Why the Coverage Figure on Your Waterproofing Tin Is Wrong (Ghana 2026)
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The coverage figure printed on a tin of waterproof coating is not a lie, but it is not a prediction of what your roof will use either. It is a theoretical number calculated for one coat, on a smooth sealed substrate, with no application losses. A real Ghanaian roof consumes roughly 1 kg per square metre across two coats — and the gap between those two numbers is why so many people run out halfway through.
How Manufacturers Arrive at a Coverage Figure
Coverage on a label is usually derived, not measured on a building. The calculation looks roughly like this: take the solids content, decide a target dry film thickness, and work out how many square metres a kilogram would cover if it were spread perfectly evenly on a surface that absorbs nothing.
Every assumption in that sentence is generous:
- Perfectly even. No human with a roller applies a perfectly uniform film.
- Absorbs nothing. Concrete is porous. A bare, rough or previously stripped slab drinks the first coat.
- One coat. Almost all published coverage is per coat, and waterproofing needs two.
- No losses. Material stays in the tray, on the roller, on the brush, and on the ground.
None of that is dishonest. It is simply a laboratory figure being read as a site figure.
The Single Biggest Trap: Per Coat vs Finished
Coverage is quoted per coat. Waterproofing needs two. If the tin says a bucket covers 20 m², that means 20 m² of one coat — so 10 m² of finished waterproofing. Halve the label figure before you order. This one misreading causes more short orders than everything else combined.
It is worth being clear about why two coats is not a sales tactic. A single coat cures unevenly, retains pinholes where the roller lifted, and leaves thin spots over aggregate. The second coat, applied perpendicular to the first, covers what the first missed. One coat is not a thinner version of waterproofing; it is decoration.
What a Ghanaian Roof Actually Consumes
We publish 1 kg per square metre across two coats as the planning figure. That is not derived from a formula — it is what our own completed jobs use, measured by counting empty buckets against measured area.
On that basis:
| Pack | Price | One coat | Finished (two coats) |
|---|---|---|---|
| 5 kg | GHS 500 | 10 m² | 5 m² |
| 10 kg | GHS 1,000 | 20 m² | 10 m² |
Publishing a lower coverage figure than we could is not good marketing. It makes our product look more expensive per square metre than a competitor quoting a theoretical number. We do it because a customer who orders from the theoretical figure runs out on the roof, and a customer who runs out on the roof has a failed job — which costs everyone more than the honest figure ever would.
The Second Trap: Plan Area vs Developed Area
Measure a roof as 10 m by 10 m and you have 100 m². That is the plan area — the shape you would see from a drone.
The developed area is every surface the membrane actually has to cover. On a roof with a parapet, that includes the vertical face of the upstand running right around the perimeter, plus kerbs, plinths and the vertical return at every penetration.
Worked through: a 100 m² roof with a 400 mm upstand around a 40 m perimeter has an extra 16 m² of vertical surface. That is 16% more area than the plan suggests, and it is the part of the roof most likely to leak.
On most Ghanaian buildings with parapets, expect the developed area to run 15–20% above plan area.
The Third Trap: Substrate Porosity
Two roofs of identical size can consume noticeably different quantities.
- Smooth, power-floated or previously sealed concrete sits at the efficient end. The first coat mostly stays on the surface.
- Rough, bare, old or freshly stripped screed sits at the thirsty end. The first coat partly disappears into the pores, and you are effectively priming before you are waterproofing.
If the slab has just had a failed coating scraped off it, budget generously. That surface is open and absorbent in a way a sealed one is not.
How to Calculate Properly — Four Steps
- Measure the developed area. Plan area, plus the perimeter multiplied by the upstand height, plus any kerbs and returns.
- Multiply by 1 kg per m². That is your baseline in kilograms.
- Add 10–15% if the slab is bare, rough or freshly stripped.
- Round up to whole buckets. Half a bucket left over costs you nothing. Half a coat short costs you the job.
Worked example. A 60 m² roof with a 32 m perimeter and a 300 mm upstand. Developed area is 60 + (32 × 0.3) = 69.6 m². At 1 kg/m² that is about 70 kg. The slab is old and bare, so add 12% — roughly 78 kg. Order 8 of the 10kg buckets.
Order from the label figure instead and you would have bought three or four. You would discover the shortfall somewhere around four in the afternoon on day two.
Why Running Out Matters More Than It Sounds
The obvious cost is a wasted trip to the supplier. The real cost is a cold joint.
A coat is meant to go on as one continuous wet film so the whole area cures as a single membrane. If you stop halfway and resume the following day, the first section has already cured. The new material sits against it rather than fusing into it, and that boundary is now the weakest line on the roof — a seam running across the middle of a system whose entire advantage over sheet membranes is that it has no seams.
Cold joints are one of the most common reasons a DIY waterproofing job fails in its first rainy season, and they are almost always caused by under-ordering rather than poor technique.
Four Questions Before You Buy Anything
- Is that coverage per coat or for the finished system? Nearly always per coat.
- What consumption in kg/m² does that imply? Divide pack weight by finished coverage. If it implies far less than 1 kg/m² on a bare roof, treat it as optimistic.
- Is my measurement plan area or developed area? If you measured off a drawing, it is plan.
- Has this slab been stripped or never coated? If yes, budget above the baseline.
The same question works on a contractor's quotation. Ask how many kilograms the price includes. A quote for a 100 m² roof that carries 40 kg of coating is not cheaper than one carrying 100 kg — it is a third of the membrane, and it will fail inside the warranty period.
Send Us Your Measurements
WhatsApp the dimensions to 0555 548 001 — area, perimeter, upstand height, and whether the slab is bare or previously coated — and we will work out the quantity before you order. It costs nothing, and it is considerably cheaper for both of us than you running short.
Polyurethane Coating 10kg — GHS 1,000 · 5kg — GHS 500 · Book a free site assessment