used for lighter beam loads. They are readily available, straightforward to bed in, and your bricklayer will be comfortable working with them. They are not suitable for heavier loads where the bearing stress is higher.

Precast concrete padstones: These are the go-to choice for most domestic beam installations. They are manufactured to a consistent compressive strength (typically C30 or above), come in standard sizes, and are available from most builders’ merchants. If you are ordering a beam, it is worth ordering the padstones at the same time.

Steel plate or fabricated padstones: For very heavy loads or where headroom is tight, a steel plate or a fabricated steel padstone may be specified. Your structural engineer will call this out on the drawings if it is needed.

Roofing slates (stacked): Some older properties have slate padstones from original construction. They are not appropriate for modern beam installations.

Padstone sizing is a structural calculation, not a rule of thumb. Your structural engineer will specify the size on the drawings based on:

The load the beam is carrying
The bearing length required (how far the beam sits onto the wall)
The compressive strength of the masonry below
The compressive strength of the padstone material

That said, it helps to understand what is being calculated. The engineer is checking that the stress on the wall beneath the padstone (load divided by bearing area) does not exceed the allowable bearing stress for that masonry type. The larger the padstone, the lower the stress on the wall below it.

A common domestic padstone for a standard RSJ over a 3-4 metre span might be 215mm x 215mm x 100mm in dense concrete. For heavier loads or longer spans, you will need something larger. Always work from the engineer’s specification rather than sizing by eye or by what the merchant has in stock.

The minimum bearing length for a steel beam onto a padstone is typically 100mm, but again this is something your engineer will confirm. Short bearing lengths concentrate load onto a smaller area and increase the stress on the padstone and the wall below.

The padstone sits directly under each end of the beam, bedded into the wall in mortar. Key points:

It should be level. A tilted padstone means the beam end will bear unevenly, which creates stress concentrations.
It should be fully bedded, with no voids beneath it. Hollow spots mean the load is only being carried on part of the padstone surface.
The padstone needs to be in the right position before the beam goes in. Once the beam is in place, getting back in to reposition a padstone is difficult.
The beam typically sits directly onto the padstone. Some engineers specify a damp proof course between the two on external walls.

On a cavity wall, the beam typically bears onto the inner leaf only unless the engineer specifies otherwise. The padstone sits in the inner leaf, and you may need to build up the wall around it to provide adequate support. Some installations use a padstone that spans across both leaves, particularly where the load is significant.

If you are working in a cavity wall, check what the engineer has specified. It is not always obvious from the drawings and it is worth confirming before the beam goes in.

Using a padstone that is too small. If the merchant does not have the specified size in stock, do not substitute a smaller one and assume it will be fine. Speak to the engineer.

Forgetting to order them. Padstones are easy to overlook when ordering materials and they are not always available for same-day delivery. Order them when you order the beam.

Poor bedding. A padstone that is not properly bedded in mortar can rock or shift over time. Take the time to get it level and fully supported.

Not checking the engineer’s drawings. The size, material, and position of padstones should be on the structural drawings. If the drawings do not specify them, go back to the engineer before proceeding.

Padstones are a small part of a beam installation but an important one. The right padstone, correctly sized, properly bedded, and in the right position, protects the wall and gives building control what they need to sign off the job cleanly. The wrong padstone, or one that has been skipped because it looked like it would be fine, is the kind of thing that causes problems further down the line.

If you are ordering a beam from us, we are happy to talk through the installation requirements. We work with builders and structural engineers regularly and understand what you need to get the job done right.

Call us on 07301 033 581 or email contact@buildersbeamsrus.co.uk.