Porcelain on pedestals
Pedestal height across a falling deck
- where
- change in pedestal height along the run (mm)
- distance from the low point to the far end (m)
- fall of the deck or membrane (%)
- fall wanted on the paving (%)
- pedestal height at the far end (mm)
- pedestal height at the low point (mm)
Explain this equation
Use this on a roof terrace or balcony where the deck falls to an outlet but the paving is to be level or flatter, to find the tallest pedestal needed.
sd is the deck fall and sp the paving fall, both in percent. Their difference is how fast the gap between deck and paving opens up. Lr is the distance from the outlet end, h0 the pedestal height there.
A 1% fall is 10 mm per metre, which is where the 10 comes from. Multiply by the run and the difference in falls to get the extra height, then add it to the starting height.
A 5 m run at 1:80 (1.25%) with level paving adds 62.5 mm, so 25 mm pedestals at the outlet become 87.5 mm at the far end. At 1:40 the same run adds 125 mm.
Watch out: if the paving is to fall more steeply than the deck, Δh comes out negative and the low point becomes the far end. Two-way falls need each direction worked out.
Works out how tall the pedestals must be at the far end of a terrace when the deck below falls to an outlet and the paving is laid flatter or level.
Enter your values to see the result.
Good to know
A 1 % fall rises 10 mm per metre, so the height change is 10 × run × (deck fall − paving fall). LABC Warranty asks for finished falls of at least 1:80 (1.25%) on roof terraces and suggests designing at 1:40 (2.5%) to allow for deflection and tolerances, and warns that the height difference must stay within the pedestal maximum. Pedestals with slope-correcting heads let the paving sit level on a falling deck.
Common mistake
Ordering one pedestal height for the whole job and finding the far end needs couplers.
Limits
Straight single-direction fall. For two-way falls work out each direction and add. Check the minimum pedestal height still leaves a clear drainage path.
Sources
- LABC Warranty Technical Manual, Section 12 Roof terraces and balconies (finished falls ≥1:80, design falls 1:40; support height within manufacturer maximum; loads to BS EN 1991-1-1)
- Marshalls, Porcelain paving 20 mm installation guide (full 1:4 mortar bed 30–40 mm, slurry primer, 3–5 mm joints, falls 1:80 / 1:40)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.