Porcelain on pedestals
Load on each pedestal (imposed load plus paving weight)
- where
- paving self-weight (kN/m²)
- tile thickness (mm)
- tile density (g/cm³)
- floor area carried by one pedestal (m²)
- tile length and width (mm)
- extra pedestals under each tile
- pedestal load from the spread imposed load (kN)
- imposed floor load (kN/m²)
- pedestal load with the point load directly above (kN)
- concentrated imposed load (kN)
- utilisation of the pedestal rating (%)
- pedestal rated load (kN)
Explain this equation
Use this to check that the pedestals chosen for a terrace are strong enough for the paving plus people and furniture.
g is the weight of the slabs in kN/m². At is the patch of floor each pedestal holds up: one tile area with corner pedestals only. qk is the spread imposed load from the Eurocode (2.5 kN/m² for a domestic balcony) and Qk the concentrated load (2.0 kN) such as a foot of a loaded chair or ladder.
t × ρ gives kg/m²; × 9.81 / 1000 turns that into kN/m². P1 spreads the imposed load evenly; P2 puts the whole concentrated load over one pedestal, which is the worst position. The larger is compared with the rating.
A 600 × 600 × 20 slab gives about 0.46 kN/m² self-weight. Spread load per pedestal is about 1.1 kN, the point-load case about 2.2 kN, around a quarter of an 8 kN pedestal rating.
Watch out: these are unfactored loads. Edge and corner pedestals carry less but often have a lower rating when the head is only part loaded. Hot tubs and planters need an engineer.
Estimates the load each pedestal carries from the paving and people or furniture, and compares it with the pedestal’s rated load.
Enter your values to see the result.
Good to know
Each pedestal carries the share of floor area around it; with corner pedestals only, that is one tile’s area. The Eurocode imposed loads for category A are 1.5 kN/m² in dwellings and 2.5 kN/m² on balconies of single-family homes, with a 2.0 kN concentrated load. The point-load case puts that 2.0 kN straight over one pedestal, which is a cautious upper bound. Buzon’s DPH pedestals, for example, quote 8 kN yield on a full head and less when only part of the head is loaded (edge and corner pedestals).
Common mistake
Comparing the load with the breaking load in the brochure instead of the working or yield value for the head area actually loaded.
Limits
Characteristic (unfactored) loads; no partial safety factors, dynamic or planter/hot-tub loads. With centre pedestals the area is shared equally, which is an approximation. A structural engineer must check the deck, insulation crush strength and roof structure.
Sources
- Structville, Live loads on buildings: Table 2 category A imposed loads (A1 dwellings 1.5 kN/m², 2.0 kN; A5 balconies in single-family dwellings 2.5 kN/m², 2.0 kN) — values as in the UK National Annex to BS EN 1991-1-1
- Buzon DPH-6-PH5 pedestal technical data sheet (full head: yield 8 kN, breaking 15 kN; pedestrian use only)
- Engineering ToolBox, Densities of solids (porcelain 2.3–2.5, granite 2.6–2.8, marble 2.6–2.8, limestone 2.7–2.8, slate 2.6–3.3, sandstone 2.1–2.4 × 10³ kg/m³)
- 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)
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.