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Padstone Sizing Calculator — Minimum Bearing Area
Calculate the minimum padstone size needed to safely distribute beam loads onto blockwork or brickwork without crushing.
Factored (ULS) beam reaction at this support, from your structural engineer's calculations
Leave at 1.0 for standard design — the BS EN 1996 partial factor (γM) is already in the bearing strengths. Increase only to add extra conservatism.
Safety notice
Structural elements must be designed or checked by a structural engineer before construction. Treat these figures as initial guidance for planning and budgeting, not a structural design.
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How We Calculate This
This calculator determines the minimum bearing area a padstone needs by dividing the beam reaction by the design bearing strength of the masonry it sits on, following the concentrated-load check in BS EN 1996-1-1 (Eurocode 6), clause 6.1.3.
The formula
Minimum bearing area (mm²) = Beam reaction (kN) × 1000 ÷ Design bearing strength (N/mm²)
The design bearing strength is fd = β × fk ÷ γM, where fk is the characteristic compressive strength of the masonry, γM is the partial material factor (UK National Annex: 3.0 for normal Category II units and Class 2 execution), and β is the concentrated-load enhancement factor. The values below take β = 1.0, with no enhancement: Eurocode 6 allows up to 1.5 only for Group 1 units loaded well away from wall ends, caps β at 1.25 for a bearing at the end of a wall (the normal case beside a new opening) where the formula itself can fall to about 1.0, and gives no enhancement for Group 2 units or shell-bedded work.
For example, a 20 kN beam reaction on a dense aggregate concrete block (design bearing strength 1.5 N/mm²) requires: 20,000 ÷ 1.5 = 13,333 mm² minimum bearing area, met by a 215 × 215mm padstone (46,225 mm²).
Design bearing strengths used
- Dense aggregate concrete block: ≈1.5 N/mm² (7.3 N/mm² unit, fk≈5.1, fk/γM = 1.7 rounded down)
- Medium-density block: ≈1.0 N/mm² (3.6 N/mm² unit, fk≈3.1)
- Lightweight / Aircrete block: ≈0.8 N/mm² (2.9 N/mm² unit, fk≈2.5)
- Engineering brick Class B: ≈3.5 N/mm² (clay unit ≥75 N/mm² per BS EN 771-1, with M12 mortar)
- Engineering brick Class A: ≈4.5 N/mm² (clay unit ≥125 N/mm² per BS EN 771-1, with M12 mortar)
fk figures are from H+H Celcon technical data (TSD03, Jun 2025) for aircrete blockwork. The values above are masonry design bearing strengths, not the raw unit crushing strengths in BS EN 771-1 — those describe the brick or block itself, not the masonry's bearing capacity, which is reduced by mortar and γM.
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. Use this tool as a sanity check — always follow the engineer's specification.
Correction
July 2026: Design bearing values were tightened to a stricter reading of BS EN 1996-1-1 and its UK National Annex: the concentrated-load enhancement is no longer assumed, so values are now conservative for bearings near wall ends.
Frequently Asked Questions
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.
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.
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.
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. If you also need to size the lintel above an opening, try our Lintel Sizing Calculator.
Engineering bricks (Class B, minimum 75 N/mm² per BS EN 771-1) 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.
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Last updated: July 2026
Verified against UK standards · estimates only, confirm with your supplier.