Most newly minted developers who try to follow the trends in the construction industry and choose metal roofing for their roofs are interested in what exactly, in addition to the truss structure, should be under the material so that the flooring can last as long as possible. The right substrate for metal roofing is, indeed, very important to ensure the durability of the coating. And what it should be and how to arrange it, we will try to give an answer.
Types of substrates
To understand what to put under the metal tile in your particular case, you need to remember that the manufacturer's recommendations should always be the priority for the developer in terms of the base for the metal tile.
However, there are two types of roofs, depending on which one or another roofing pie design is usually chosen:
- Cold - for unheated attic spaces.
- Warm roofs - for residential (mansard) under-roofing premises.
The design of the underlay for a cold roof, starting from the metal tile and towards the attic space, should be as follows:
- Lathing and counter lathing, which serve as a frame for fastening the roofing material and create an air gap between the coating and the waterproofing film to remove possible leaks or condensation.
- Waterproofing film under the metal tile, which serves as a barrier to moisture penetrating from the outside.
- Naturally, rafters.
- Attic upholstery.
For a warm roof, this design looks a little more complicated:
- Lathing and counter lathing.
- Waterproofing. Here, it should provide both protection against possible roof leaks and condensate that forms on the inside of the metal tile coating due to the temperature difference between the environment and the living under-roof space. Among other things, the functions of the waterproofing film in this case also include ensuring the removal of water vapor penetrating into the roofing cake under the metal tile from the interior. Thanks to this, the insulation remains dry and does not lose its thermal characteristics.
- A ventilation gap of 2-4 cm is required between the anti-condensation waterproofing and the insulation to ensure the normal functioning of the first.
- Insulation placed between the rafters and serving as a heat insulator of the interior. Its thickness is chosen depending on the climate in the construction area.
- A vapor barrier film that serves as a barrier against the penetration of water vapor into the roofing pie from the interior.
- Sheathing of the attic (residential) premises.
It is these designs of a roofing pie for a particular type of roof that are considered the most optimal.
Metal tile and roofing felt substrate
One of the most relevant regarding roofing today is the question of whether roofing material can be laid under a metal tile. It is rather difficult to give an unambiguous answer to it, since, again, it all depends on the design of the roof and its operating conditions.
For example, for a newly built cold (important!) roof, roofing material can be laid under a metal tile instead of a waterproofing film, since, subject to sufficient ventilation of a non-residential attic, vapor permeability of the waterproofing material is not required.
However, the obligatory presence of an air ventilated layer between the metal tile and the roofing material is important here.
If we are talking about an old roof covered with roofing felt, on top of which it is planned to install a metal tile flooring, then everything here depends on the functions of the attic.
If it is non-residential and well ventilated, the metal tile is laid on the roofing material with an air ventilation layer provided by the sheathing device on top of the roofing material.
Advice! In any other cases, the installation of roofing material under a metal tile (or tiles on a roofing material) is not recommended, since this is fraught not only with leaks, but also with premature failure of the roofing and truss system.
Roof waterproofing
Now let's talk more about the functional layers of the roofing cake and start with waterproofing.
The waterproofing material under the metal tile should ensure the performance of such functions as protecting the insulation and the roof structure as a whole from precipitation penetrating into the air gap during strong winds or through cracks in the roof, as well as preventing the penetration of moisture that condenses on the metal tile coating.
There are 2 types of installation of roofing waterproofing:
- The installation of a waterproofing film with a ventilation gap of 2-4 cm between the heat-insulating material and the waterproofing.
- The device of the film with laying directly on the insulation. This case involves the use of special membrane-diffusion films. Such films, like their counterparts, work as a barrier to moisture that penetrates from the outside, while passing it in the form of steam coming from inside the building.
Membrane-diffusion films waterproofing under metal roofing have a higher cost, but have a number of undeniable advantages:
- They are laid directly on the insulation, and this allows you to reduce the thickness of the roofing cake and, accordingly, lay a thicker (“warm”) layer of insulation.
- The membranes perfectly protect from the wind, thereby reducing heat loss through the insulation.
- The membrane, which is vapor-permeable from the inside, is waterproof from the outside. Thanks to this, the roof is able to "breathe".
Heat and vapor barrier
Insulation for metal tiles, as a rule, is selected with a thickness of 15-20 cm, depending on the climatic conditions of the building area.
In this case, it is desirable that this thickness be several slabs of smaller thickness (3-4 5 cm thick slabs) laid in a checkerboard pattern. They are laid tightly to the rafters to avoid the formation of cold bridges, while sealing the joints.
The vapor barrier serves as a barrier against moisture entering the insulation from inside the premises. It is laid with an overlap of 10 cm, attached to the rafters and is the bottom layer of the entire roofing pie, which is subsequently covered with the interior decoration of the attic (mansard room).
The finishing material is attached to the rails installed under the bottom of the vapor barrier to prevent contact between the film and the finish. Typically, vapor barrier films consist of 1-2 layers of polyethylene, which are reinforced with a special mesh.
Ventilation of the space under the roof deck
Ventilation under the roof during the installation of the latter is a necessity, since it provides:
- removal of moisture from the space under the roof;
- equalization of temperature over the entire area of the roof (protects against the formation of frost in winter over the installation of heating appliances);
- reduces the level of heat penetration from the sun's rays.
In this case, the air inflow is provided through the filing of the cornice overhang, and the air outlet is provided through a ventilated roof ridge or mounted point ventilation elements (aerators).
Now, having considered the question of what is put under the metal tile, we would like to recommend that you do not save on the installation of a roofing pie, and even more so do not engage in amateur activities, since only in this case your roof will last for at least the life of the metal tile declared by the manufacturer.
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