Reliable waterproofing of metal roofs of garages, hangars and sheds is a task that requires a competent approach to the choice of binding material. Ruberoid remains one of the most affordable and time-tested solutions, however, its installation on a steel surface has its own nuances. The main problem is that metal is subject to thermal expansion, and the roofing material itself requires specific adhesive, which will not leak in summer and will not crack in winter.
To successfully complete the work, it is necessary to select a composition capable of creating a monolithic connection between the porous base of the roofing sheet and the smooth metal surface. Mistakes at the stage of choosing glue or mastic can lead to swelling of the coating, leaks and the need for expensive repairs after just a couple of seasons. In this article we will look at which chemical compositions are best suited for this purpose and how to properly prepare the base.
The modern building materials market offers many solutions, from classic bitumen resins to high-tech polymer adhesives. It's important to understandthat there is no universal remedy, and the choice depends on the type of metal, roof slope and climatic operating conditions. Properly selected technology will create a durable “pie” that will protect the structure for decades.
Requirements for adhesives for metal substrates
Metal, be it galvanized steel, corrugated sheet or smooth sheet, has high thermal conductivity and a coefficient of linear expansion. This means that when heated by the sun, it increases significantly in size and shrinks at night. Adhesive composition for roofing felt it must have high elasticity to compensate for these movements without breaking. If the glue is too hard, it will simply peel off from the metal at the first temperature load.
The second critical parameter is heat resistance. In summer, the black surface of the roofing material can heat up to 70–80 degrees Celsius, and the metal underneath can heat up even more. Ordinary low-quality bitumen mastics can float, causing the sheet to slip. Therefore, for gluing to metal it is often recommended to use materials based on modified bitumen or special polymer compositions with increased heat resistance.
It is also worth considering the corrosive activity of some compounds. Aggressive solvents contained in cheap adhesives can damage the protective zinc layer of the metal, triggering the rusting process under the coating. It is critical to choose compounds with anti-corrosion additives or a neutral chemical formula. Only an integrated approach will ensure the durability of the structure.
- 🌡️ Heat resistance: the material must withstand heating up to +85°C without fluidity.
- 🔗 Adhesion: high adhesion to both smooth metal and roofing felt.
- 💧 Moisture resistance: Completely impermeable to water after polymerization.
- 🛡️ Anti-corrosion: absence of components that destroy the zinc coating.
Overview of types of adhesives and mastics
The basis for fastening roofing felt is most often bituminous materials. Bitumen mastics divided into cold and hot. Cold mastics are ready for use immediately after mixing and contain solvents that evaporate, fixing the coating. They are easy to use, but take time to dry and often have a strong odor. Hot mastics (bitumen) require preheating to 160–180 degrees, which creates a very strong monolith, but is dangerous when working at height and requires special equipment.
A modern alternative is polymer adhesives and rubber-based primers. They provide stronger adhesion to metal due to deep penetration into the surface microrelief. Such compounds are often called “liquid rubber”. They create an elastic membrane that perfectly compensates for vibrations and thermal expansion of the metal. However, their cost is significantly higher than that of traditional bitumen.
Separately worth mentioning self-adhesive modifications roofing felt, which have an already applied sticky layer. To fix them on metal, it is often enough to simply heat the base with a hair dryer or use special activating primers. This simplifies installation, but requires a perfectly flat metal surface without defects.
⚠️ Attention: Never use pure bitumen without additives (tar) for stickers on metal without first priming. It is too fragile at low temperatures and will quickly become covered with cracks, losing its tightness.
Preparing the metal surface
The quality of adhesion between roofing felt and metal depends 80% on the preparation of the base. Even the most expensive glue will not stick to a rusty, dusty or greasy surface. The first step should always be mechanical cleaning. Use wire brushes or sanders to remove loose rust, old paint and dirt. The surface should become matte and rough.
After mechanical cleaning, it is necessary to degrease the metal. Suitable solvents for this purpose are: White spirit, Nefras or special degreasers. Pay special attention to welds and places where screws are attached - this is where oil and flux residues most often accumulate, which interfere with adhesion.
The final and mandatory step is the application primer (primers). The primer binds dust residues, evens out absorbency and creates a sticky film that improves the contact of the mastic with the metal. For metal roofs, primers based on bitumen solutions in organic solvents are best suited, as they dry faster and penetrate deeper into the pores.
☑️ Checklist for preparing the foundation
Installation technology: step-by-step instructions
The process of gluing roofing felt to metal requires consistency. First, the canvas is cut to the size of the slope, taking into account the overlap. Then glue or mastic is applied. If cold mastic is used, it is applied with a comb spatula in an even layer 1–2 mm thick. Do not apply the entire roof at once - the glue may have time to dry before laying the sheet.
The roofing material sheet is laid on the prepared base and carefully rolled with a heavy roller or pressed with feet (in soft shoes), moving from the center to the edges. This is necessary to squeeze out air bubbles. The air under the coating is the main enemy of waterproofing, since when heated it expands and tears off the material.
Joints of canvases and junctions with vertical elements (parapets, pipes) require special attention. They are additionally coated with mastic and often reinforced with strips of roofing material or special sealing tapes. For reliability, you can use hot method precisely at the seams, even if the main field was glued with cold mastic.
| Material type | Consumption (kg/m²) | Drying time | Application temperature |
|---|---|---|---|
| Cold mastic | 1.0 – 1.5 | 12–24 hours | from +5°C to +30°C |
| Hot bitumen | 1.5 – 2.0 | Instantly | from -10°C (with warming up) |
| Polymer glue | 0.5 – 0.8 | 24–48 hours | from +5°C to +35°C |
| Bitumen primer | 0.3 – 0.5 | 2–4 hours | from +5°C to +40°C |
Deposition method: features of working with metal
Fusion of roofing felt is a technology in which the lower bitumen layer of the sheet is melted with a gas burner. When working with a metal base, this method has its own characteristics. The metal has high thermal conductivity, so it quickly removes heat from molten bitumen. This can lead to the glue “setting” prematurely, without having time to penetrate into the uneven areas.
To avoid this, the metal must be preheated. However, there is a danger here: thin metal can become deformed (“strains”) due to local overheating, and the galvanized coating can burn out, losing its protective properties. Therefore, when fusing metal, a more gentle burner mode is used and the temperature is constantly monitored.
Safety precautions When welding onto metal, it must be strictly observed. Sparks from a burner can easily burn through old wood structures under metal or ignite insulation. Additionally, working with open flames on a slippery metal roof requires the use of safety ropes.
Nuances of fusing on galvanization
When fusing on galvanized metal, there is a risk of damaging the zinc layer due to overheating. Zinc melts at a temperature of about 420°C, which is quite achievable with the flame of a propane torch. Damage to the zinc will cause rapid corrosion in that area. It is recommended to use special fiberglass pads or switch to the adhesive method for galvanized surfaces.
Common errors and ways to resolve them
One of the most common mistakes is applying mastic to a wet base. Water, trapped under a layer of bitumen, turns into steam when heated and blows up bubbles, which then burst. If you notice any swelling immediately after installation, you can carefully pierce them, squeeze out the air and re-glue them with mastic.
Another mistake is saving on primer. An attempt to glue roofing felt directly onto metal without a primer is doomed to failure. Adhesion will be weak, and the first strong wind or frost will tear off the coating. This can only be corrected by completely removing the canvas, cleaning and repeating the process with a primer.
Insufficient blade pressure also leads to problems. If air pockets remain between the metal and roofing felt, moisture condenses in them, causing corrosion of the metal below. High-quality rolling using a roller is not a formality, but a necessity.
Expert advice: To improve adhesion on smooth metal sheets, you can pre-glue a layer of fiberglass or geotextile onto the mastic, creating a reinforcing layer that will increase the contact area.
The main secret to success is thorough cleaning and degreasing of the metal before applying the primer. Without this step, even the most expensive glue does not guarantee durability.
⚠️ Attention: Do not carry out work on roofing felt stickers in the rain or when air humidity is above 85%. Moisture condensing on cold metal will make adhesion impossible.
FAQ: Frequently asked questions
Is it possible to glue roofing felt to rusty metal?
Strongly not recommended. Rust is an iron oxide that has no load-bearing capacity and continues to degrade the metal. The glue holds on to the rust itself, which will quickly peel off along with the coating. It is necessary to strip the rust down to the metal, treat it with a rust converter and prime it.
What is the consumption of mastic per 1 square meter?
Consumption depends on the viscosity of the material and the roughness of the base. On average, for cold mastic the consumption is 1–1.5 kg/m² when applied in one layer. For hot bitumen, consumption may be higher due to volatilization of light fractions when heated.
Do I need to heat the roofing felt before laying it?
If you use cold mastic, there is no need to heat the roofing material; it should be at room temperature for better elasticity. If the fusing method is used, then it is the lower bitumen layer of the canvas that is heated until a glossy shine appears.
How can I replace the primer if I don’t have one?
In emergency cases, the primer can be prepared independently by dissolving the bitumen in gasoline or solvent (ratio 1:3 by weight) until it becomes liquid milk. However, factory primers contain special additives for adhesion, so a homemade analogue may be inferior in quality.