Short answerTo waterproof a flat roof yourself: clean and repair the surface, wait until it's dry, apply an elastic waterproofing coating in several thin coats starting from the edges, until you reach between 1 and 1,5 kg/m², and don't walk on it until it has cured. With Cool Roof Therm, that's around 0,8–1,25 litres per square metre in total.
Example: a 30 m² flat roof needs between 25 and 38 litres, that is, two 15 L tins, plus two 4 L tins if you want to reach the maximum load.
A flat roof with cracks or worn-out waterproofing eventually shows up on the top-floor ceiling: a ring that appears after rain and grows with every storm. If the surface is sound, waterproofing it with an elastic coating is a job you can do yourself over a weekend. Here's the full process, how to calculate the product so you don't run short, and when it isn't a DIY job.
Before you start: is this your case?
This method works when water is getting in through the surface of the roof: hairline cracks, open joints, an old porous finish. Check for these signs:
- The stain on the ceiling below appears or grows hours or days after rain.
- On the roof you can see cracks, unsealed joints or areas where the previous finish is crazed.
- The surface (mortar, tiles, previous paint) is sound and well bonded.
It isn't a DIY job if the slab has cracks that keep growing, if there are hollow or sunken areas, if the bitumen membrane has lifted across the whole surface or if water is getting in through a broken downpipe or a badly finished drain. In those cases, a professional first; the coating comes afterwards. If you're unsure, send us photos of the roof and of the ceiling below.
Which product to use
Cool Roof Therm is a water-based elastic waterproofing coating applied “in situ”, for horizontal outdoor surfaces: flat roofs, terraces, domes, vaults, overhangs and party walls. Here's what its technical data sheet says:
- Fully waterproof against rainwater.
- Works its way into small cracks to form a continuous, waterproof and elastic layer.
- High elasticity and tensile strength (300–500 % elongation).
- Strong adhesion to cement mortars and tiled floors.
- Can be walked on once it has cured.
- No tar, asphalt or plasticisers.
It's sold in 4 L (28,14 €) and 15 L (97,26 €), in White, Grey, Red, Tile Red and Green, at the same price in every colour. On-screen colours may differ from the real thing.
Cool Roof Therm
Elastic waterproofing coating for flat roofs and terraces. It penetrates small cracks, forming a continuous layer.4 L (≈10 m²) and 15 L (≈40 m²) · 5 colours
How much product you need
This is where most people go wrong. The data sheet gives an approximate coverage per container (around 10 m² for the 4 L and around 40 m² for the 15 L) and, at the same time, says to apply as many coats as needed until you reach between 1 and 1,5 kg/m². To avoid running short, calculate using that total load.
The product's density is 1,20 kg/L, so:
- 1 kg/m² ≈ 0,83 L/m²
- 1,5 kg/m² ≈ 1,25 L/m²
The sum is: square metres × 0,83 = minimum litres and square metres × 1,25 = litres for the maximum load. Add 10–15 % if the surface is rough or very porous.
| Flat roof | Litres (1–1,5 kg/m²) | Suggested tins | Price |
|---|---|---|---|
| 10 m² (small terrace) | 8,3–12,5 L | 3 × 4 L | 84,42 € |
| 20 m² | 16,6–25 L | 1 × 15 L + 1 × 4 L (19 L) | 125,40 € |
| 30 m² | 25–37,5 L | 2 × 15 L (30 L) | 194,52 € |
| 50 m² | 41,5–62,5 L | 3 × 15 L (45 L) | 291,78 € |
The tin counts cover at least the minimum load. To reach 1,5 kg/m², add more product: on the 30 m² roof, two more 4 L tins (38 L in total). Measure carefully: length by width of the roof, plus the parapets or low walls up to the height you're going to cover.
Step by step
1. Choose the day
The temperature of the air and of the surface must be between +5 °C and +40 °C. Don't apply in direct sunlight or on a surface heated up by the sun: in summer, work first thing in the morning or late in the afternoon. Check the forecast: you need several days without rain.
2. Clean thoroughly
Sweep and remove leaves, moss, dust and loose debris. Remove any previous paint or finish that isn't well bonded. The surface must be dry, sound, even and free of anything that would stop the product from gripping. Clear drains and gutters so the water runs off properly.
3. Repair
Fill holes and go over joints and chipped areas with repair mortar, and let it cure as its manufacturer specifies. Hairline cracks are covered by the coating itself, which works its way into them; open cracks have to be repaired first.
4. Protect what you're not painting
Cover window and door frames, glass and anything else that isn't being treated.
5. Apply in thin coats
- Shake the container: the product may thicken without losing its properties.
- It comes ready to use. Work in sections and start with the edges, the junctions with parapets and around drains.
- Spread the load by passing the roller over it several times without overloading it. Don't paint over areas that are drying and avoid lap marks.
- Apply several thin coats until you reach 1–1,5 kg/m². Too much product per coat can cause micro-cracking: more coats are better than thick coats.
- Let each coat dry before the next.
6. Let it cure before walking on it
It can be walked on once it has cured. Until then, don't step on it, don't put plant pots or furniture on it and protect the roof if rain is expected.
7. Finish inside
Once you've stopped the water getting in and the ceiling below is dry, it's time for the inside. Prepare the stained area with Sellador Total L44, which regulates absorption and improves adhesion of the finish, and paint with a breathable paint: Pared Seca or MATE5000 AntiHumedad, depending on the case. If you paint before it's dry, the stain will come back.
Common mistakes
- Calculating only from the coverage per container and ending up below 1 kg/m².
- Applying a single thick coat to finish sooner: it can micro-crack.
- Painting onto a damp surface or in direct midday sun.
- Leaving the junctions until last. Parapets, corners and drains are where most water gets in: start with them.
- Painting the ceiling below too early. The cause first. Then the finish.
Maintenance
Check the roof every autumn, before the first rains: drains clean, free of leaves and soil, and no spots where the coating has been damaged by knocks or by dragging furniture. A small repair in time prevents a new stain below.
If your problem doesn't come from the roof but from a wall or a window, read the water ingress page. And if it's mould on the ceiling with no ring and no link to the rain, it's probably condensation: see mould in the bedroom.
Guía escrita y revisada por el Equipo Humix Paint con los datos de las fichas técnicas de cada producto. La pintura actúa en la superficie: si el agua sube del suelo o entra de fuera, primero se trata la causa.
