The point of contact between the chimney duct and the roofing is the part of the roof most exposed to leakage. Therefore, it is necessary to ensure the most precise and professionally performed chimney flashing. How to perform a tight connection between the chimney and the roof? What materials to choose?
Neat chimney flashing, performed by an expert using high-quality materials, will minimize the risk of slits appearing around the chimney. The most popular method of waterproofing a chimney is a metal sheet flashing. What types of metal sheets are used for flashings and what roofing will they fit to?
Material for chimney flashing
For metal chimney flashing, flat, coated sheets made of aluminum, steel and copper are used, both in galvanized and zinc-titanium versions. Sheet metal waterproofing can be used for any type of roofing in the form of ceramic or steel tiles, sheet metal and bituminous shingles. Sheet metal on such roofs will be an effective and durable chimney waterproofing, and at the same time will allow to maintain the aesthetic continuity of the roofing.
Some roofers prepare the individual elements needed for chimney flashing themselves. However, the ready-made waterproofing components make their work easier. GZK chimney flashing system products from Fakro are a good example. The system flashing made of aluminum sheet can be used in a roof with a slope between 15 and 70 degrees, finished with any type of roofing. Simple and quick installation as well as high durability and resistance to changing weather conditions speaks in favour of it.
How to perform a tight chimney flashing?
Professionally performed chimney flashing ensures full tightness of the roofing. Metal sheet chimney duct waterproofing starts with preparing the space around the chimney and flashing elements, followed by installing and waterproofing the chimney flashing.
- Preparation for chimney flashing
Before a chimney duct is properly waterproofed, it is necessary to protect it with a layer of pre-coating. For this purpose we use a roofing membrane or tar paper. This material should be stretched on a chimney duct, starting from the front of a chimney, then on its sides and finally on the back, that is according to the direction of rainwater flow. The membrane or roofing paper is fixed to the chimney with a tape that is resistant to high temperatures.
- Preparation of flashing elements
Four elements, i.e. the front, rear and two side strips, are cut out of the sheet metal for chimney flashing. Depending on the height of the chimney, we may need one or two strips of sheet metal. When adjusting the measurements of the flashing elements, it should be taken into consideration that the parts situated on the sheathing must be at least 10 cm away from the eaves and 20 cm from the ridge. Vertical parts, on the other hand, should protrude over the roofing at least 15 cm.
- Installation of a chimney flashing
Prepared fragments of a metal sheet are attached to a chimney and roofing, and also form the so-called standing seam from connecting metal sheets together. During the assembly we use roofing screws, which have a rubber washer. We start the chimney flashing from its front, bottom part, then the sides and back – this way the rainwater will be prevented from getting inside.
- Sealing a chimney flashing
All connecting points are sealed very carefully with special roofing silicone. The use of this type of silicone guarantees a tight and heat-resistant connection of the chimney duct to the roof.
What mistakes should be avoided during chimney flashing?
Improperly performed chimney flashing may result in leakage or heat loss through the gaps created at the connections between the chimney duct and the roof. The most common mistakes occur already at the stage of preparing the space around the chimney. The pre-coating layer not adhered to the chimney and left without being rolled up may cause leaks. It will also be a big mistake to use inappropriate materials, e.g. using cement mortar instead of roofing silicone. Roofing silicone is not only resistant to high temperatures, but also flexible, which is very important for a constantly working chimney and roof. In order to ensure tightness and durability of the connection between the roof and the chimney, it is worth choosing high-quality materials. Therefore, it is a mistake to save on a chimney flashing. Waterproofing the sheet metal chimney is a difficult job, and it is not worth using the cheapest materials and employing unreliable or cheap specialists to do it.