What is a padstone and when do I need one?

A padstone is a concrete or stone block placed under a beam (typically a steel or timber beam) where it bears on masonry. It spreads the concentrated point load over a larger area so it does not exceed the crushing strength of the blocks or bricks. You need a padstone wherever a beam sits on blockwork or brickwork — it is a Building Regulations requirement.

What is the standard padstone size in the UK?

The most common domestic padstone is 440 × 215 × 140mm dense concrete. This is essentially a dense concrete block and fits neatly into standard blockwork courses. For heavier loads, 440 × 215 × 215mm or larger custom padstones may be needed. Always check the structural engineer's specification.

What strength of concrete padstone do I need?

Padstones should be made from dense concrete with a minimum compressive strength of 30-40 N/mm². This is significantly stronger than standard building blocks (which are typically 3.6-7 N/mm²). Pre-cast padstones from builders merchants meet this requirement. Do not use lightweight or aerated blocks as padstones.

How do I know the beam load for my padstone calculation?

The beam load (reaction force at each support) will be specified by your structural engineer on their calculations and drawings. It is typically expressed in kN (kilonewtons). For a rough guide, a standard domestic floor beam spanning 4m might have reactions of 15-30 kN at each end, but always use the engineer's actual figures.

Can I use engineering bricks instead of a concrete padstone?

Engineering bricks (Class B, minimum 48.5 N/mm²) can be used to form a padstone area by building up courses under the bearing point. However, pre-cast concrete padstones are more reliable and easier to install. Your structural engineer will specify what is acceptable for your situation.

Padstone Sizing Calculator — Minimum Bearing Area for Beams UK

How We Calculate This

This calculator determines the minimum area a padstone needs by dividing the beam load by the crushing strength of the bearing material, with a safety factor applied.

The formula

Minimum area (mm²) = Beam load (kN) × 1000 ÷ (Crushing strength ÷ Safety factor)

For example, a 20 kN beam load on medium-density blocks (3.6 N/mm²) with a safety factor of 1.5 requires: 20,000 ÷ (3.6 ÷ 1.5) = 8,333 mm² minimum padstone area.

Crushing strengths used

Dense concrete block: 7 N/mm² (7.3 N/mm² is common)
Medium-density block: 3.6 N/mm² (most common inner-leaf block)
Lightweight / Aircrete block: 2.8 N/mm² (Celcon, Thermalite type)
Engineering brick (Class B): ~5 N/mm² net bearing area

Building Regulations

Padstones are required under Building Regulations Approved Document A (Structure) wherever concentrated loads are applied to masonry. The structural engineer's calculations will specify the required bearing area and padstone specification. Building Control will inspect padstone installation before the beam is placed.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.