Formwork for a 10 cm Slab - Shores, Beams and Panels

System formwork vs traditional - one calculator for both

Same 400 sq ft slab, two different approaches. System formwork uses prefabricated aluminum panels and H20 beams - fast to set up, expensive to rent. Traditional formwork uses lumber boards on wooden joists - cheaper per square foot but takes twice the labor hours. This calculator handles both: pick your type, enter slab dimensions and thickness, and get a complete material list - shores, beams, panels (or boards) and concrete volume - with optional cost estimates.

Before and after - what proper shore spacing changes

BEFORE - guessing shore count

Shores placed by eye, often 5 ft apart
6 in slab weighs 75 lb/sq ft - overloads wide spacing
Mid-span deflection, cracking, slab failure risk
Extra material ordered "just in case" - 30-40% waste

AFTER - calculated shore layout

Shore grid at 3.3-4.9 ft (based on actual load)
Each shore carries exactly 4,500 lbf capacity
Zero over-shoring, zero under-shoring
Material surplus 5-15% - just enough for cuts

How to use the calculator - step by step

Choose formwork type - system (prefab panels + beams) or traditional (lumber boards + joists). System formwork is faster; traditional is cheaper on small projects.
Enter slab length and width - clear span between interior wall faces, in meters. Convert from feet: divide by 3.281.
Enter story height - from finished floor to the underside of the slab being poured. This determines shore/prop length.
Enter slab thickness - in centimeters. Thicker slabs are heavier and require closer shore spacing. Typical residential: 12-20 cm (5-8 in).
Select material surplus - 5% for simple rectangles, 10% typical, 15% for slabs with stairwells, penetrations or irregular shapes.
Read the results - shore count with grid layout, beam quantities, panel/board count, concrete volume in m3 and cu yd, and optional monthly rental cost.

Shore spacing reference by slab thickness

Thicker slabs carry more dead load, which means shores must be closer together. The calculator uses 4,500 lbf (20 kN) per shore capacity.

Slab thickness	Load (kN/m2)	Shore spacing	Shores per 100 sq ft
5 in (12 cm)	5.0	4.9 ft (1.5 m)	~5
6 in (15 cm)	5.75	4.6 ft (1.4 m)	~6
8 in (20 cm)	7.0	4.1 ft (1.25 m)	~7
10 in (25 cm)	8.25	3.6 ft (1.1 m)	~9
12 in (30 cm)	9.5	3.0 ft (0.9 m)	~12

Practical examples

Example 1: Standard house slab, system formwork - 8 x 5 m, height 2.8 m, 15 cm thick, 10% surplus
Result: 24 shores (grid 6x4, every 1.4 m), 4+17 beams, 59 panels, 6.0 m3 concrete

Example 2: Garage slab, traditional formwork - 6 x 4 m, height 2.5 m, 12 cm thick, 5% surplus
Result: 20 shores (grid 5x4, every 1.5 m), 5+16 joists, 168 lin ft boards, 2.9 m3 concrete

Example 3: Heavy commercial slab - 12 x 8 m, height 3.5 m, 25 cm thick, 15% surplus
Result: 130 shores (grid 11x12, every 1.1 m), 9+25 beams, 148 panels, 24.0 m3 concrete + high-story warning

Example 4: Small balcony slab - 3 x 1.5 m, height 2.7 m, 12 cm thick, 5% surplus
Result: 6 shores (grid 3x2), 2+7 beams, 7 panels, 0.5 m3 concrete

Example 5: Cost estimate - 40 m2 slab, system formwork at $15/m2 + $8/shore (28 shores)
Result: formwork $660 + shores $224 = $884/month rental

FAQ - Frequently asked questions

How long should formwork stay in place after pouring?

ACI 347 recommends leaving formwork in place for 7 days minimum for slabs spanning under 10 ft, and 14 days for longer spans. In cold weather (below 50F / 10C), extend to 21 days. Some shores (reshores) must remain for 28 days or until concrete reaches 75% of design strength. Never strip formwork based on time alone - verify concrete strength first.

When should I use system formwork instead of traditional?

System formwork pays off on projects over 500 sq ft or when you need to pour multiple slabs. Setup is 3-4 times faster than traditional lumber, the surface finish is smoother, and rental includes engineering support. Traditional formwork is cheaper for one-off small slabs (under 250 sq ft) where you already own the lumber.

What is the maximum shore height without bracing?

Most adjustable steel shores (acrow props) can extend to 11.5 ft (3.5 m) without horizontal bracing. Above that height, you must install cross-braces every 6.5 ft (2.0 m) of vertical height. For heights above 16 ft (5 m), use scaffolding towers or engineered shoring systems instead of individual props.

How does slab thickness affect the number of shores?

Concrete weighs about 150 lb/cu ft. A 6 in slab puts 75 lb/sq ft on the formwork. A 10 in slab puts 125 lb/sq ft - a 67% increase. Since each shore has a fixed capacity (typically 4,500 lbf), thicker slabs require closer shore spacing and more shores. Going from 6 in to 10 in thickness roughly doubles the shore count.

Do I need a structural engineer for formwork design?

For residential slabs spanning under 16 ft with standard thickness (5-8 in), this calculator provides adequate estimates. For spans over 16 ft, slabs thicker than 10 in, post-tensioned slabs, or any commercial project, have a structural engineer review the shoring layout. System formwork suppliers (Doka, PERI) typically include engineering as part of the rental package.

Can I reuse traditional formwork boards?

Yes, but expect 2-3 uses maximum from standard lumber boards before they warp or absorb too much moisture. Plywood sheathing (3/4 in) lasts 4-6 pours with proper oiling. System formwork panels are designed for 50-100+ cycles, which is why renting makes sense despite the higher per-use cost.

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Formwork for a 10 cm concrete slab - how many shores?

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