Padstone Sizing Calculator — Minimum Bearing Area

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.