Chimney passage through metal roofing

Recommendation points

• What is the roofing system
• How to choose a cut
• Making an opening for a chimney
• Strengthening the rafter system
• Rebuilding the roof pie
• Installation of cutting

Due to the complex shape of the surface, the roof cutting of metal tiles is carried out according to a more complex scheme than for other coatings. We offer you to learn how to correctly execute the passage of the chimney through the roof in such cases, without affecting the functions of the roofing and the supporting system.

What is the roofing system

Unlike ventilation ducts, the passage of the heating chimney almost always requires intervention in the roof truss system. This is due to the fact that due to the action of high temperatures around the chimney, it is required to equip a protective zone in which the presence of combustible materials, in particular wood, is strictly not allowed. What exactly will be the width of this zone depends on the type of chimney.

Strictly speaking, the degree of interference in the supporting system does not depend on its type, but on the place where the pipe passes. There are three most common cases:

1. In the slope of the attic with free access.
2. On a skate.
3. In the slope of a closed attic and other hard-to-reach places.

The main specificity of work here is that in the first two cases, the rafter system remains available for work. If access to the under-roof space is limited, the sequence of actions is reversed: first, the roof covering, waterproofing and lathing are dismantled on a large area, after which the floor is disassembled and the passage zone is arranged. This must be taken into account in advance..

The flue duct passes through the roof as part of its booster section – the final straight pipe section, in which the necessary thrust is created. Therefore, the zone of passage through the roof in the projection exactly coincides with the cutting of the floor, the pipe should not have any turns above. This somewhat complicates the work, because in the rafter system there are convenient places for arranging the passage, but not many and it is not always possible to place the boiler strictly under them.

The most successful circumstance is considered when the pipe passes between two rafter legs at such a distance from the ridge node, where there are no crossbars, no puffs and girders. The lower along the slope, the more work arises in re-laying the roofing, but if the passage is too high, you will have to interfere with the ridge knot and somehow part it, making a gap in the girders. Therefore, if it is possible to shift the place of passage through the ceiling, it is better to think over this moment in advance. The passage of the chimney through the valley or in the immediate vicinity of the roof windows is considered categorically unacceptable..

How to choose a cut

There are two types of roof gaps – for round and rectangular chimneys. In the latter case, the sealed apron is assembled with his own hand from the roofing iron. Despite the laboriousness, this option is considered the most reliable. Cuttings for round pipes are made at the factory, they look like a corrugated cuff with wide brim. For a high-quality and tight connection, the pipe passage must be insulated with two aprons: an internal and an external decorative.

For the passage of pipes of circular cross-section through metal tiles, flexible cuts made of heat-resistant synthetic rubber are used. A more exotic option, predominantly of European production, is composite aluminum cuts. In both cases, the essence of their work is that they repeat the complex profile of the metal tile and are attached to it both mechanically and with a sealing adhesive..

Self-production of cutting requires a certain skill and the simplest set of tools for roofing work – a mallet and a mandrel with a right angle. Both the lower and upper apron are made up of four Z-shaped bars. At the corners, the planks are joined either by a fold or on the folds, which is preferable. Cutting of strips of roofing steel should be carried out without overheating of the edges, in addition, the edges adjacent to the chimney and the roof should have a turn, that is, a single rolling.

Making an opening for a chimney

Fire safety requirements provide for an indent from the chimney body to the elements of the rafter system and the roof pie, made of combustible or reflow materials. The size of the gap is determined by the presence of non-combustible insulation at the chimney duct, the thermal conductivity of which does not exceed 0.3 m²* ° С / W. So, for brick and ceramic wells, the indent should be at least 250 mm, while for a certified sandwich pipe, 130 mm is enough.

Taking into account the dimensions of the chimney, it is quite simple to determine the size of the opening. This is followed by marking the details of the inner lining and lathing. Steam-proof and waterproofing materials are cut with an envelope and wrapped inside the cake. Further, the inner surfaces of the opening should be finished with a waterproofing material that is not sensitive to heat, as a rule, aluminum or copper foil is used for this purpose..

Please note that by this time the metal tile must be dismantled around the passage. The best option is considered to be dismantling starting from the ridge and removing one sheet in each side of the opening. Thus, the passage in the roof must be completely free at the time of installing the chimney. For roofs for metal tiles, the following procedure is characteristic: first, the rafter system is prepared at the place of the pipe passage, then, according to the general scheme, the roof pie is arranged, after which the opening is filled and the covering with cutting is mounted.

Strengthening the rafter system

In the simplest case, the opening is limited by two spacers installed between the rafters. Thus, a square is formed in the roof pie, the walls of which are spaced at a distance not less than the standard. For light metal roofs, a rare step of rafters is characteristic, therefore, usually the horizontal dimension of the opening is excessively large. It can be shortened by inserting a couple more vertical ones between the horizontal struts. This is desirable not only from the point of view of reducing the gap in thermal insulation: even in a cold roof, the opening of the minimum permissible size will cause less difficulties with the installation of non-combustible lathing and fastening the metal tile in place.

In difficult cases, the rafter leg has to be cut. Depending on the height of the passage, a tightening, a crossbar, or several runs are also subject to dismantling. The general method of strengthening the rafter system is as follows: in the place of the cut, the rafter leg is fastened from above and below to the neighboring ones by means of horizontal struts. Further, between these spacers, vertical beams are inserted, replacing the rafter leg at the passage. The same method can be applied to other elements of the support system, located in parallel: purlins or, for example, high tightening.

Reinforcement with retaining struts is rarely practiced. Basically, this is advisable when the slope of the slope is above 35 °, when it is required to cut out sufficiently long fragments of the rafter system. In such cases, a supporting system is arranged around the opening, often it is of a hanging type. To ensure sufficient strength, it is customary to connect opposite legs with puffs on which the racks will rest.

Rebuilding the roof pie

When the opening is prepared, the pipe is fixed in it with steel guy wires. Next, a set of lathing is made from non-combustible materials – a steel corner or a galvanized profile. The step and location of these elements are selected based on the conditions for fastening the metal tile and cutting under the pipe. Often, two horizontally directed inserts are sufficient, which are connected to each other almost closely on both sides of the pipe.

Reverse sealing of the opening is made in the direction from bottom to top. For a cold roof, this stage is skipped: the opening pie is not filled with insulation. Otherwise, a mat of dense mineral wool, cut into two parts, of an appropriate size is inserted into it. Additional fastening of the insulation can be provided with the help of a steel reinforcing mesh lined from below to the wooden spacers limiting the opening. The vapor barrier is not installed in this area, it is replaced with aluminum foil.

Further embedding is carried out from the side of the roof. Half a meter from the chimney, the waterproofing is changed to a sheet of galvanized iron, in which a hole is made along the pipe diameter, expanded to an ellipse in accordance with the slope angle. The edges of the sheet are sealed with bitumen mastic and fixed to the crate with self-tapping screws, in this place the sheet should have an overlap of at least 10 cm over the overall waterproofing.

Installation of cutting

Cutting for rectangular chimneys must have a sealed lower apron and a protective and decorative upper one. For these purposes, the lower prefabricated collar must not only be tightly adjacent to the metal sheet, but also to the brick well itself. To do this, a groove is cut in the masonry along an inclined ring, which, taking into account the seams in the shallowest place, should have a depth of at least 15–20 mm. The height of the groove cutting must be chosen so that in horizontal sections it does not pass along the seam of the masonry. In this case, the inclined sections are cut parallel to the plane of the slope.

The cutting is fastened to the metal sheet and the walls of the well using roofing screws and dowels with a wide plastic mushroom head. Before inserting the curved shelf into the groove, the latter is abundantly filled with silicone or other weather-resistant sealant. The apron is assembled in the following sequence: first the lower segment, then the side ones fixed with 70–80 mm “tongues” rolled at the bottom. The upper segment of the apron is attached last, its edges are wrapped on the sides. All corner joints should be carefully sealed with bitumen mastic, including outside.

The upper baffle is assembled together with the roof covering. First, a row of metal tiles is attached, adjacent to the chimney, after which the lower apron strip is installed. It, like the others, does not need to be fixed in a gutter, rather frequent fixing with small dowels and greasing of thick sealant. After fixing the lower plank, the set of sheets of metal tiles continues to the extreme row, which is entirely adjacent to the side walls of the chimney, that is, the roofing should not be laid higher along the slope. Before proceeding, you need to secure the top strip of the apron, and then the side ones. Thus, the metal tile is attached over the outer groove only in the upper part, and on the sides and bottom it is hidden under the slats, for which a couple of vertical cuts should be made at the higher corners and their edges should be processed.

As for the factory cut-outs, they are fixed in full accordance with the enclosed installation instructions. First, the cap of the required diameter is cut off, after which the cuff is put on the pipe. For the lower and upper cuts, the sequence of actions is the same: bending the flange according to the shape of the surface, laying a bitumen cord and frequent fastening with self-tapping screws. The difference between the more accurate cutting is a higher cap with an angular arrangement in accordance with the slope of the slope.