What is mastic for roofing felt and why is it needed?
Roofing felt is one of the most popular materials for waterproofing roofs of garages, sheds and outbuildings. But even high-quality roofing material eventually separates at the joints, cracks or peels off from the base. This is where it comes to the rescue mastic for gluing roofing felt - a special adhesive composition that not only fixes the canvases to each other, but also creates an additional protective layer.
Unlike traditional methods (for example, fusing with a gas torch), mastic allows you to work without open flame, which is especially important when repairing roofs with wooden sheathing or near flammable materials. In addition, modern formulations have elasticity, which prevents repeated cracking during seasonal deformations of the roof.
But not all mastics are equally effective. Some are suitable only for gluing new sheets, others are suitable for repairing old coatings with cracks. In this article we will look at how to choose the right composition, avoid mistakes during application and extend the service life of roofing felt roofing.
Types of mastics for roofing felt: comparison and features
All mastics for roofing felt are divided into two large groups: bituminous and polymer-bitumen. The former are cheaper, but less durable, the latter are more expensive, but can withstand extreme temperatures and mechanical stress. Let's take a closer look.
1. Hot bitumen mastics - require warming up 160–180°C before application. Their main advantage is high adhesion strength to roofing felt. However, working with them is difficult: you need a gas burner or a special boiler, plus the risk of burns. Suitable for large projects (for example, flat roofs of garage cooperatives), but inconvenient for minor repairs.
2. Cold bitumen mastics — ready to use immediately after purchase. They contain solvents (for example, white spirit), which evaporate after application, leaving an elastic film. The downside is the strong smell and long drying time (up to 24 hours). Popular brands: TechnoNIKOL No. 21, Bitumast.
3. Polymer-bitumen mastics — contain rubber or latex additives, which gives them unique frost resistance (down to −40°C) and resistance to UV radiation. They can be applied even to a damp surface, and their service life reaches 15 years. Market leaders: TechnoNIKOL No. 33, Gidroizol.
4. Rubber-bitumen mastics - ideal for repairing old roofs, as they fill cracks and chips. They have high elasticity, but are more expensive than analogues. Example: Slatex.
- 🔥 Hot mastics - maximum strength, but difficult to apply.
- ❄️ Cold bitumen - convenient for minor repairs, but takes a long time to dry.
- 🧪 Polymer-bitumen - the best choice for durability.
- 🩹 Rubber-bitumen - saving old roofs with cracks.
How to choose mastic: 5 key criteria
The choice of mastic depends on the type of work, climatic conditions and the condition of the roofing material. Here's what to look for:
1. Temperature range. If you live in a region with frosts below −25°C, take polymer-bitumen compositions. For southern regions, ordinary bitumen mastic is suitable.
2. Method of application. For repairing small areas (for example, cracks around a ventilation pipe), cold mastics are more convenient. To lay new roofing felt over a large area - hot.
3. Adhesion to the substrate. If the roof is concrete or metal, you need a mastic with high adhesion (for example, TechnoNIKOL No. 24). Any composition is suitable for wooden sheathing.
4. Drying time. Hot mastics set in 2–4 hours, cold ones - up to a day. Keep this in mind if you plan to work in the rain.
5. UV resistance. Bitumen without polymer additives is destroyed under the sun in 3–5 years. For open roofs, choose mastics with UV filters (for example, Gidroizol Ultra).
| Criterion | Hot bitumen | Cold bitumen | Polymer-bitumen |
|---|---|---|---|
| Price (per 1 kg) | 80–120 ₽ | 150–250 ₽ | 300–500 ₽ |
| Application temperature | from +5°C | from −10°C | from −20°C |
| Service life | 5–7 years | 7–10 years | 10–15 years |
| Crack resistance | Average | Low | High |
⚠️ Attention: Do not use mastics based on cheap bitumen BN-70/30 for gluing grades of roofing felt RKP-350 or RKK-420. They will not provide sufficient adhesion, and the seams will come apart after 1-2 seasons.
Step-by-step instructions: how to glue roofing felt with mastic
Before starting work, prepare the following tools: a metal spatula, a brush with stiff bristles, a roller, a gas burner (if you are using hot mastic), and protective equipment (gloves, a respirator).
Step 1: Surface Cleaning. Remove debris, dust and old mastic from the roof. If the roofing material has peeled off, cut off the bubbles with a knife and clean the base with a wire brush. For better adhesion, you can treat the surface with a primer (for example, TechnoNIKOL No. 01).
Step 2. Preparing the mastic. Melt the hot mastic in a cauldron to the temperature 160–180°C (not higher, otherwise the bitumen will lose its properties!). Just stir the cold one.
Step 3. Applying the first layer. Using a brush or spatula, cover a 10–15 cm wide joint of roofing felt with mastic. If you are repairing a crack, apply the compound with an overlap of 5 cm along the edges.
Step 4. Laying roofing felt. Press the new sheet onto the mastic and roll it with a roller, squeezing out the air. For reliability, coat the seam on top with another layer.
Step 5. Quality check. After 24 hours, check the seams for bubbles or untaped areas. Repeat treatment if necessary.
Clean the surface from dirt and old mastic|Heat the mastic (if hot) to the required temperature|Apply primer to porous substrates (concrete, wood)|Check the width of the overlap (at least 10 cm)|Use a roller to remove air-->
⚠️ Attention: Never apply hot mastic to wet roofing felt - this will cause blistering and peeling. The humidity of the base can be checked with a special moisture meter or plastic film (if condensation appears under it overnight, the roof needs to be dried).
Common mistakes and how to avoid them
Even experienced craftsmen sometimes make mistakes that shorten the service life of roofing felt roofing. Here are the most common of them:
1. Saving on mastic. A thin layer (less than 1 mm) will not provide a seal. The optimal thickness is 1.5–2 mm.
2. Ignoring the primer. On porous surfaces (concrete, cement screed), mastic without a primer is absorbed and loses its adhesive properties.
3. Work in rain or fog. Moisture under the roofing felt will lead to rotting and swelling. Optimal weather is dry, windless, at temperatures above +5°C.
4. Using expired mastic. Bitumen compounds have a shelf life (usually 12 months). Expired mastic loses its elasticity and cracks.
5. Improper storage. If a jar of mastic was stored in the cold, the bitumen may delaminate. Before use, it must be kept for 24 hours at room temperature.
- 💰 Consequences of savings: in a year the seams will come apart and the repair will have to be repeated.
- 🌧️ Working in the rain: The water under the roofing felt will freeze in winter and tear the coating.
- ⏳ Expired mastic: Instead of 10 years, it will last a maximum of 2 seasons.
Before buying mastic, check its production date on the jar. If the composition was released more than a year ago, ask the seller for a quality certificate - some brands (for example, TechnoNIKOL) retain their properties longer thanks to preservatives.
Comparison of popular brands of mastics
The Russian market is dominated by several manufacturers of roofing felt mastics. Their products differ in price, composition and application. Let's look at the most popular brands:
TechnoNIKOL - leader in assortment. Mastics of this brand are divided into series: №21 (universal cold), №24 (for metal), №33 (polymer for extreme conditions). The main plus is stable quality, the minus is the high price.
Bitumast - a budget option with good adhesion. Suitable for temporary repairs or small roofs. Disadvantage: low frost resistance (up to −15°C).
Gidroizol — specializes in mastics for difficult substrates (for example, rusty metal). Composition Gidroizol Ultra contains anti-corrosion additives.
Slatex — premium rubber-bitumen mastics. Used to repair old roofs with numerous cracks. The price is justified by the service life (up to 20 years).
| Brand | Model | Type | Price (per 10 kg) | Features |
|---|---|---|---|---|
| TechnoNIKOL | №21 | Cold bitumen | 1 800 ₽ | Universal, for any reason |
| Bitumast | Standard | Cold bitumen | 1 200 ₽ | Budget, for temporary repairs |
| Gidroizol | Ultra | Polymer-bitumen | 2 500 ₽ | Anti-corrosion additives, for metal |
| Slatex | Pro | Rubber-bitumen | 3 800 ₽ | For old roofs with cracks |
For most garage roofs, the best choice is TechnoNIKOL No. 21 or Gidroizol Ultra. They combine reasonable price and reliability, and their characteristics are enough to protect against precipitation for 10+ years.
Alternative methods of gluing roofing felt
Mastic is not the only way to connect sheets of roofing felt. In some cases it is more convenient to use other methods:
1. Fusing with a gas torch. Suitable for new roofs where large areas need to be connected. The downside is the risk of fire and the need for skills in working with a burner.
2. Bitumen tape (for example, Nicoband). Convenient for quickly repairing small cracks or joints around pipes. Does not require heating, but is less durable than mastic.
3. Liquid rubber. Modern material based on bitumen and polymers. Spray applied to form a seamless finish. The downside is the high price (from 500 ₽/kg) and the need for special equipment.
4. Glue for roofing felt (for example, K-36). Used for gluing roofing felt to vertical surfaces (for example, a parapet). Not suitable for horizontal seams.
- 🔥 Burner: fast, but dangerous for wooden roofs.
- 📦 Bitumen tape: ideal for emergency repairs.
- 🎨 Liquid rubber: best option for complex shapes (eg domes).
When is mastic not suitable?
If the roof has a slope of more than 30°, the mastic may run off before drying. In this case, it is better to use mechanical fastening (nails with rubber washers) or fusing with a torch.
FAQ: Frequently asked questions about mastic for roofing felt
Is it possible to apply mastic to old roofing felt?
Yes, but first you need to clean the surface from dirt, moss and loose areas. If the roofing material is very worn (holes have appeared), it is better to apply a patch of new material, covering the edges with mastic.
How long does it take for mastic to dry?
Hot mastic sets in 2–4 hours, but complete polymerization takes up to 24 hours. Cold compounds take longer to dry - from 12 to 48 hours, depending on temperature and humidity.
Is it possible to paint the mastic after drying?
Yes, but only with special paints for bitumen surfaces (for example, Bitumast Paint). Regular façade paints will peel off after a few months.
How to remove mastic from instruments?
Fresh mastic can be washed with white spirit or kerosene. Dried - only mechanically (with a scraper) or with special washes (for example, TechnoNIKOL bitumen removal).
How to store an open jar of mastic?
Close the lid tightly and store in a cool place (such as a basement). If the mastic has thickened, it can be diluted with a solvent (no more than 5% of the volume), but this will worsen the properties of the composition.