How to calculate deck or patio slope ?
A percentage gradient compares the vertical fall with the horizontal run. The drainage direction then identifies which end is lower.
Formula used
Reverse calculation: gradient (%) = fall ÷ horizontal run × 100. The equivalent angle is arctan(gradient / 100).
Worked example
Over a 5 m horizontal run, a 2% gradient creates 0.10 m of fall, equal to 10 cm or 100 mm.
How should the result be interpreted?
The result describes geometry; it is not an installation specification. Compare it with the requirements for the finish, waterproofing or drainage system and the rules applying to the build.
Check before you start
- Measure the run horizontally, even when the base is already sloping.
- Select the unit first, then enter only the number.
- Mark the high and low points physically before setting the levels.
Frequently asked questions
Is a 2% gradient the same as 2 degrees?
No. A 2% gradient is an angle of about 1.15°. Percentage is the ratio of vertical fall to horizontal run.
What gradient should I use for a deck or patio?
There is no single value for every system. Use the requirement for the finish, waterproofing, drainage and project configuration, and check the applicable technical documents.
Does changing the drainage direction change the result?
No. The percentage and total fall stay the same. Direction places the high and low points so you can verify where water will be collected or discharged.