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As a heat conductor, zinc makes roofs particularly sensitive to condensation in case of reported insulation. It is then necessary to increase the ventilation of the under-roof air space. For that, it is enough to put down flats.

put a zinc flap

Hot air has the capacity to store more moisture than cold air. When the air inside a building is in contact with the roof, and if the outside temperature is colder, a condensation phenomenon appears. This is called the "dew point". Water vapor in the air condenses into droplets on the surface of the nearest materials. This natural phenomenon is visible especially in winter, when there is a big difference between the outside temperature and that of a heated house.

Raise the "dew point"

zinc hatch

Airing each room daily renews the ambient air. The drier outside air replaces the indoor humid air, limiting the risk of condensation, especially in cold weather. When the attic is isolated under crawling or at the right of the high floor, the water vapor must necessarily be limited. Otherwise, it degrades the insulation, the frame, then the lining.
To counter this phenomenon, it is necessary to ventilate the underside of the cover to raise the dew point and neutralize any condensation or at least delay the appearance.
The installation of a ventilation accessory, called a flap, on a zinc roof is mandatory (DTU 40.41). For one simple reason: Zinc sheets are placed continuously on the trestle so that the cover is completely airtight. These roofs have an air gap under the support, 40 mm thick, or 60 mm if the length of the ramp is greater than 12 m.
The role of the gates is to ensure good air circulation in this ventilation space.

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A calculated distribution

When insulation work is undertaken, ventilate the air space under the roof. The number of doors is dependent on the position of the insulation.
In the case of an insulation under crawling, it is necessary to count a door for 25.5 m ² of roof surface projected horizontally. An air inlet is mandatory at the bottom of the ramp. It is usually done through a ventilated sewer strip.
The catches ensure the air outlets at the top.
In the case of thermal insulation on the floor of a lost roof, a roof hatch is required for 42.5 m² projected roof area. They are distributed over the entire cover, ensuring the air inlet at the bottom and the air outlet at the top. Having a higher number of doors in the lower part of the roof will put the height in relative pressure and prevent the entry of dust and powdery snow.

take the measures of the pet door

Present the cat flap at the desired location and draw its contours.
Then draw a second contour, 20 mm inside the first trace, to define the zinc readings that will be folded.

zinc notch

Using a zigzag claw, cut the zinc at the interior marks.
Use a ruler to make straight cuts.

raised around the opening

Using a zinc shear, cut the corners 10 mm wide.
This cutout makes it possible to take the reading around the opening.

straighten the edges of the flap

Form the three sides of the triangle with flat pliers.
Straighten each edge to obtain an angle of approximately 45°.

ensure ventilation under the roof

Cut out the trench under the zinc sheet (here with a handsaw).
The opening in the zinc sheet creates the air supply needed for good ventilation under-roof.

perfect sealing

Use the zinc drop to cut three triangular sections (called "gussets").
They are intended to close the angles to complete the seal.

solder the dish of zinc

Adjust the folding and cutting of the gussets.
Solder them on the zinc plate to the zinc-plated tip, with a tin bolt.
Perform the welding while progressing from the bottom to the top.

application of a stripper

Place the cat flap on its landmarks.
Brush the stripper on the zinc sheet before welding.

solder the boiler

To solder the cat flap, start at the corners and then solder the lengths.

Tips for DIYers

- The opening made in the zinc sheet, at the location of the flap, must have a reading of 10 mm (or 50 mm if the slope is between 2.5 and 5%, in the case of curved roofing). This statement must not be in contact with the grid to prevent rain penetration.
- The models of cat flap to be favored are those with an outwardly inclined grill more effective against infiltrations and the passage of insects.


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