Suggested Guidelines for Watertight Chimney Construction

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Suggested Guidelines for Watertight Chimney Construction

Suggested Guidelines for Watertight Chimney Construction

A chimney is one of the most vulnerable parts of a structure because it is exposed to weathering on all sides. If the proper steps are not taken, both on the drawing board and in the field, water can enter the chimney and cause efflorescence and eventually freeze thaw damage. However, if industry guidelines are followed, chimneys can be designed to be watertight for decades. As with most forms of masonry construction, steps should be taken to prevent moisture damage. Two illustrations are referenced to here. They are available for hyperlinking on this page but are not threaded in because of their size. Click on figures to view them full size.

Roof / Chimney intersection The intersection between the chimney and the roof is a critical spot where several trades have to come together to provide a watertight joint. Base flashing should be installed on top of the roof sheathing, extending horizontally under roof shingles for a minimum of 4 inches. As shown in figure 1, flashing should be turned up a minimum of four inches before it is inserted into a raked mortar joint and caulked. As shown with the dotted lines, the stepped flashing segments should be lapped at least three inches and sealed. Prefabricated corners are also suggested to properly protect the intersection.

Chimney Cap Look at figure 2 for a graphical representation of how a cap is constructed. The chimney cap should be designed to keep moisture from entering the system. Caps should not be constructed as a mortar wash, which is simply parging the surface with mortar. The chimney cap should be made with cast in place concrete, precast concrete or stone. The chimney cap should be sloped away from the flue to direct water out of harms way. The cap should overhang the chimney wall at least two inches and should have a drip edge cast into the overhang. Rain water is now directed off of the top of the chimney and drips off of the overhang. Chimneys can also be capped off with brick arches. Metal chimney caps is another way but personally i don’t like metal caps they blow off and can be quite noisy when it is raining.

Because the concrete cap and clay flue liner are different materials having different thermal expansion characteristics, there should be a void left between the cap and the flue. This void should be filled with a compressible material and caulked. This allows both materials to move independently, while the sealant keeps moisture out.

All of the measures discussed above are focused on keeping water out of the chimney. Although these measures can protect the chimney from moisture, they are not foolproof. Some water will still find it’s way past the cap. For this reason, through wall flashing should be placed under the cap as a second line of defense against moisture penetration. The flashing is adhered to the flue liner, extends horizontally under the cap and is extended beyond the face of the wall.

There are several kinds of Chimney Caps. One option is to use an old chimney crown which are installed in europe. They look like chess pieces. Another way to cap a chimney is to install top damper which can fit over a clay liner it is important to make sure the damper is as big as possible so that you don’t choke the fireplace. These dampers help stop down drafting, animals and rain from coming down the chimney. Prefab concrete caps are also used. Screens can be installed to keep animals out and sparks from blowing on to the roof.

Another type of capping is concrete slabs which can be either formed and poured or pre-cast with a drip edge over hanging the chimney to keep the water off the top of the chimney and running down the flue. When the top of the chimney does not have a cap moisture soaks into the cement collar and the frost causes the cement to expand and contract allowing the cement collar and the brickwork to loosen up. Chimney capping is very important.

If you need a dry, leak-proof chimney built for you, or you have problems with a leaking chimney, or you need chimney leak repair in Vancouver, contact us.

This article is helpful for those having the following problems:

  • chimney leaks through brick
  • chimney leaks when it rains
  • chimney leaks inside fireplace


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