Decide where water can discharge
- 1Finished level
- 2Sloping base
- 3Drainage away from the building
Locate walls, thresholds, low points, channels, outlets, joints and boundaries. Choose a route that protects the building and complies with local drainage and neighbour rules.
Survey the existing levels
Measure several points rather than only two ends. Record humps and hollows that could hold water beneath boards, slabs or a bonded finish.
Keep one fixed datum. The distance used in the calculation is the horizontal run measured in the direction of fall.
Establish the design fall
Consult the project design and technical literature for the finish, supporting base and drainage products. Distinguish a locally applicable requirement, industry guidance and a manufacturer specification.
No single fall suits every patio or deck. An open-jointed walking surface may look level while the layer below still needs to drain.
Calculate the level difference
- 1Finished level
- 2Sloping base
- 3Drainage away from the building
Level difference equals horizontal distance multiplied by the percentage and divided by 100. Keep units consistent, then convert the result to the unit used for setting out.
Arithmetic example only: a project value of 1.5% over 4 m gives 0.06 m, or 60 mm, of fall. This does not prescribe 1.5% for your project.
Transfer levels to the work
Mark high and low points, run string lines and add intermediate checks. For concrete, transfer levels to the formwork; for pedestals, level the support grid without reversing drainage below.
Check before and after installation
Use a straightedge and level, then confirm that outlets, joints and inspection access remain open. A local water test is appropriate only when the substrate and products can be wetted at that stage.
Calculate the drained areaTesCalculs→