Setting the thinnest-point thickness of roof screed-to-fall layers (and kitchen / bathroom floor falls): design first, code minimums where unstated - roof material falls preferably not under 20 mm, indoor concrete falls not under 30 mm, mortar falls 10-20 mm

Topic Positioning

Setting the "thinnest-point thickness" of roof material fall screeds and indoor (kitchen / bathroom) floor fall screeds: design first, code minimums where unstated, with the national and Beijing floors and the fall gradients collected.

Core Conclusions

Order: the drawings state it - follow the design; unstated - take the code minimum.

1. Roof basis (national)

• GB 50345-2012 Technical Code for Roof Engineering, clause 5.2.4: "fall screeding shall follow the roof drainage direction and the design gradient; the thinnest point of the fall screed is preferably not less than 20 mm." The commentary: per clause 4.3.1 material falls preferably run 2%, so the substrate is strung and levelled by instrument or gradient rule per the drainage mode; the 20 mm addresses the first metre from the fall start - lightweight fall materials shape poorly, so a 1:2.5 cement mortar may complete that reach, lightweight material resuming beyond; at 2% over 1 m the rise is 20 mm.
• GB 50207-2012 Code for Acceptance of Roof Engineering Quality, clause 4.2.2: "fall screeds preferably lightweight aggregate concrete; laid in layers and duly compacted, surface even." Commentary: to lighten the roof load and ease work, lightweight aggregate concrete may serve - cement-expanded perlite is unsuitable; the thinnest point shall meet the design (design thickness first).

2. Indoor kitchen / bathroom basis (Beijing local, stricter)

• DB11/T 1811-2020 Technical Regulation for Waterproofing of Kitchens and Bathrooms, clause 5.2.3: concrete fall screeds not under 30 mm at the thinnest; mortar fall screeds not under 20 mm. Where the levelling screed doubles as the fall screed, C20 fine-gravel concrete applies; where a filling course carries pipes, it preferably merges with the fall screed, lightweight aggregate concrete preferred.
• T/CBDA 51-2021 Technical Regulation for Housing Decoration Works, clause 11.3.7: concrete fall screeds preferably not under 30 mm at the thinnest; cement-mortar fall screeds preferably not under 10 mm.

3. Gradients and companion provisions

• DB11/T 643-2021 Technical Regulation for Roof Insulation, clause 5.1.2: ordinary roofs take structural falls not under 3%; material falls preferably lightweight, low-absorption, reasonably strong material at 2%; valley and gutter falls lengthwise not under 1%, gutter drop not over 200 mm, 5% within 500 mm of outlets, metal gutters not under 0.5%. Fall materials stay light and low-absorption (ceramsite concrete, water-repellent perlite mortar) to cut roof load.
• DB11/T 1832.9-2022 Construction Technology Code for Building Works, Part 9: roof works, clause 3.4.4: string the falls with pads per the drainage direction and design gradient, screed strips along drainage at 1.5 m centres; work the flashing at gutters and outlets; then lay the fall screed. Thinnest point preferably not under 20 mm.

4. Tie-in with pricing: where the fall screed measures by volume (or area x thickness), the thickness drives the quantities - fix the thinnest point, compute the mean thickness on the design gradient, then apply the items and the per-increment / decrement adjustment items.

Applicability Boundaries

• GB 50345-2012's 20 mm floor addresses MATERIAL-fall roofs; structurally-fallen roofs are outside it (structural falls not under 3%).
• Indoor wet-room falls (DB11/T 1811-2020; T/CBDA 51-2021) run above the roof floor - never substitute 20 mm; the mortar floors read "not under 20 mm" and "preferably not under 10 mm" respectively - execute the stricter of the locally applicable.
• Stated design thicknesses govern; code floors apply only unstated; stricter locals govern over nationals.
• Fall materials are code-bound: GB 50207-2012 bars cement-expanded perlite - flag a design naming it and propose lightweight aggregate concrete / water-repellent perlite mortar.

Further Reading

• National: GB 50345-2012 clauses 4.3.1 (2% material falls) and 5.2.4 (20 mm); GB 50207-2012 clause 4.2.2.
• Beijing: DB11/T 1811-2020 clause 5.2.3; T/CBDA 51-2021 clause 11.3.7; DB11/T 643-2021 clause 5.1.2; DB11/T 1832.9-2022 clause 3.4.4.
• Companion: roof and waterproofing measurement rules; mean fall-screed thickness and item banding.