Saturday morning, 7 AM. The tile pallet arrived Friday, the concrete pump is booked for Monday, and the plumber is halfway through laying PEX loops in the screed. Three trades, three material orders, three chances to run short. The roofer needs battens - but how many? The concrete crew needs shores - but at what spacing? The plumber needs PEX - but how many feet per loop before pressure drop kills the flow?
None of these numbers are hard to calculate. They just require knowing the right formula, the right spacing rule, and the right safety margin. Which is exactly what the three calculators below handle.
Battens first - because the roofer is already on the scaffold
Clay tile on a gable roof. Two slopes, each 650 cm long (ridge to eave) and 1,200 cm wide. The calculator returns a batten gauge of 33 cm (13 inches) - the standard for most clay tile profiles. That gives 20 battens per slope, 40 total, and 480 linear meters (1,575 ft) of 1.5 x 2.5 in timber.
Switch the material to plain tile (double-lap) and the spacing drops to 15 cm. Same roof, same dimensions - but now you need 82 battens instead of 40. That is more than double the timber and double the nailing time. The material choice drives the batten count more than the roof size does.
And if the roofing is asphalt shingles? No battens at all. The calculator switches to full sheathing mode and tells you how many 4x8 ft OSB panels you need instead.
| Roofing material | Batten spacing | Battens (this roof) | Linear feet |
|---|---|---|---|
| Clay tile | 33 cm | 40 | 1,575 |
| Concrete tile | 32 cm | 42 | 1,654 |
| Metal tile | 35 cm | 38 | 1,496 |
| Plain tile (double lap) | 15 cm | 82 | 3,232 |
| Asphalt shingles | Full sheathing | - | 41 OSB sheets |
35 shores under a 40 m2 slab - and the math behind the grid
The concrete pump is scheduled for Monday. The slab: 8 x 5 meters, 15 cm thick, story height 2.8 m. System formwork with prefab panels. The calculator returns 35 shores on a 7 x 5 grid, spaced 1.5 m (4.9 ft) apart, plus 5 primary beams and 17 secondary beams.
Where does the 1.5 m spacing come from? Each standard adjustable shore handles 20 kN (4,500 lbf). A 15 cm slab loads the formwork with 5.75 kN/m2 (concrete weight + formwork + construction traffic). That means each shore supports 3.48 m2 at most - and the square root of 3.48 is 1.86 m. The calculator caps it at 1.5 m for safety. Increase the slab to 25 cm and the load jumps to 8.25 kN/m2, the spacing drops to 1.1 m, and the shore count nearly doubles.
How much does it cost? With a formwork rental rate around $15/m2/month and shores at $8/piece/month, this 40 m2 slab runs about $940/month in formwork rental. Formwork stays up for at least 7-14 days after the pour, so plan for a full month of rental minimum.
Quick reference - shore count by slab thickness for the same 40 m2 area:
| Slab thickness | Shore spacing | Shore count |
|---|---|---|
| 10 cm slab | 1.5 m | 35 |
| 12 cm slab | 1.5 m | 35 |
| 15 cm slab | 1.5 m | 35 |
| 20 cm slab | 1.45 m | 42 |
| 25 cm slab | 1.25 m | 56 |
| 30 cm slab | 1.05 m | 72 |
600 feet of PEX disappear into an 80 m2 floor
While the formwork crew works overhead, the plumber is downstairs laying radiant floor tubing. The heated area: 80 m2 (860 sq ft) with 8 in (20 cm) spacing between loops - standard for living rooms and bedrooms. The calculator returns 479 meters (1,572 ft) of 5/8 in PEX, split into 4 loops with 16 elbows, 8 couplings and a 4-port manifold.
Why 4 loops and not 1? Each 5/8 in PEX loop maxes out at 330 ft (100 m). Beyond that, friction loss in the tubing exceeds what the circulator pump can overcome, and the far end of the loop stays cold. The 80 m2 floor at 20 cm spacing needs roughly 400 m of tubing net - add 4 m per loop for manifold connections and the 15% surplus, and you land at 479 m split across 4 circuits.
Tighten the spacing to 4 in (10 cm) - common for bathrooms and under-window zones - and the same 80 m2 eats 950+ meters of PEX. More than double. The spacing choice is the single biggest driver of material cost in radiant floor installations.
Quick reference - PEX tubing by heated area at 6 in (15 cm) standard spacing:
| Heated area | PEX length | Loops |
|---|---|---|
| 30 m2 | ~245 m | 3 |
| 50 m2 | ~406 m | 4 |
| 80 m2 | ~637 m | 6 |
| 100 m2 | ~791 m | 7 |
| 150 m2 | ~1,176 m | 11 |
| 200 m2 | ~1,560 m | 14 |
The plumbing calculator handles more than radiant floors. Switch the system type to cold water supply and it calculates pipe runs to fixtures with risers, elbows, tees and couplings. Drain/waste mode adds cleanouts and venting. Gas mode flags the legal requirement for a licensed installer.
Three trades, one morning, zero return trips
The roofer has 40 battens and 500 meters of timber on site. The concrete crew has 35 shores, 22 beams and 59 panels staged in the right order. The plumber has 4 rolls of PEX, a 4-port manifold, 16 elbows and 8 couplings laid out by the screed. Nobody is calling the supplier. Nobody is waiting for a van. The truck comes once, the work starts on time, and the surplus is 10-15% - not 30%.
That is what a ten-minute calculation prevents: the two-hour round trip, the $50 small-order surcharge, and the idle crew standing around the kettle while you drive.
Tools discussed in this article
Roof Batten Spacing Calculator - batten gauge, piece count and linear footage for clay tile, concrete tile, metal, plain tile, shingles and rolled roofing. Switches to OSB sheathing for shingle roofs.
Slab Formwork Calculator - shore count, beam quantities, panel count and concrete volume for system or traditional formwork. Includes optional rental cost estimate.
Plumbing Pipe Calculator - pipe footage, fittings and diameter recommendation for cold water, hot water, drain, radiant floor, radiator heating and gas systems.
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