When a beam — whether steel, concrete or timber — transfers its load into a masonry wall, the stress concentration at the bearing point can crush or crack the masonry beneath. A padstone solves this by spreading that point load over a larger surface area, keeping stress within safe limits for the blockwork or brickwork below.
Padstones are a small but frequently overlooked structural element. Omitting them, undersizing them, or using the wrong material is a common cause of cracking around lintels and beams — sometimes appearing months or years after a beam was installed.
Why Padstones Are Needed
The compressive strength of standard concrete blockwork is typically 3.5–7.0 N/mm² (depending on block type). A loaded steel beam might deliver 50–150 kN at its end bearing. Without a padstone, that load passes through only the end section of the steel onto the masonry — a contact area that can be as small as 100 mm × 100 mm. The resulting stress easily exceeds the blockwork’s capacity.
A padstone made from dense concrete (characteristic compressive strength 25–30 N/mm²) or Class B engineering brick (compressive strength ≥ 50 N/mm²) distributes that same load over a larger footprint, reducing the stress to within safe limits.
The padstone also provides:
- A flat, level, mortar-bedded seating surface for the beam
- Protection against local spalling of the inner leaf when the beam deflects under load
- A defined bearing zone that can be inspected and recorded
Concrete vs Engineering Brick Padstones
Two materials dominate UK padstone practice:
| Type | Compressive Strength | Cost (each) | When Used |
|---|---|---|---|
| Precast dense concrete block | 25–40 N/mm² | £8–£35 | Standard choice for most steel and timber beams |
| Engineering brick (Class B) | ≥ 50 N/mm² | £1–£3 per brick | Light loads, smaller bearings, historic masonry repairs |
| Engineering brick (Class A) | ≥ 75 N/mm² | £2–£5 per brick | Heavy loads or aggressive exposure |
| In-situ concrete pad | 25–35 N/mm² | Materials only (labour varies) | Large or irregular bearings; formed in shuttering |
| Steel bearing plate | N/A (distributes load) | £15–£60 | Under very heavy steel columns or beams |
Engineering bricks are often used in pairs or multiples, bedded in a 1:3 cement:sand mortar, to achieve the required bearing area. Their high density and low water absorption also resist moisture wicking in damp-prone locations.
Precast concrete padstones are the most common choice for domestic beam insertions. They are supplied in standard sizes (see below) and can be ordered from builders’ merchants or structural suppliers. They are bedded in full-bed cement mortar before the beam is set on top.
Sizing Padstones
The padstone must be large enough to reduce the bearing stress to within the allowable capacity of the masonry beneath. Sizing is a structural engineering calculation, but the following rules of thumb give a starting point for standard domestic scenarios:
Minimum bearing length (the dimension along the beam span) is typically 100–150 mm for lintels and beams in domestic masonry. For heavier steel beams (UB 203 and above), 150–200 mm bearing is common.
Padstone footprint must be sized so that: load ÷ padstone area ≤ allowable bearing stress of the masonry below. Structural engineers use BS EN 1996-1-1 (Eurocode 6) and the UK National Annex for masonry design.
Typical precast padstone sizes stocked by UK suppliers:
| Padstone size (L × W × H, mm) | Typical application |
|---|---|
| 215 × 140 × 75 | Light timber joists and lintels |
| 215 × 215 × 75 | Standard domestic steel lintels and beam ends |
| 440 × 215 × 75 | Heavier beams over door/window openings |
| 440 × 215 × 140 | Larger UB sections in loft conversions |
| 440 × 215 × 215 | Steel beams with high point loads |
| 600 × 215 × 215 | Heavy structural beams in extensions |
For a standard loft conversion ridge beam or an RSJ replacing an internal wall in a 1930s terrace, a 440 × 215 × 140 mm or 440 × 215 × 215 mm precast padstone is typically specified. Your structural engineer should confirm this.
Spread Bearings and Alternative Solutions
Where an inner leaf is thin, or the masonry below is in poor condition, a padstone alone may not be sufficient. Options include:
Spreader plates: galvanised or grade S275 steel plates welded to or placed beneath the beam end. These spread load over a wider area than a padstone and are sometimes preferred in heavily loaded commercial scenarios.
Column bases: for point loads from posts or columns, a reinforced concrete column base or proprietary steel base plate is used rather than a padstone.
Propped bearings: where two beams meet at a wall pocket, a proprietary joist hanger or beam shoe (e.g. Simpson Strong-Tie or Mitek) is used instead of a bearing pad. These transfer load to the masonry via face-fixings rather than compression.
Wall Plates
A wall plate is a horizontal timber member (typically 100 × 75 mm or 75 × 50 mm regularised treated softwood) bedded in mortar on top of an inner leaf or ring beam. It distributes concentrated rafter, truss or joist loads along the wall line, and provides a fixing surface for the structural members above.
Wall plates are standard practice in traditional roof construction:
- At eaves level: on top of the inner leaf of the cavity wall, supporting rafters or truss tails
- At floor level: where joists are to be fixed to a wall (in some masonry details)
Wall plates must be:
- Treated with a preservative compliant with BS 8417:2011 (timber preservation) — UC3 or UC4 treatment class depending on exposure
- Bedded in a full bed of 1:6 cement:sand or 1:5 lime-based mortar
- Kept clear of the cavity and away from the outer leaf
- Continuous along the wall where possible, with scarf joints staggered
Wall Plate Straps
One of the most important and most frequently missed details in UK domestic construction is the holding-down strap connecting wall plates to the masonry below. These are required by Building Regulations Part A to resist wind uplift on roofs.
Straps are galvanised mild steel (typically 30 × 5 mm section, 1,000 mm or 1,200 mm long). They fold over the wall plate and are built into the inner leaf of the cavity wall at approximately 2.0 m centres (or at every other rafter position, whichever is closer).
| Strap requirement | Spacing |
|---|---|
| Holding-down straps (wall plate to masonry) | Max 2.0 m centres |
| Rafter ties (rafter to ceiling joist, where not trussed) | At every rafter |
| Gable wall straps (to anchor roof to gable) | Max 2.0 m centres vertically |
Omitting holding-down straps is a serious structural deficiency. In high wind exposure zones (exposed coastal and upland areas), strap spacing may need to be reduced — refer to BS EN 1995-1-1 and the Timber Research and Development Association (TRADA) guides.
Used with Steel and Timber Beams
The padstone sits below the beam end, the beam sits on the padstone, and — in most domestic steel installations — a packing plate or thin grout bed levels the beam perfectly. For timber beams, the padstone is usually topped directly by the beam end without packing.
The sequence for a standard RSJ installation:
- Build the masonry to the correct height, incorporating the padstone in the bed joint (or bedding the padstone on top of the masonry as a separate unit).
- Prop the existing structure.
- Cut the wall opening and set the padstone(s) level, with full mortar bedding.
- Lower the steel onto the padstones (two operatives minimum; mechanical assistance for heavy sections).
- Allow mortar to achieve sufficient strength before removing props (typically 48 hours minimum in warm conditions).
The padstone is always specified by the structural engineer who designs the beam. If a padstone size, specification or bearing length is not stated in the structural engineer’s drawings or calculations, ask before proceeding.
