Roofing felt remains one of the most popular materials for waterproofing roofs of garages, sheds and outbuildings. Its availability, ease of installation and reliability when installed correctly make it a favorite among budget solutions. However consumption of roofing material per 1 m² of roofing is the question that causes the most controversy. Some masters claim that one layer is enough, others insist on 3-4 layers, and still others simply buy “by eye” and then buy what is missing. The result is unnecessary waste or, worse, leaks due to a lack of material.
In this article we will look at roofing material consumption rates depending on the type of roof, the number of layers and even weather conditions. You will learn how to calculate the exact number of rolls, avoid being deceived by sellers (who often round up the footage in their favor) and save money without sacrificing quality. We will also reveal the secrets of professionals that can reduce material consumption by 15-20% without losing reliability.
Spoiler: on a flat roof when laying in 2 layers, you will need 20-30% more roofing material than on a pitched roof with an angle of 30°. And that's not all - even the direction of the wind in your region has an impact!
Why can the consumption of roofing felt per 1 m² differ by 2 times?
Let's start with the main thing: there is no universal number, which would be suitable for all roofs. Consumption depends on five key factors:
- 📏 Roof type - flat, single or double slope. On flat roofs, consumption is always higher due to the need for more overlap.
- 🔢 Number of layers — minimum standard for a garage: 2 layers. For residential buildings or roofs with high loads - 3-4 layers.
- 📐 Tilt angle — the steeper the slope, the less overlap (and, accordingly, less consumption). On roofs with a slope >45°, the overlap can be only 7-10 cm.
- 🌡️ Climatic conditions — in regions with strong winds or heavy precipitation, overlaps increase by 20-30%.
- 🛠️ Material quality - cheap roofing material (RKP-350) breaks during installation, which leads to overspending. Stamps RKK-420 or RKP-400 fit tighter.
For example, for flat roof garage with an area of 20 m² when laying in 2 layers you will need about 44-50 m² roofing material (taking into account overlaps of 10-15 cm). And for pitched roof with an angle of 30° the same area will be enough for 40-42 m². The difference is almost 20%!
Roofing material consumption rates: table for different types of roofing
To avoid guessing how many rolls to buy, use this table. It indicates consumption of roofing material per 1 m² of roof taking into account the number of layers and type of roof. The data is based on SNiP II-26-76 and the practice of professional roofers.
| Roof type | Tilt angle | 1 layer (m²) | 2 layers (m²) | 3 layers (m²) |
|---|---|---|---|---|
| Flat | 0-5° | 1.15 | 2.30 | 3.45 |
| Single-pitch | 5-15° | 1.10 | 2.20 | 3.30 |
| Gable | 15-30° | 1.05 | 2.10 | 3.15 |
| Sloping (steep) | 30-45° | 1.00 | 2.00 | 3.00 |
How to use the table? Multiply the area of your roof by the factor from the column with the required number of layers. For example, for a gable roof of 30 m² laid in 2 layers: 30 × 2.10 = 63 m² roofing felt.
⚠️ Attention! If you are using roofing felt with coarse sprinkles (for example, RKK-350B), add +10% to the calculation. Large fractions increase the thickness of the material, which requires more overlap.
How to calculate the number of rolls: step-by-step instructions
Now let's move on to practice. To avoid making a mistake with the number of rolls, follow this algorithm:
- Measure the roof area (length × width of each slope). For complex shapes, divide into rectangles and add up the areas.
- Determine the number of layers:
- 🏪 1st layer - temporary protection (maximum 2-3 years).
- 🏠 2 layers - standard for garages and sheds (service life 5-7 years).
- 🏡 3-4 layers - for residential buildings or roofs with high loads (10+ years).
Calculation example for a gable roof 4x5 m (angle 20°, 2 layers, roll 15 m²):
- Area of one slope:
4 × 5 = 20 m². - Total roof area:
20 × 2 = 40 m². - Coefficient for 2 layers (see table):
2.10. - Total area of roofing material:
40 × 2.10 = 84 m². - Number of rolls:
84 / 15 = 5.6 → 6 rolls. - 🔹 Optimize overlaps. On pitched roofs with an angle of >30°, the side overlap can be reduced to 7-8 cm (instead of the standard 10 cm). This reduces consumption by 12-15%.
- 🔹 Use a backing layer. Instead of two layers of roofing felt, lay one layer glass insulation or bicrosta below and one layer of roofing felt on top. It's cheaper and more durable.
- 🔹 Cut the roofing felt along the chalk line. If you guide the knife “by eye”, the edges will turn out uneven, and you will have to increase the overlap. Chalk markings help you cut accurately.
- 🔹 Lay along the slope. On pitched roofs the rolls should go parallel to the drainage line. This reduces the number of cross joints.
- 🔹 Buy roofing felt 1 m wide. Standard rolls 1x10 m are more convenient to work with than wide rolls (1.5x10 m), as they are easier to lift onto the roof and cut.
- ❌ Incorrect overlap. Too small (less than 7 cm) will lead to water leakage, too large (>15 cm) will result in excess roofing material consumption by 25-30%.
- ❌ Laying on a wet base. A wet surface prevents the bitumen from setting properly. After a year or two, the coating will begin to “walk” and tear.
- ❌ Ignoring temperature. Roofing material must be laid when
+5°C…+25°C. In the heat it melts, in the cold it cracks. - ❌ No primer. Without a primer (bitumen primer), the adhesion of roofing material to the base is reduced by 40%. This leads to swelling and peeling.
- ❌ Using nails instead of mastic. Fastening with nails destroys the waterproofing. Allowed only along the edges (every 50 cm).
- Flat roof (0-5°): 10-15 cm.
- Pitched (5-30°): 8-10 cm.
- Steep (>30°): 7-8 cm.
Specify the area of the roll (10 or 15 m²)|Measure the roof, taking into account overhangs and parapets|Add +1 roll for defects and joints|Check the production date (shelf life - 1 year)|Choose a brand taking into account the climate (for cold regions - RKK, for hot regions - RKP)-->
Saving secrets: how to reduce roofing material consumption by 20%
Professional roofers know several tricks that allow them to save material without losing quality. Here are the most effective:
Critical error: saving on the number of layers. One layer of roofing material will last a maximum of 3 years, after which it will begin to let water through. The repair will cost more than the 2-3 rolls saved.
If the roof has a complex shape (for example, with valleys or dormer windows), add +15% of the material to the calculation. These areas require additional pieces to seal.
Common installation mistakes and their consequences
Even experienced craftsmen sometimes make mistakes that lead to excess material consumption or leaks. Here are the most dangerous of them:
⚠️ Attention! If you are laying roofing felt on old covering, be sure to clean it of debris and heat it with a gas burner. Otherwise, the new layer will lie unevenly, and consumption will increase by 30-40%.
What to do if the roofing material is torn during installation?
If the gap is small (up to 5 cm), seal it with a piece of roofing felt with a 10 cm overlap, heating the edges with a burner. In case of major damage, cut out the defective area and insert a patch, gluing with mastic from the bottom and top. Never leave even small holes - they will become sources of leaks.
Which roofing material to choose: comparison of brands and their consumption
Not all roofing felts are the same. Not only the service life depends on the brand, but also consumption per 1 m². For example, RKP-350 thinner and breaks during installation, so it takes 10-15% more than RKK-420. The table below compares popular brands:
| Brand | Density (g/m²) | Consumption per 1 m² (2 layers) | Service life | Price (per roll 15 m²) |
|---|---|---|---|---|
| RKP-350 | 350 | 2.35 m² | 3-5 years | 400-500 ₽ |
| RKK-350 | 350 | 2.20 m² | 5-7 years | 500-600 ₽ |
| RKP-400 | 400 | 2.15 m² | 7-10 years | 600-750 ₽ |
| RKK-420 | 420 | 2.10 m² | 10-12 years | 750-900 ₽ |
| Glass ruberoid (RM-420) | 420 | 2.00 m² | 12-15 years | 1000-1200 ₽ |
Conclusion: cheap RKP-350 costs more in the long run due to high consumption and short service life. The best option in terms of price/quality ratio - RKK-420.
For roofs with high loads (for example, on which people will walk), choose roofing felt with reinforcing mesh (for example, RM-420). It is more expensive, but will last 2 times longer and will not break during installation.
Frequently asked questions about roofing material consumption
How much roofing felt is needed for a 3x6 m garage roof?
Roof area: 3 × 6 = 18 m². For a flat roof with 2 layers, the coefficient is 2.30. Total consumption: 18 × 2.30 = 41.4 m². For a roll of 15 m² you will need 41.4 / 15 ≈ 3 rolls (round up). We recommend taking 4 rolls with a reserve.
Is it possible to lay roofing felt in 1 layer?
Technically it is possible, but this option is only suitable for temporary protection (maximum 2-3 years). A minimum of 2 coats is required for durability. One layer will quickly crack under the influence of UV rays and temperature changes.
How to calculate roofing felt for a roof with a slope of 45°?
For steep slopes (30-45°) the flow coefficient is minimal. For example, for a roof of 20 m² in 2 layers: 20 × 2.00 = 40 m². This is equivalent to 3 rolls of 15 m² (including scraps).
How to fasten roofing felt: mastic or nails?
Optimally - bitumen mastic (cold or hot). Nails are used only to secure edges (every 50 cm) in windy areas. Mastic provides 100% waterproofing, nails provide only mechanical fastening.
What kind of overlap should you make when laying?
Depends on the roof angle:
In regions with strong winds, the overlap is increased by 20%.