What to measure before ordering chimney flashings

Mark the eaves side and the ridge side. Chimney flashings behave differently at the front and at the back, because water reaches these places by different routes. When measuring, also note the type of wall finish. Brickwork may require different preparation from an approved cladding of a system chimney. It must not be assumed that the casing can be cut at will.

The front apron drains water onto the covering below the chimney. The side elements lead it along its walls. The rear part takes up water flowing in from the upper part of the roof slope. In addition there is the solution for the top termination at the wall. Chimney flashings should therefore be priced as a detail, not as four random angle sections.

In the order it is worth describing each element and its side. With a wider chimney, additional shaping of the water path behind the obstacle may be needed. This is decided by the design, and the contractor assessing the roof evaluates it. It should not be assumed in advance that a flat strip of the same width is sufficient behind every chimney.

The joints must lead water towards the eaves, without the contacts opening under its inflow. This is a principle that must already be taken into account in the drawing. Merely adding sealant to a wrongly turned lap does not produce a correct solution. Water should also not remain in a pocket formed by an unnecessary fold.

Chimney flashings are joined in accordance with the covering technology. With standing-seam panels, solutions different from those used with the popular tile-effect metal roofing are applied. The offer should therefore clearly state the intended use of the set. The phrase “for every roof” will not replace checking the actual connection.

The chimney and the roof structure may move relative to each other. In addition, sheet metal changes dimensions under the influence of temperature. In many solutions, the part connected to the covering is separated from the upper cover fixed to the chimney. This makes it possible to maintain continuity of the cover without rigidly tying all the parts together.

Such a division should not be regarded as an unnecessary addition to the price. If the design provides for a cover or expansion strip, it has a specific function. The method of fixing and sealing it must be matched to the chimney material. Also agree the requirements for the service penetration, because ordinary external flashing does not replace a proper chimney solution.

Chimney flashings can be made from galvanised, aluzinc and colour-coated sheet metal. The protection of the material should correspond to the conditions in this zone of the roof. Moisture, contamination and contact with adjacent materials must be taken into account. A colour match with the ridge tiles alone does not mean a technically correct choice.

For a coated roof, it is worth comparing a sample of the surface, not just the colour code. Gloss and texture may differ. On flashing with many bends, the permissible working of the material also matters. The contractor should know the requirements for radius and bending conditions so as not to damage the protective layer at the folds.

When pricing chimney flashings, specify whether the scope covers the sheet metal alone, the fabrication of the elements or also the installation. In a renovation, the removal of old flashings and repair of the masonry may be added. These works have a real effect on the cost. Omitting them makes it difficult to compare two apparently similar offers.

In the order, enter the number of chimneys, the dimensions of each and the type of covering. Unusual sheet-metal flashings should be marked with numbers corresponding to the drawings. Barge boards, gutter strips and starter profiles should be itemised separately. This makes it easy to identify which items are chimney elements and which belong to the remaining roof finish.

Before the work is completed, the connections on every side of the chimney should be inspected. The direction of the laps, the condition of the surface and the closure of the top joint are checked. The free flow of drainage behind the chimney and the removal of filings are also important. Photographs of the successive layers make it easier to assess later the places that will be covered.

Chimney flashings require later inspections together with the whole roof. It is worth checking the seals and contamination around the penetration in accordance with the recommendations for the materials used. The purchase should include access to information on maintenance. This is more practical than basing the decision on a general promise of maintenance-free tightness.