High-quality waterproofing of a garage is not just a way to keep the roof intact, but a necessity that extends the life of the entire structure. If you are thinking about what to glue roofing felt on, which means the time has come for a major overhaul or planned replacement of the roofing pie. Mistakes at this stage can be costly: leaks will lead to damage to property indoors and destruction of concrete floor slabs.
The modern market offers many solutions, from classic bitumen to polymer modifiers. The choice of a specific composition depends on the type of base, weather conditions during installation and the budget of the work. Bitumen mastics remain the leader in this niche due to their affordability and time-tested reliability, but new rubber or polyurethane-based compounds are gaining popularity.
In this article we will analyze in detail which glue to choose for different types of roofing felt, how to prepare the surface and why applying primer is a critical step that cannot be ignored. Proper adhesion of materials ensures that the roof will last for decades and will not require repairs next spring.
Classification of adhesives for roofing felt
Before you go to a hardware store, you need to clearly understand that roofing felt is a different material, and it requires specific adhesives. The main division occurs according to the type of binder: it can be pure bitumen, a bitumen-polymer mixture or a synthetic resin. For deposited materials Glue as such is not needed, gas burners are sufficient, but gluing traditional cardboard or glass base requires a liquid composition.
The most popular are bitumen mastics, which are divided into cold and hot. Cold ones are ready for use immediately after opening the bucket, which simplifies work at home. Hot ones require preheating to a liquid state, which creates additional risks and requires the presence of an open fire or powerful heating equipment on the roof.
It is also worth paying attention to polymer adhesives, which provide higher elasticity of the seam. If your garage is subject to vibration or thermal deformation, the hard bitumen layer may crack, while polymer mastic will stretch and contract along with the base, maintaining tightness.
- 🔥 Hot bitumens: They require heating up to 150–200 degrees, provide instant traction, but are dangerous to work with and cool down quickly.
- 💧 Cold mastics: contain solvents, the evaporation of which ensures hardening, are safer, but take longer to dry.
- 🧪 Polymer composites: They have high adhesion to concrete and metal, are resistant to UV radiation, but are more expensive than classic bitumen.
The choice between these categories is often dictated by ambient temperature. In winter, cold mastics can thicken, and hot mastics can cool too quickly when applied. In summer, volatile solvents in cold compositions evaporate too quickly, not giving time for quality installation of the canvas.
Primers: the basis for reliable adhesion
Many beginners make the fatal mistake of skipping the priming step. Primer (or bitumen primer) is not just “water”, but a complex chemical composition that penetrates the pores of the concrete screed. Without it, the mastic lays on top as a film, which over time can peel off along with dust and small fractions of concrete.
The function of the primer is to remove dust from the surface and create a sticky intermediate layer. It binds dust residues that cannot be removed with a brush and creates a monolithic connection between the base and the main waterproofing carpet. This is especially true for garage roofs, since the concrete is often old and loose.
The primer can be applied with a brush, roller or spray. It is important to let it dry completely before applying mastic - this usually takes 1 to 4 hours depending on the weather. If you apply glue to a wet primer, the solvent will “boil”, forming bubbles that will break the seal.
Check whether the primer is dry by placing a piece of polyethylene on it. If condensation has not formed under it and the film has not turned white, you can begin gluing the roofing felt.
There are ready-made water-based primers that are odorless and safe for indoor areas, which is handy if you're repairing the roof of an attached garage. Oil analogues are more effective for old, greasy surfaces, but require good ventilation.
Technology for preparing the base for laying
Even the most expensive glue will not hold the roofing felt if the base is poorly prepared. The first step is to inspect the surface: remove all protruding elements, knock down concrete sagging and seal deep cracks with repair mortar. The surface should be as flat as possible so that air pockets do not form under the canvas.
Cleaning from debris, dirt and oils is a mandatory step. If roofing felt has already been placed on the roof of the garage, it must be completely dismantled or, in extreme cases, thoroughly cleaned and primed. The presence of old, swollen waterproofing under a new layer is a guarantee of a recurrence of the problem after one season.
Base moisture is a critical parameter. Bitumen mastics don't like water. If you glue roofing felt to wet concrete, the water will turn into steam when heated by the sun and tear the coating from the inside. You can check the humidity using a simple folk method: stick a square of film with tape overnight. In the morning there should be no moisture under it.
☑️ Preparing the garage roof
Pay special attention to corners and junctions with walls or ventilation pipes. Moisture and dirt often accumulate in these places. They need to be thoroughly cleaned and coated with mastic with a reinforcing mesh before laying the main strips of roofing felt.
Comparison of characteristics of adhesive materials
To finally make your choice, it is worth comparing the main parameters of popular formulations. The table below will help you navigate the differences between hot and cold mastics, as well as specialized adhesives.
| Parameter | Hot mastic | Cold mastic | Polymer glue |
|---|---|---|---|
| Application temperature | Above +5°C (requires heating) | -20°C to +40°C | From +5°C to +30°C |
| Drying time | Instantly when cooled | 12–24 hours | 24–48 hours |
| Consumption per m² | 1.5–2.0 kg | 0.5–1.0 kg | 0.3–0.5 kg |
| Fire hazard | High (open fire) | Medium (solvent vapor) | Low |
As can be seen from the table, cold mastics are superior in convenience and safety, although they take longer to dry. Hot compounds are good because they allow you to continue working immediately, but they require serious preparation and compliance with safety precautions. Polymer adhesives are the choice for those who make “for themselves” and want maximum durability without unnecessary fuss with fire.
Cost also plays a role: hot bitumen (if you cook it yourself from tar) is the cheapest option, but labor-intensive. Ready-made cold mastics in buckets are more expensive, but save time and effort. Polymer compositions - premium segment.
For self-repair of a garage without professional equipment, the best choice is ready-made cold bitumen-polymer mastics in buckets.
Step-by-step instructions for sticking roofing felt
The installation process begins with cutting the material. Roofing felt is cut into strips of the required length, taking into account the overlap. The standard overlap is 10–15 cm in length and width. If the roof has a slope, installation begins from the lowest point, rising upward so that water flows over the surface without flowing under the joints.
The mastic is applied with a notched trowel or wide brush. The layer should be uniform, without gaps. You should not spread the entire roof at once - the mastic may evaporate while you roll out the roll. It is optimal to treat the area for 1-2 canvases.
After applying the glue, roofing felt is laid. It must be pressed firmly and smoothed with a heavy roller or brush, moving from the center to the edges to expel the air. Pay special attention to the seams: they need to be coated with additional mastic and pressed firmly.
⚠️ Attention: Never walk on freshly laid roofing felt wearing soft shoes or carrying sharp objects. Even a small hole made by a heel will become a source of leakage within a year.
If multi-layer installation is used (which is recommended for garages), the second layer is laid with the seams offset from the first. This covers weak points and creates a monolithic carpet. The glue between layers is applied in the same way as on the base.
The nuances of working in different weather conditions
Weather is a roofer's biggest enemy. Ideal conditions for working with bitumen mastics are dry, warm weather without wind. Rain during work is fatal: water will fall under the waterproofing layer and remain there forever.
At low temperatures, cold mastics thicken and become difficult to spread. In this case, the bucket of mastic can be taken into a warm room or garage overnight, but it is strictly forbidden to heat it over an open fire - the solvent vapors are explosive. In the summer, in the heat, the mastic may become too liquid and drain from vertical areas.
Wind is another problem. It blows solvents out of the mastic, speeding up its drying, and can knock over a light gas burner if you use the hot method. When working in windy weather, you need to have a partner who will hold the canvas.
What to do if it rains?
If rain catches you while applying mastic, work should be stopped immediately and the area covered with film. If it rains after gluing, but before drying (the mastic is still sticky), the seams may be damaged - they will have to be re-glued after complete drying.
Air humidity also matters. At high humidity (>85%), bitumen emulsions may turn white and lose their properties, as water from the air condenses in the adhesive layer. On such days, it is better to refrain from work or use special moisture-repellent primers.
Frequently asked questions (FAQ)
Is it possible to glue roofing felt onto old roofing felt?
This is technically possible, but not recommended. The old layer can hide blisters and moisture. If dismantling is not possible, the old layer must be thoroughly cleaned, dried with a torch, all bubbles sealed and be sure to be primed with a primer for bitumen surfaces.
How many layers of roofing felt are needed for a garage roof?
For a flat garage roof, it is optimal to use 2 layers. The first layer is a lining layer (marked with the letter “P”, for example, RPP), the second is a roofing layer with protective coating (marked with the letter “K”, for example, RKK). One layer will quickly deteriorate when exposed to the sun.
How to dilute thickened cold mastic?
Cold bitumen mastic can be diluted with white spirit, gasoline galoshes or solvent. The solvent should be added in small portions and mixed thoroughly. Hot mastic cannot be diluted, it is only heated.
Do I need to wait for the primer to dry?
Yes, definitely. The primer should dry completely (no longer sticky and no longer black), which usually takes a few hours. Applying mastic to a wet primer will cause the coating to peel off.
What is the consumption of mastic per 1 square meter?
On average, consumption is 1–2 kg/m² for the first layer (primer) and 0.5–1 kg/m² for subsequent layers when gluing roofing material. The exact flow rate depends on the porosity of the base and the viscosity of the material.