Kumar IronUpdated
In this guide
What coverage means
Coverage is the amount of product needed to cover one square metre with one coat. Most data sheets write it as kg per sq m per coat. Some write it as sq m per kg or per litre. These are the same idea turned upside down: if a sheet says 2 sq m per kg, that is 0.5 kg per sq m per coat.
To convert to square feet, remember that 1 sq m is about 10.76 sq ft. So a rate of 1.2 kg per sq m is about 0.11 kg per sq ft per coat.
What changes the number
- Product type. Cementitious slurries, polymer coatings and membranes have very different rates.
- Number of coats. Most sheets give a rate per coat, so the total is per coat times coats.
- Surface porosity. A rough, open or porous slab soaks up more product than a smooth, sealed one. Many sheets give a range for this reason.
- Required thickness. Some sheets are written for a target dry film thickness, so the rate rises with the thickness you need.
- Water head. Tanks and basements under water pressure often need more coats or more material per coat.
- Extra work at corners and parapets, mesh and application method, such as brush or roller loss.
Typical rates printed on data sheets
These are examples of the kind of figures printed on manufacturer sheets. They are not for any one product, and your sheet's figure is the one that counts.
| Product type | Rate printed on sheet | Note |
|---|---|---|
| Polymer modified cementitious slurry | About 0.75 to 0.9 kg per sq m per coat | Varies with substrate roughness, minimum two coats |
| Polymer modified cementitious coating | About 1.0 kg per sq m per coat, up to 1.5 in splash areas | Some sheets add a third coat for high water ingress |
| Waterproofing coating for water head | About 1.0 to 2.0 kg per sq m per coat | Rate rises with water head |
| Flexible polymer coating | About 1.75 to 1.85 kg per sq m per coat | Figures tied to a target dry film thickness, check the sheet |
What to read on the data sheet
- Coverage basis: per coat, or total for the whole system.
- Number of coats and the dry film thickness they achieve.
- Surface type the figure assumes, and whether it allows for rough or porous bases.
- Pack size, mixing ratio for two part products, and pot life once mixed.
- Recoat time, cure time and any water test wait.
- The sheet's date and version. Old sheets get revised.
The formula
Quantity (kg) = roof area (sq m) x coverage per coat (kg per sq m) x number of coats x 1.10
The 1.10 adds 10 percent wastage for spills, tool loss, uneven base and the extra cut at corners. Then divide the total by the pack size and round up to whole packs. Add the parapet strip and cove area to the roof area before you multiply, measured as height times length.
Worked example (made-up figure, example only)
The coverage below is invented for illustration. It is not the figure for any real product. Use your own product's data sheet.
Take a 50 sq m terrace, two coats, an example coverage of 1.2 kg per sq m per coat, and a 20 kg pack. Total product for the coats: 50 x 1.2 x 2 = 120 kg. With 10 percent wastage: 120 x 1.10 = 132 kg. Packs needed: 132 / 20 = 6.6, so order 7 packs. In square feet, 50 sq m is about 538 sq ft, so the same job is about 0.25 kg per sq ft for the whole system.
| Roof area | Product needed | Packs of 20 kg |
|---|---|---|
| 30 sq m | 79.2 kg | 4 packs |
| 50 sq m | 132 kg | 7 packs |
| 100 sq m | 264 kg | 14 packs |
Mistakes that cause shortage or waste
- Buying by area without counting the coats. Two coats need about twice the product of one.
- Forgetting the parapet, cove and drain areas.
- Ordering too little, so the last coat is from a different batch and shows a line.
- Diluting to stretch the pack, which lowers the film thickness the sheet expects.
- Assuming a rough old slab takes the printed rate. It often takes more.
Frequently asked questions
- How many sq ft does one kg of waterproofing cover?
- It depends on the product and the number of coats. Divide the sheet's kg per sq m per coat by 10.76 to get kg per sq ft per coat, then multiply by the number of coats.
- Why should I buy 10 percent extra?
- Wastage, spills, uneven base and corner work use up more product than the flat area suggests. Ten percent is a sensible margin for a roof.
- Is the coverage the same for every brand?
- No. Each manufacturer tests its own product and writes its own rate. Compare only the sheets of the products you are considering, using the same basis.
- Do I count the parapet in the area?
- Yes. Measure the parapet height and length and add it, and add the cove and drain areas too. Then apply the formula.