Mesh, spacers, chairs and tying
Spacers or chairs for an area at given centres
- where
- spacers per m²
- spacer centres each way (mm)
- spacers to order
- area of the mat (m²)
- allowance for edges, laps and breakage (%)
Explain this equation
Estimates the number of spacers or chairs needed for a mat of reinforcement at the centres you choose.
sx and sy are the spacer centres in each direction, so n is the number of spacers in each square metre. A is the area of the mat and w an extra allowance for edges, laps and breakage. N is the number to order.
A square metre is 10⁶ mm², so dividing that by the area each spacer looks after, sx × sy, gives spacers per m². Multiplying by the area and the allowance, then rounding up, gives a whole number to order.
Sense check: spacers at 1 m centres give one per m², so 50 m² with 10 per cent extra is 55. At 500 mm centres each way you need four times as many.
Watch out: light mesh and areas that will be walked on need closer centres than heavy bar, and spacers should be staggered so they do not form a line of weakness.
Estimates how many spacers or chairs a mat of reinforcement needs from the spacing you set them at.
Enter your values to see the result.
Good to know
Spacing is an input because it depends on the bar size, the load from foot traffic and concrete, and the support type. LABC guidance for foundations is no more than 1 m centres for spacers; supplier guidance for loose bars is a chair every 500–800 mm along each bar, and about five chairs per 4.8 × 2.4 m mesh sheet. BS 7973-2 sets out how spacers and chairs should be fixed and staggered. Use heavy-duty spacers for bars over 20 mm and closer centres for light mesh that sags between supports.
Common mistake
Spacing spacers too far apart under light mesh, so it is trodden down to the formation and loses its cover.
Limits
Regular grid over a rectangular area. Edges, openings, laps and top-mat chairs on cantilevers may need extra supports. Follow the specification where it gives spacer centres.
Sources
- BS 7973-2:2001 Spacers and chairs for steel reinforcement and their specification. Fixing and application of spacers and chairs and tying of reinforcement
- LABC Warranty, How to get it right: using reinforcement in your foundations (spacers at no more than 1 m centres; mesh lap rule of thumb two wire spacings + 50 mm; minimum covers)
- Next Day Steel, Concrete spacer calculator (chairs every 500–800 mm along loose bars; 5 chairs per 4.8 × 2.4 m mesh sheet)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.