# Detailing

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.

- Category page: https://jianzhujisuan.com/en/c/detailing
- Data checked: 2026-10-25

## Tools in this group

- [Roof Drainage Calculator](https://jianzhujisuan.com/en/roof-drainage): Catchment area to flow to downpipe count.
- [Roof Slope Layer Calculator](https://jianzhujisuan.com/en/roof-slope-layer): Average thickness and volume from span and slope.
- [Insulation Thickness Calculator](https://jianzhujisuan.com/en/insulation-thickness): Insulation thickness from a target U-value.
- [Movement Joint Width Calculator](https://jianzhujisuan.com/en/expansion-joint-width): Joint width from thermal movement plus allowances.

## Questions

### 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.