How much roofing membrane do you need?
The question we get most often is not whether the membrane will last. It's how many buckets are needed. Here's the math, and where people typically miscalculate.
Basic arithmetic
A 20 kg bucket covers up to 13.3 m². Take the area of the roof, divide by 13.3 and round up to the nearest whole bucket.
- 30 m² garage roof: 3 buckets
- 50 m² extension: 4 buckets
- 100 m² flat roof: 8 buckets
- 150 m² city property: 12 buckets
Always round up. There is no good way to finish a roof in three quarters of a bucket.
Target taken, not the floor plan
This is where most frames are located. The floor plan of the house is not the same as the roof surface, and on a flat roof there is almost always more surface area than the drawing shows.
Remember the upslopes
The edge up against a wall, fire ridge or neighboring building is also a surface that must have a membrane. If the edge is 30 cm high and runs 20 meters, that's 6 m² on top. That's half a bucket that people forget.
Remember the details
Skylights, penetrations, vents and drains take more material per square meter than the flat part because you work closer together and add a little extra at the transitions. Add 10 percent if the roof has a lot of detail.
What drives up consumption?
The 13.3 m² per bucket is on a regular, even roof surface. The substrate changes the number:
- Rough or porous roofing felt absorbs more than a smooth surface.
- Slate granules on older roofing felt provide more small cavities to fill
- Moss and dirt must be removed first, otherwise the membrane will lie on the dirt instead of on the roof.
- Many edges and details mean more material per square meter
A newer, smooth roof lands close to 13.3. An older roofing felt roof with granules is typically slightly lower, and that's another reason to round up.
An example from reality
A 46 m² summer house roof with a 25 cm rise all the way around, 28 meters. The surface gives 46 m², the rise gives 7 m², a total of 53 m². Divided by 13.3, it is 3.98, or 4 buckets. With two skylights in the roof, we would take 5, so there is something to go around the edges.
Use the calculator instead of an estimate
On product page You can enter the square footage of the roof and get both the number of buckets and the price immediately. It rounds up in the same way as above.
What if you buy too little?
You can easily reorder, but try to install the entire roof in one go. The membrane is seamless when installed wet on wet, and it is precisely the seamless surface that means there is nothing for water to get in. If you still have to divide it, divide it at a natural edge and not in the middle of the surface.
If you are in doubt about your roof, send us the measurements and we will calculate it.
