Masonry Chimney Water Leaks

March 29, 2022
William Hussel

This article was written by Bill Hussel, owner of Hansen And Sons chimney and fireplace Madison Wisconsin. Bill is an experienced Chimney business owner for the last 40 years with an engineering degree and holds a rare federally issued 1998 Patent covering “Chimney Moisture Seals”. This an educational white paper that will cover the various aspects of Chimney water leaks, their source of origin, and their repair solutions.

I.  Internal vs External Water Leaks

To begin, there are two major categories of water leaks: internal water leaks and external water leaks. Certainly the most well known category of water leak is the external water leak meaning the water enters the chimney from the OUTSIDE of the home.  Water filters through or along the chimney exterior with evidence of water dripping or moisture on the inside of the home near the chimney.  The second category of water leaks is internal water leaks which means the water is coming from a source INTERNAL to the home and is usually water formed during the combustion process of gas or fuel oil - usually the furnace or water heater. The combustion moisture condenses on the inside of the of the cold chimney flue, leaks out of the chimney system and forms moisture on the inside of the home usually soaking through drywall or plaster near the chimney. This latter source of water leak is commonly misdiagnosed and expensive external repairs to the roof or chimney are often made and do not solve this internal moisture problem.

II. Top Chimney Crown

With respect to external water leaks, probably the most common single external source is from the chimney top crown. This is a vulnerable area of the chimney because it is horizontal which allows water and snow to remain in place for longer periods of time – compared to a vertical surface - which allows absorption into the masonry surface causing cracks and deterioration due to freeze thaw cycles. The other common cardinal sin with regard to chimney crowns is that this is the last portion of the chimney constructed by masons building the original chimney and  is usually hastily put together with incorrect designs and incorrect materials. The Crown must be made of concrete which is a hard and more durable/stronger  product and not mortar which is used to cement brick or stone units together. The Wisconsin Dwelling code requires that the chimney crown be a minimum of 2 inches thick  have a 1 inch minimum overhang with a drip groove to shed water away from the vertical sides of the chimney. Rarely is this full design used on chimneys. Therefore, once deterioration occurs, water can easily enter the cracked and deteriorated  areas of the crown and easily enter the home.

III. Metal Flashing/Saddles

Probably the next most common source of external water leaks Is due too improper flashing design or flashing deterioration along with improper designed or lack there-of saddles.  Basically, Metal chimney flashings are the portion of the chimney between the chimney and the roof shingles installed on all four sides at the chimney base.  The bottom portion of the flashing is made of two parts, one of which is interwoven with each row of shingles called step flashing, with the second outer flashing (call counter flashing ) covering the step flashing with the top edge inserted into the chimney structure. As mentioned, leakage can occur due to improper design of the flashing which fails to channel the water away from the chimney structure or deterioration of the flashing or the sealant used at the top edge of the flashing. The saddle is a critical part of the overall chimney design, is required on all chimneys more than 30 inches wide (that side facing the roof ridge) and is used to divert water to both sides of the chimney without allowing the water to collect and dam up behind the chimney. Roofers or flashing installers are well known for skipping this critical item because it is difficult and labor-intensive to install.

IV.  Mortar/Bricks/Stone

The next most common cause of external water leaks is the deterioration or improper construction Using mortar to cement the bricks or stone together.  Certainly the deterioration or cracking of mortar and/or bricks/stones will allow water to easily enter into the chimney and later leak into the home. Even though tuck pointing is a common solution by simply grinding out the top half inch to three-quarter inch mortar and replacing with new, this procedure can not be if the entire bed of mortar between the outside units is deteriorated. If this is the case, the chimney must be disassembled and rebuilt. A lesser known cause of exterior water leaks in the sides of the chimney is where the installation technique was improper and a solid 4 inch bed of mortar was not installed completely across all masonry units. If an improper amount of mortar is used, water will usually penetrate the masonry surface  and leak into the home. Also, the deterioration of brick or stone can also allow moisture to penetrate through the masonry units.  Many times if the chimney is in good solid construction condition and no cracks or  deterioration needs to be repaired, the water leak can be eliminated with the use of Chimney Saver TM, a special formulated and patented chimney waterproofing material designed specifically for waterproofing chimneys.

V.  Top Flue Tile/Rain Cap

Next most common source of external water leaks in chimneys is the very top flue tile cracked or deteriorated thereby allowing water to easily enter and into the home. The Wisconsin Uniform Dwelling code requires that a “slip joint” be installed between each flue tile and the top concrete crown to allow movement of the flue tile without stressing the concrete crown there by cracking the flu tile and/or crown. Rarely do masons ever include this important item in their Chimney construction. Also of critical importance is installation of a stainless steel chimney rain cap to shed water away from the interior area of the flue tile yet allowing combustion gases to safely escape. The top chimney rain cap should also have a heavy duty animal screen to prevent critters from entering the chimney and or home.

VI.  Corbeled Chimney Shoulders

This area of the chimney is lesser known for external water leaks, but can also be a serious problem. This is the section  of the chimney where the width is gradually brought in.  The exterior water leakage can occur due to deterioration or improper design of this area.  Certainly the best design for reducing the width of an external Chimney is to use a one piece sill stone with 1” overhangs on all sides - to cover the reduction area. Brick corbeling is not a good long-term solution because bricks should only be used in constructing “vertical” surfaces, not horizontal nor sloped. Certainly corbeled areas can be repaired with mortar and chimney saver for a lower cost solution, but using one piece sill stone is a better longer-term solution.

VII.  Foundation

For fireplaces located in lower living areas or basements below ground level, water leakage is Common whereby the exterior foundation of the home is deteriorated or cracking and allows ground water to seep into the home. The easiest way to differentiate between water leaking through the foundation vs the chimney above, is to use a garden hose to flood the area of the ground outside the fireplace location and determine if water leaks into the home. If needed, type repair is usually best performed by a basement foundation repair firm.