Detailing calculators

4 detailing calculators: roof drainage downpipes, roof screed falls, insulation thickness from a target U-value, and movement joint width. With code sources and jurisdiction notes.

4 tools, most used first

◺Roof Slope Layer CalculatorAverage thickness and volume from span and slope.☂Roof Drainage CalculatorCatchment area to flow to downpipe count.▨Insulation Thickness CalculatorInsulation thickness from a target U-value.⇹Movement Joint Width CalculatorJoint width from thermal movement plus allowances.

What this group solves

This group sizes the building fabric: how a roof drains, how thick the fall screed and insulation go, how wide a movement joint needs to be. The formulas are ordinary physics; the inputs are jurisdiction-specific. Storm intensity comes from a local formula, U-value limits from a climate zone, joint spacing from the structural type and joint width from a standard detail. Move country and the formula holds while every number changes.

All four tools name their source. Downpipe capacity comes from GB 50015 Table 4.9.22, where plastic and cast iron are two separate tables and a nominal DN100 differs by 36% between them. Insulation is back-calculated with GB 50176 surface resistances, so Ri is 0.11 where EN ISO 6946 would use 0.13. Movement joints are controlled by the spacing limits in GB/T 50010 Table 8.1.1, with the width detailed at 20-30 mm rather than computed straight from thermal movement.

The way to use this group is to fix the jurisdiction-specific inputs first and apply the formula second. For the same 1000 m² roof at 300 L/(s·ha), plastic De110 downpipes need two and cast iron DN100 needs three — the formula is untouched, only the material changed. Confirm the pipe material, the surface resistances, the masonry type and the climate zone before reading the answer, or the result will look precise while being based on the wrong table.

Which one for your case

I know the roof area and local storm intensity, and need the downpipe count	Roof Drainage Calculator
I know the roof area and fall, and need the screed volume	Roof Slope Layer Calculator
I have a target U-value and need the insulation thickness	Insulation Thickness Calculator
The structure is too long and I need joint width and spacing	Movement Joint Width Calculator

Where this group goes wrong

Sizing downpipes from a blanket "DN100 carries about 15 L/s" without checking the material — plastic De110×3.2 carries 15.98 L/s but cast iron DN100 only 11.77, a 36% gap. Sizing to the larger figure leaves the roof a third short of downpipes and it ponds in a storm.
Mixing surface resistances between codes — GB 50176 uses Ri 0.11 while EN ISO 6946 uses Rsi 0.13, which shifts the thickness by about 1 mm and leaves the energy report disagreeing with the detail.
Doubling thermal movement and calling it the joint width — a 60 m structure over 30°C works out at 56 mm where a standard expansion joint is detailed at 20-30 mm. An oversized joint forces a different waterstop and cover plate, and still leaks air and water.

FAQ

How many roof downpipes do I need?

Start with the design flow: Q = runoff coefficient × catchment area × storm intensity ÷ 10000, in L/s, then divide by the capacity of one downpipe and round up. The critical point is that capacity must be read from the table for the actual material — plastic De110×3.2 carries 15.98 L/s while cast iron DN100 carries only 11.77, a 36% difference, so mixing them undercounts the downpipes by a third and the roof ponds in a storm. The storm intensity must also come from the local formula, never a national average.

How thick should the insulation be?

Back-calculate from the target U-value: R = 1/U, subtract the internal and external surface resistances and the base layer, and what remains is the resistance the insulation must provide; multiply by the corrected conductivity for the thickness. Surface resistance is jurisdiction-specific — GB 50176 uses Ri 0.11 and Re 0.04 for walls, while EN ISO 6946 uses Rsi 0.13 and Rse 0.04. Apply a 1.15-1.30 correction to conductivity for moisture, ageing and site compression.

How wide should a movement joint be?

Codes control the maximum spacing between joints, not the width. GB/T 50010 Table 8.1.1 gives 55 m indoors and 45 m exposed for frames, 45/30 m for shear walls and 30/20 m for retaining and basement walls, with the width detailed at 20-30 mm. Thermal movement ΔL = α·ΔT·L is there to check whether that detailed width is sufficient, not to be used as the width itself — they are two different quantities.

What size should roof downpipes be?

Divide the design flow by the capacity of one downpipe and round up. Design flow = runoff coefficient × catchment area × storm intensity ÷ 10000, in L/s. Capacity must come from the table for the actual material (GB 50015 Table 4.9.22): plastic De110 carries 15.98 L/s and De160 44.43 L/s, while cast iron DN100 carries 11.77 L/s and DN150 34.69 L/s. The same nominal DN100 differs by 36% between the two.

The insulation comes out only a few tens of millimetres — is that right?

Usually yes. A board with conductivity 0.045 needs only 45 mm to give 1.0 m²·K/W. When the answer looks thin, suspect the inputs instead: the target U-value may be a "best practice" figure rather than the mandatory local limit, the 1.15-1.30 correction for moisture, ageing and compression may have been left out, or the base layer resistance may have been counted twice. Check all three before fixing the thickness.

Do I calculate movement joints, or just take a standard detail?

Take the standard detail. Codes control the maximum spacing — GB/T 50010 gives 55 m indoors for frames, 45 m for shear walls and 30 m for basement walls — and the width is detailed at 20-30 mm. Thermal movement ΔL = α·ΔT·L exists to check whether that detailed width is sufficient, not to be used as the width. Only very long structures or exposed decks with a large temperature range need a dedicated calculation.

Related data tables

Pipe size chart (DN to inch)DN8 to DN600: the inch equivalent, outside diameter, common wall, weight per metre and external area for insulation take-off.

Other groups

Concrete Rebar Masonry Finishes Earthwork Temporary Works Construction Costing M&E Material weights Area and Volume Unit conversion Code Lookup Foundations Structural Design Steel Design

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.