- Cold attic heating technology
- Rafter system
- Roof waterproofing
- Installation of lathing
- Ventilation of a cold attic
Most modern houses have a cold loft. Compared to a warm attic or attic, such a roof is less expensive in terms of materials and installation time, but do not let the simplicity of the device fool you. To avoid mistakes in the construction of a cold roof, use the instructions from our website.
Cold attic heating technology
Warm air always moves upward, penetrating even the smallest cracks and gaps in the ceiling. To preserve maximum heat inside the house, a sufficiently powerful heat protection belt with a thickness of up to 300-350 mm is arranged above the ceiling. Thermal insulation is laid as tightly as possible in order to slow down the rising hot flow as much as possible.
The insulation needs to be protected from getting wet, so the steam, which is carried by the warm air, must be blocked. For these purposes, the ceiling is covered from the inside with a vapor barrier – a special membrane that allows gases to pass through, but retains evaporated moisture, and rmnt.ru has already talked about these materials in more detail in a separate article. On the inside, the heat insulator has a sufficiently high temperature and therefore condensation on the vapor barrier does not fall out.
The outer side of the insulation is cold. It is ventilated with outside air, so that moisture does not accumulate in the insulation. The attic floor is the only step of thermal protection; no other cold attic structures perform this function.
The main problem with a cold roof is that condensation occurs on the back of the roof during temperature changes. Since the air in the attic remains warm for a long time after sunset, and the roof almost immediately acquires the temperature of the outdoor environment, the appearance of moisture drops is an inevitable phenomenon. Falling down, they fall on the overlap cake, moistening it. This leads to a heavier filler, loss of heat-insulating properties, dampness and mold. That is why, when installing the roof of a cold attic, the installation of under-roof waterproofing is mandatory..
Since insulation will not fit between the rafter legs, their step and section can be chosen absolutely freely. The kalk.pro service will help in this matter: in the “Roofs” section, you can model and calculate load-bearing elements and lathing for any type of roof.
The most suitable lumber for the device of rafter legs is considered to be a board 50–70 mm thick and 100–200 mm wide. Based on the weight of the roof and the predicted snow load, the pitch between the rafters of such a section is adjusted in the range of 60–120 cm. It must be borne in mind that the rafters during installation are reinforced with a counter lattice 30–40 mm thick, so it is reasonable to initially choose a slightly smaller width of the board.
When installing the rafter system, it is imperative to put your legs in common planes in order to subsequently avoid problems with leveling the battens. For these purposes, it is recommended to rigidly fasten the extreme rafters to the spacers, pull the cords in the upper and lower parts of the ramp, and then put the rest of the legs along them. The rafters should be temporarily fastened from the inside of the roof until the waterproofing and counter-batten with lathing are installed on the outside..
The best choice for waterproofing cold roofs is a film with microperforation. This is one of the cheapest and easiest materials to install, which completely solves the problem of condensation. During installation, a roll of film is rolled across the slope of the ramp, starting from the bottom. Each subsequent sheet should be laid with an overlap on the bottom one with a width of about 70 mm.
Installation of the hydro-barrier is best done in calm weather. Temporary fastening is performed with staples to the upper edge of the rafter legs, while it is necessary to adjust the force of the stapler spring so that the staple does not press close to the film, pushing it, but forms a gap of about 0.5 mm. The film must be fixed in such a way that a sag of about 20–25 mm forms between the rafters. This will prevent water from soaking the counter grill, collecting at the lowest point..
When the entire ramp is covered with a hydro-barrier, you need to nail in a counter-lattice – slats of the same thickness as the rafter legs. Before nailing the rail, the side with which it is adjacent to the rafters can be coated with any alkyd enamel. Sawn materials for counter-lattice and lathing should be pre-treated with fire-bioprotective impregnation. It is recommended to fasten the counter-grill either with brushed nails or anodized screws. Black hardened self-tapping screws are not suitable for this purpose..
The counter grille performs several functions at once. Firstly, it reliably presses the hydraulic barrier, while blocking punctures in the attachment points. Secondly, due to the increased distance between the coating and the waterproofing, a path for an upward air flow appears, due to which the formed moisture evaporates quickly.
Installation of lathing
The type and method of installation of the transverse battens depends on the roofing used. Slate, profiled sheet, metal and bitumen shingles are well suited for cold roofing. It all depends on aesthetic preferences and the slope of the slopes..
In total, two types of lathing for a cold roof can be distinguished: solid and plank. Solid sheathing works well for soft tiles and materials such as profiled sheeting that make a lot of noise in the rain. Tight pressing of the coating to a solid massive base eliminates the unwanted acoustic effect and greatly facilitates the installation of the roof.
Continuous crate is made either from OSB of 3 and 4 classes, or from moisture-resistant plywood on phenol-formaldehyde resins. The minimum plywood thickness is 8 mm with a rafter leg pitch of 60 cm. OSB has a looser structure, for reliable fixation of fasteners in it, the thickness must be at least 12 mm.
It is recommended to install a solid crate, starting from the front gable, laying solid sheets and shifting the extensions to the least visible part of the slope. It is imperative to leave a deformation gap of about 10–15 mm between the plates. If the joint between the slabs is not in the area of the rafter leg, fastening through a backing strip 20-25 mm thick is allowed according to the same principle as fastening horizontal joints.
When all sheets are secured, the joints must be filled with non-hardening bituminous mastic or a special roofing sealant. After sealing, it is recommended to check the sheathing plane against the tensioned cord, if necessary, installing pads of different thicknesses in the main attachment points.
The device of the board crate is much simpler. Both edged and unedged boards are suitable for it, it is only important not to use materials that are not calibrated in thickness. Sheathing boards are nailed perpendicular to the rafters. In this case, two factors must be taken into account:
- The main boards, to which the covering of the selected format is attached, should be located on the overlap line between the sheets. For these purposes, it is recommended to select the widest boards.
- In the range between the main boards, several additional ones should be installed, taking into account the degree of filling of the crate recommended by the manufacturer and the number of intermediate rows of fastening.
In the case of choosing in favor of an unedged board, it must be placed with a wider plane upward. It is better not to use screws and self-tapping screws for fastening, ordinary nails are better than any other fasteners.
Ventilation of a cold attic
The last issue in arranging a cold roof is the ventilation of the attic. It should be adjustable so that in the summer heat at the level of the attic floor the temperature is close to the air temperature, and in the event of a sharp cold snap there is no abundant condensation. In winter, the ventilation is shut off, which makes it possible to exclude intensive blowing of the insulation and save as much heat as possible..
As a rule, in a private house, an entrance door to the attic and one openable window in the upper part of the pediment on the opposite side is sufficient. If the roof is hip, a small ledge must be made under the window.
For roofs over 300 m2 organization of additional vents is recommended. They cut into the top of the ramp and help remove air entering the attic in the Mauerlat area. To ensure full air circulation, perforated panels should be inserted every 70–80 cm when soffits are sewn up. You can avoid installing air vents when covering the roof with a material with a wavy profile. On the line of convergence of the slopes, you need to leave a gap of about 100-140 mm, which is covered with a wide ridge.