Build-up and material compatibility of composite waterproofing (sheet plus coating): the coating preferably below the sheet, prohibited material combinations and substrate dryness conditions (GB 50345-2012 clause 4.5.4 + GB 50207-2012 clause 6.4.1 + DB11/T 1945-2021 clause 5.2.9)

Topic Positioning

The build-up and material compatibility requirements when sheet waterproofing and coating waterproofing are combined - the definition of composite waterproofing, whether the coating or the sheet sits below, prohibited combinations, substrate dryness conditions and separation layers.

Core Conclusions

1. GB 50345-2012 Technical Code for Roof Engineering, clause 4.5.4 "composite waterproofing design shall comply", original wording:

• 1 the selected waterproof sheets and waterproof coatings shall be compatible;
• 2 the waterproof coating film is preferably set beneath the waterproof sheet;
• 3 evaporation-curing waterproof coatings must not be used as bonding material for waterproof sheets;
• 4 torch-on (heat-fusion) waterproof sheets must not be applied over water-based or synthetic-polymer coating films;
• 5 with water-based or cement-based waterproof coatings, sheets are cold-adhered only after the film has through-dried. Commentary: "composite waterproofing means a waterproof layer formed of mutually compatible sheets and coatings. In service the two materials must be compatible in properties and must not corrode each other, and during construction must not interfere with each other. Hence this clause: evaporation-curing coatings must not bond sheets, otherwise film quality suffers; torch-on sheets must not go over water-based or synthetic-polymer coatings, otherwise sheet laying destroys the film; water-based or cement-based coatings need the film dried before sheet laying, otherwise the film forms poorly and may fail to become a flexible waterproof membrane. Where two waterproof materials are incompatible or mutually corrosive, a separation layer is set, chosen per the upper material's demands on the substrate."

2. GB 50207-2012 Code for Acceptance of Roof Engineering Quality, clause 6.4.1: where sheets and coatings are combined, the coating waterproof layer is preferably set beneath the sheet waterproof layer. Commentary: "placing the coating below the sheet chiefly exploits the coating's high bond strength, its ability to bridge substrate cracks, and its seamless, monolithic nature; it equally exploits the sheet's strength, puncture resistance, even thickness and long service life."
3. DB11/T 1945-2021 Technical Standard for Roof Waterproofing, clause 5.2.9: the composite waterproofing build-up preferably places the coating below and the sheet above.
4. The three codes agree: coating below, sheet above; the sheet and coating must be compatible in properties and must not corrode each other.

Applicability Boundaries

• Applies to design, construction and acceptance of combined sheet-plus-coating (composite waterproofing) details in roof and waterproofing works; not to single-material single-layer protection (a single sheet or single coating pass).
• Nature of the wording differs: coating below the sheet is "preferably" (recommended); "evaporation-curing coatings must not bond sheets" and "torch-on sheets must not go over water-based or synthetic-polymer films" are prohibitions not to be violated.
• The two passes are sequenced: water-based or cement-based coatings require the film to through-dry before cold-adhering sheets.
• Where two waterproof materials are incompatible or mutually corrosive, a separation layer is set, its detail chosen per the upper material's demands on the substrate.
• How composite waterproofing counts towards the number of protection passes, and whether the sheet and coating layers are measured as separate items, follow the waterproofing schedule on the drawings and the local measurement and pricing rules.

Further Reading

• GB 50345-2012 Technical Code for Roof Engineering, clause 4.5.4 and commentary.
• GB 50207-2012 Code for Acceptance of Roof Engineering Quality, clause 6.4.1 and commentary (waterproofing and sealing works).
• DB11/T 1945-2021 Technical Standard for Roof Waterproofing, clause 5.2.9 (flat-roof waterproofing design).