Beam and Block Slab Calculator
Estimate a beam and block (joist and hollow block) slab: number and length of precast joists, infill blocks, topping concrete in m³ and welded mesh for any room size, block size and joist spacing.
How it works
block rows = ⌈width ÷ spacing⌉ · joists = rows − 1 (edge rows bear on the walls) · joist length = span + 2 × bearing · blocks = rows × ⌈span ÷ block length⌉ × (1 + waste) · concrete = area × (topping + rib fill) × (1 + waste), rib fill ≈ 0.14 m × (block height − 0.12 m) ÷ spacing + 0.003 m³/m² · mesh = area × 1.15
A beam and block floor — vigueta y bovedilla in Mexico and Spain — uses precast prestressed joists set at a fixed spacing, with lightweight blocks of concrete, clay or polystyrene spanning between them as permanent formwork. A thin cast-in-place topping, reinforced with welded mesh, ties everything into a slab. The number of joists follows from the width and spacing, their length from the span plus bearing on each wall, the blocks from rows times blocks per row, and the concrete from the topping thickness plus the small volume that fills the ribs above each joist. Joist size and spacing must come from the manufacturer's span tables for your load.
Worked example
A 4 × 5 m room with joists at 70 cm, 25 cm long 15 cm blocks, a 5 cm topping and 10 cm bearing: ⌈5 ÷ 0.70⌉ = 8 rows of blocks with 7 joists between them, each 4.20 m long (29.4 m total); 8 × 16 = 128 blocks, 135 with 5% waste; concrete = 20 m² × (0.05 + 0.009) × 1.05 = 1.24 m³; mesh 23.0 m².
Sources: The Concrete Centre — precast floors · EN 15037-1 Precast concrete products — Beam-and-block floor systems
Frequently asked questions
How many joists do I need for a beam and block slab?
Divide the width across the joists by the spacing (usually 70 cm) and round up to get the rows of blocks; there is one joist fewer than rows when the edge blocks bear on the walls or tie beams. A 5 m wide room at 70 cm needs 7 joists.
How many blocks per square metre?
One divided by (spacing × block length): with 70 cm spacing and 25 cm blocks it is 5.71 blocks/m²; with 60 × 20 cm it is 8.3.
How much concrete does a beam and block slab use?
Mostly the topping: 5 cm is 0.05 m³/m², plus roughly 0.01–0.03 m³/m² to fill the ribs depending on block height — about 0.06–0.08 m³/m² in total, far less than a 10–12 cm solid slab.
How long can the joists span?
Typically 3–6 m for houses, depending on the joist size, prestress, spacing and the load; always confirm with the manufacturer's table and add temporary props at mid-span while the topping cures.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.