Under-slab membranes: water, vapour or soil gas, and what each one changes on the day
A loading-dock slab drawing shows a sheet below the concrete, while the delivery label describes a vapour retarder. The mechanical layout also includes a soil-gas system. The crew calls all three items waterproofing, but their approved purposes and connections have not been checked.
Identify what the specified under-slab layer must control before selecting its details or describing it as waterproofing. Bulk water, vapour movement and soil-gas control are different duties; confirm which functions the approved assembly provides. Health Canada’s guidance also states that a soil-gas barrier alone is not an effective stand-alone radon reduction strategy. A membrane label does not prove that the complete project design or required radon follow-up has been provided.
Read the layer’s purpose before the pour
- Trace the approved assembly. Bring the slab sections, specification, approved submittal and relevant soil-gas details together. Ask the responsible designer to identify the layer’s required functions where the information is unclear or inconsistent.
- Match the delivered material. Confirm the product and accessories against the accepted assembly. Do not substitute a sheet described for one duty because its colour or thickness resembles the specified material for another.
- Locate the interfaces. Mark seams, edges, penetrations, pits and slab-to-wall connections on the plan. Use the applicable approved details for each function rather than assuming an apparently overlapped sheet completes the required control layer.
- Coordinate other system components. Identify the relevant mechanical work, sumps or soil-gas provisions shown by the design and assign their handovers. Keep their required assessment, commissioning or later testing visible; the membrane inspection does not replace those activities.
- Protect the installed layer. Agree the approved access and protection arrangements with the reinforcing and concrete leads. Record damage at its location and obtain the actual repair requirements before the next work hides it.
- Arrange the required pre-pour checks. Retain current details, material records and located correction evidence for the specified reviewers. Confirm the authorized release and boundaries, then pass outstanding later obligations into the project’s handover records.
Common mistakes
- Calling every under-slab sheet waterproofing without checking its purpose.
- Treating similar appearance as product equivalence.
- Using membrane completion as proof that an entire soil-gas system is effective.
Checklist
Before the slab conceals the layer
- Required functions confirmed from the design.
- Delivered material and accessories matched.
- Approved interfaces located on the plan.
- Other system components and later checks tracked.
- Protection, repairs and pre-pour release recorded.
Check your understanding
The soil-gas membrane is installed. Does that alone prove the finished building’s radon level is controlled?
