Padstone Calculator for RSJ Installations – Size, Strength, and Concrete Mix Guide

Padstones are crucial but often overlooked components of RSJ beam installations. These concrete blocks distribute the concentrated load from your steel beam across masonry or foundations, preventing crushing and structural failure. Getting padstone sizing wrong can lead to beam settlement, wall cracking, or even catastrophic collapse.

What is a Padstone?

A padstone (also called bearing pad or bearing plate) is a strong concrete or stone block placed beneath each end of an RSJ beam. Its primary functions are:

Load Distribution

RSJ beams concentrate enormous loads onto small bearing areas. A typical 4m beam supporting a floor might impose 15-25 kN (1,500-2,500 kg) on each end. Standard bricks and blocks cannot safely handle these concentrated stresses without padstones.

Example Without Padstone:

203x133x25 beam, 20 kN end reaction, bearing on standard concrete block:

• Bearing area: 133mm × 150mm = 19,950 mm² = 0.02 m²
• Bearing pressure: 20 kN / 0.02 m² = 1,000 kN/m² (1.0 N/mm²)
• Standard block strength: 3.6 N/mm² (7N class)

While technically the block can handle this (3.6 > 1.0), there’s minimal safety factor, and point loading can cause local crushing.

Example With Padstone:

Same beam on 400 × 300 × 150mm padstone:

• Padstone spreads load to: 400mm × 300mm = 120,000 mm² = 0.12 m²
• Bearing pressure: 20 kN / 0.12 m² = 167 kN/m² (0.17 N/mm²)
• Safety factor: 3.6 / 0.17 = 21 (excellent!)

Span Transitions

Padstones bridge between steel (with high strength but small contact area) and masonry (with lower strength but larger area), creating a safe transition.

Level Bearing Surface

Masonry walls are rarely perfectly level. Padstones provide a flat, level surface ensuring uniform load distribution across the entire beam bearing width.

Determining Required Padstone Size

Step 1: Calculate End Reaction Forces

For a simply supported beam with uniform load:

End reaction = (Total load on beam) / 2

Example:

• Beam span: 4.0m
• Uniform distributed load: 10 kN/m
• Total load: 10 × 4 = 40 kN
• Each end reaction: 40 / 2 = 20 kN

For beams with point loads or complex loading, consult your structural engineer’s calculations.

Step 2: Determine Masonry Bearing Capacity

Common UK masonry strengths:

Material	Typical Bearing Capacity
Engineering brick (Class B)	0.6-0.8 N/mm²
Standard concrete block (7N)	0.3-0.5 N/mm²
Dense concrete block (10N)	0.5-0.7 N/mm²
Autoclaved aerated block	0.2-0.3 N/mm²
Solid wall (old brick/stone)	0.4-0.6 N/mm² (verify condition)

Use conservative values unless you have verified testing results.

Step 3: Calculate Required Padstone Base Area

Required area = End reaction / Allowable bearing pressure

Example:

• End reaction: 20 kN = 20,000 N
• Masonry capacity: 0.4 N/mm²
• Required area: 20,000 / 0.4 = 50,000 mm²

This equals approximately 224mm × 224mm, so specify minimum 250 × 250mm

With safety factor incorporated: 300 × 300mm or 400 × 300mm preferred

Step 4: Verify Padstone Dimensions

Length (along beam):

• Minimum: Beam flange width + 50mm
• For 203x133 beam: 133 + 50 = 183mm minimum
• Practical: 300-450mm

Width (perpendicular to beam):

• Determined by required bearing area
• Usually 215-300mm (matching wall thickness)
• Practical: 215, 300, or 450mm

Thickness (height):

• Minimum: 100mm for light loads
• Standard: 150mm for typical domestic
• Heavy loads: 200-300mm
• Most common: 150mm or 215mm

Standard Padstone Sizes by Application

Small Openings (Single Door, Light Load)

Beam size: 152x127x37 or 178x102x19 End reaction: 8-12 kN Padstone: 300 × 215 × 150mm Concrete grade: C20/25 minimum

Medium Openings (Typical Knockthrough)

Beam size: 203x133x25 or 203x133x30 End reaction: 12-20 kN Padstone: 400 × 300 × 150mm or 450 × 300 × 150mm Concrete grade: C25/30

Large Openings (Wide Spans)

Beam size: 254x146x31 or larger End reaction: 20-35 kN ** Padstone:** 600 × 300 × 215mm or 600 × 450 × 215mm Concrete grade: C25/30 or C30/37

Very Heavy Loads

Beam size: 305mm UB and above End reaction: 35+ kN Padstone: 800 × 450 × 300mm or larger Concrete grade: C30/37 Note: May require reinforcement bars

Concrete Specifications for Padstones

Concrete Grades

C20/25 (20 N/mm² cube strength):

• Light residential use
• Suitable for small beams
• Common for DIY cast-in-place

C25/30 (25 N/mm² cube strength):

• Standard residential specification
• Recommended for most applications
• Good durability

C30/37 (30 N/mm² cube strength):

• Heavy-duty applications
• Commercial work
• Harsh environments (frost, chemicals)

Mix Ratios for DIY Concrete

If mixing on-site (small padstones only):

C20/25 equivalent:

• 1 part cement
• 2 parts sharp sand
• 3 parts 20mm aggregate
• 0.5-0.55 parts water
• Mix ratio: 1:2:3

C25/30 equivalent:

• 1 part cement
• 1.5 parts sharp sand
• 2.5 parts 20mm aggregate
• 0.45-0.50 parts water
• Mix ratio: 1:1.5:2.5

For important structural work, use ready-mix concrete – better quality control and certified strength.

Ready-Mix Concrete

Advantages:

• Certified strength
• Consistent quality
• Faster placement
• Building Control acceptance

Disadvantages:

• Minimum order (often 1m³ or more)
• Higher cost for small quantities
• Access required for mixer truck

Cost:

• £100-140 per m³ delivered (2026 UK prices)
• Typical padstone (0.02 m³): Only cost-effective if doing multiple

Installation Steps

Preparation

1. Mark Bearing Locations: Measure accurately from structural engineer’s drawings. Each beam end needs minimum 100-150mm bearing.

2. Create Padstone Recess:

• Remove bricks/blocks to create pocket
• Depth: Padstone thickness + 10mm for mortar bed
• Width/length: Padstone dimensions + 10mm clearance

3. Clean and Prepare:

• Remove all debris and dust
• Dampen surface (not saturated) for better bond
• Ensure base is level

Casting Padstones

Method 1: Pre-cast (Recommended)

Buy or make padstones in advance:

Formwork:

• Use 18mm plywood or shuttering ply
• Oil or plastic sheet for release
• Ensure right-angle corners

Pouring:

• Fill formwork in layers
• Vibrate or tamp to remove air pockets
• Strike off level with top
• Cure for minimum 7 days before use (14-28 days ideal)

Method 2: Cast In-Situ

Pour directly into prepared pocket:

• Build temporary formwork in pocket
• Ensure formwork is well-supported
• Pour and finish as above
• Wait 7-14 days before loading

Padstone Installation

1. Mortar Bed:

• Lay 10mm thick bed of 1:3 cement:sand mortar
• Use spirit level to achieve perfect horizontal
• Set padstone onto fresh mortar

2. Position and Level:

• Press padstone into mortar bed
• Check level in both directions
• Adjust before mortar sets
• Critical: Top surface must be perfectly level

3. Seating the Beam:

• Place beam onto padstone
• Verify minimum 100mm bearing
• Check beam level
• Shim with steel plate if needed (not wood!)

4. Make Good:

• Fill gaps around padstone with mortar or concrete
• Ensure good bond to existing masonry
• Point neatly

Common Mistakes to Avoid

1. Undersized Padstones

Using too-small padstones is dangerous:

Case Study: DIYer used 215 × 215 × 100mm instead of specified 450 × 300 × 150mm:

• Bearing pressure increased by 200%
• Concrete block beneath cracked within 6 months
• Beam settled 8mm, causing ceiling cracks
• Repair cost: £3,500 vs. £80 for correct padstone

Always follow structural engineer specifications exactly.

2. Poor Concrete Quality

Weak or poorly mixed concrete fails prematurely:

Issues:

• Too much water (reduces strength by 20-40%)
• Insufficient cement (weak mix)
• Not enough curing time (loaded too soon)
• Poor compaction (honeycombing, voids)

Solution: Use ready-mix or carefully control DIY mixing ratios.

3. Inadequate Curing

Concrete gains strength over time:

Age	Strength (% of 28-day)
3 days	40-50%
7 days	65-75%
14 days	85-90%
28 days	100%

Loading padstones before adequate curing causes crushing.

Minimum: Wait 7 days before installing beam (14 days preferred)

4. No Moisture Barrier

Direct contact between steel and damp concrete causes corrosion:

Solution:

• Place DPC (damp-proof course) between padstone and beam
• Use bitumen felt or plastic DPC
• Extends beam life significantly

5. Ignoring Existing Foundations

Padstones transfer load to foundations. Weak or shallow foundations may need strengthening.

Warning signs:

• Cracks in existing walls near bearing points
• Previous settlement issues
• Old or unknown foundation types
• Soft ground conditions

Consult structural engineer if foundations are uncertain.

Precast vs. Cast-in-Place Padstones

Precast Padstones

Advantages:

• Consistent factory quality
• Immediate strength (if properly cured before delivery)
• Clean installation
• Building Control approval easier

Disadvantages:

• Higher cost (£25-80 each depending on size)
• Limited standard sizes
• Delivery required
• May not fit non-standard locations

Cost Examples (UK 2026):

• 300 × 215 × 150mm: £28-40
• 450 × 300 × 150mm: £45-65
• 600 × 300 × 215mm: £65-90

Suppliers: Builder’s merchants, specialist concrete companies

Cast-in-Place

Advantages:

• Custom sizes for unusual situations
• Lower material cost (if doing other concrete work)
• No delivery logistics

Disadvantages:

• Time required for curing before beam install
• Risk of poor quality control
• Messy on-site
• Building Control may require testing samples

Best for: Remote locations, custom sizes, multiple padstones

Reinforcement Requirements

Most residential padstones don’t require reinforcement, but heavy loads may:

When to Reinforce

• End reactions >35 kN
• Padstones >600mm in any dimension
• Poor quality masonry beneath
• Structural engineer specifies reinforcement

Typical Reinforcement

Light reinforcement:

• A142 mesh (6mm bars at 200mm centers)
• Place in middle third of padstone depth

Heavy reinforcement:

• Engineered bar arrangement
• 10-12mm diameter bars
• Minimum 25mm concrete cover

Cost: Add £10-25 for materials per padstone

Building Regulations and Approval

UK Requirements

Padstone details typically appear on structural engineer drawings:

Building Control wants:

• Padstone dimensions clearly marked
• Concrete grade specified
• Bearing area calculations
• Installation method noted

Inspection points:

• Before pouring (if cast in-situ)
• After installation, before beam seating
• Final inspection with beam in place

Common Rejection Reasons

• Undersized padstones
• No concrete grade specified
• Inadequate curing time documented
• Pad stones not level
• Insufficient bearing length

Cost Analysis

Material Costs (UK 2026)

Pre-cast padstones:

• Small (300 × 215 × 150): £30-40 each
• Medium (450 × 300 × 150): £50-70 each
• Large (600 × 300 × 215): £70-95 each

DIY casting materials:

• Cement (25kg bag): £6-8
• Sharp sand (bulk bag): £45-60
• 20mm aggregate (bulk bag): £55-70
• Formwork (reusable): £20-40

Single padstone DIY: £8-15 materials (plus labor, equipment, time)

Typical project (two padstones for 4m opening):

• Precast option: £100-140
• DIY option: £30-50 materials + significant time

Labor Costs

If hiring builder:

• Remove bricks and create recesses: £80-150
• Install precast padstones: £60-120
• Cast in-situ padstones: £120-200

DIY: Reasonable project for competent DIYer with supervision

Conclusion

Proper padstone sizing and installation is essential for safe RSJ beam support. Calculate required bearing areas based on end reactions and masonry strength, specify appropriate concrete grades, and allow adequate curing time. While precast padstones cost more initially, they provide quality assurance and time savings that usually justify the expense.

Quick Reference:

• Small beams (152-178mm): 300 × 215 × 150mm, C20/25
• Medium beams (203mm): 450 × 300 × 150mm, C25/30
• Large beams (254mm+): 600 × 300 × 215mm, C25/30

Use our calculator above to determine exact requirements for your project, then verify with your structural engineer’s specifications.

Disclaimer: Padstone calculations should be verified by a structural engineer. This guide provides general information only and does not replace professional engineering advice or Building Regulations compliance.