Assembly inputs
Results
Layer contribution
| Layer | Thickness | Density | Load | Share |
|---|
Worked example — a domestic concrete floor
The default assembly, built up layer by layer: a 180 mm reinforced concrete slab with screed and finish, over a 25 m² room.
Layer by layer. Each layer is density × thickness × g:
Floor finish 15 mm × 2000 kg/m³ = 0.294 kPa
Screed 60 mm × 2100 kg/m³ = 1.236 kPa
Concrete slab 180 mm × 2400 kg/m³ = 4.236 kPa
Layers total = 5.766 kPa
Add the things you cannot draw. Services, ceiling, partitions allowance: 0.25 kPa. Total 6.016 kPa, or 588 kg/m². Over 25 m² that is 150.4 kN — about 15 tonnes sitting on the supporting beams and walls.
Read the shares. The slab alone is 70% of the load. Trimming 20 mm off it saves 0.47 kPa — more than the entire floor finish. That is the value of the contribution table: it tells you which layer is worth arguing about.
A useful sanity check: a normal-weight reinforced concrete slab contributes roughly 0.24 kPa per 10 mm of thickness. Multiply slab depth in millimetres by 0.024 and you have the self-weight in kPa within a percent. If a result is far from that, the density or thickness is probably wrong.
Typical material densities
Representative values consistent with EN 1991-1-1 Annex A. Always prefer a supplier's figure for a specified product.
| Material | Density (kg/m³) | Per 10 mm (kPa) |
|---|---|---|
| Reinforced concrete (normal weight) | 2400–2500 | 0.235–0.245 |
| Plain concrete | 2300–2400 | 0.226–0.235 |
| Lightweight aggregate concrete | 1400–2000 | 0.137–0.196 |
| Cement/sand screed | 2000–2100 | 0.196–0.206 |
| Solid clay brickwork | 1800–2100 | 0.177–0.206 |
| Aerated concrete block | 500–900 | 0.049–0.088 |
| Structural steel | 7850 | 0.770 |
| Softwood timber | 350–500 | 0.034–0.049 |
| Glulam / LVL | 450–600 | 0.044–0.059 |
| Plywood / OSB | 550–700 | 0.054–0.069 |
| Gypsum plasterboard | 700–900 | 0.069–0.088 |
| Ceramic floor tile | 2000–2300 | 0.196–0.226 |
| Mineral wool insulation | 20–150 | 0.002–0.015 |
| Rigid foam insulation (PIR/EPS) | 15–40 | 0.001–0.004 |
| Glass (float) | 2500 | 0.245 |
| Asphalt / bituminous roofing | 2100–2400 | 0.206–0.235 |
| Gravel ballast (loose) | 1500–1800 | 0.147–0.177 |
| Saturated soil / green roof substrate | 1800–2200 | 0.177–0.216 |
Insulation is worth noting for the opposite reason to concrete: 200 mm of mineral wool adds perhaps 0.05 kPa. It is thermally dominant and structurally irrelevant, and including it changes nothing.
Assumptions and where this stops being valid
It is unfactored characteristic load. The output is Gk. Design requires load factors and combinations — 1.35Gk + 1.5Qk at ultimate limit state under EN 1990, or 1.2D + 1.6L under ASCE 7. Never take this number into a member check unfactored.
It is load per square metre, not a load path. Converting an area load into beam and column reactions needs tributary widths, continuity and two-way action. A 6.0 kPa floor does not put 6.0 kPa on every support.
Partitions are a judgement call, not a layer. Movable partitions are commonly handled as an equivalent uniformly distributed load — EN 1991-1-1 permits 0.5 to 1.2 kPa depending on partition self-weight, added to the imposed load rather than the dead load. Fixed partitions are dead load and should be entered. Getting this wrong in either direction is one of the more common errors.
Services allowances vary widely. A domestic ceiling with light services might be 0.15–0.25 kPa. A commercial soffit with ductwork, sprinklers, cable tray and a suspended ceiling is commonly 0.5–1.0 kPa. Use the project's services load, not a habit.
Density ranges are real. Concrete density varies with aggregate and reinforcement ratio; timber varies with species and moisture content. Where the result is marginal, use the unfavourable end of the range rather than the mean.
Standards and what to do next
Europe: EN 1991-1-1 (Eurocode 1, Part 1-1) covers densities and self-weight, with nominal density tables in Annex A. EN 1990 gives the combination rules and partial factors.
United States: ASCE/SEI 7, Chapter 3 for dead loads, Chapter 2 for combinations. The International Building Code references it directly.
Next steps: take the characteristic load into a load combination, distribute it to the supporting members with the correct tributary areas, and check deflection as well as strength — on floors, deflection and vibration usually govern before bending stress. For long-span or lightweight floors, check footfall vibration separately; a floor can satisfy every strength and deflection limit and still be unacceptably lively.
Calculation basis
Mass per area = density × thickness
Layer load = mass per area × 9.80665 / 1000
Total dead load = Σ layer loads + additional permanent load
One kilopascal is one kilonewton per square metre. The psf result is converted from the same calculated pressure.
Using the calculator
- Use actual specified material densities where possible.
- Enter finishes, screeds, membranes and other fixed layers separately.
- The layer order does not affect the arithmetic; each contribution is calculated independently.
- Do not use the total as a complete structural design load combination.
Frequently asked questions
Dead load is the permanent weight of the structure and fixed materials, such as concrete, screed, roofing, finishes and permanently installed equipment.
No. Occupants, movable furniture, storage, snow and other variable actions are not dead loads and must be considered separately.
Layer load equals material density multiplied by thickness and gravity, so incorrect density assumptions directly change the result.