Recently asked by a new customer about solving a problem with a leaking wood siding-style chimney. This chimney was leaking from the trim board joints.
Under most circumstances, high-quality caulking will do the job, but you see it as just one option. Some other options would be to install one of two types of crowns (or chimney tops).
- measure, then shape bend, and apply a metal crown flashing or
- apply a cemented crown covering material that could further be protected with a weather seal coating.
Now when it comes to just using caulking to fill suspect water entry points (without adding anything additional to the top), you will have two basic quality choices.
This type of caulk is an excellent choice for most porous surfaces because of its flexibility and adhesion properties, ease of use, and water cleanup. Shorter-lived products won’t expand and contract as well. It can bridge gaps of up to 1/2 inch wide and deep, though it will usually require two applications or the use of a foam backer rod to do this effectively.
Elastomeric caulk
Elastomeric caulk can expand and contract as much as 200% and is claimed to last as much as 50 years by some suppliers/manufacturers.
Elastomeric caulk dries quickly, so you need to tool (that is, press in with a rounded corner plastic or metal tool) this caulk immediately after application.
The reason for this recommendation is that Elastomeric caulking is very long-lasting, which is what you want in a situation where you don’t want repeated repairs every few years but every few decades. Also, know that rain, snow, or other inclement weather will affect the effectiveness during product application.
Polyurethane caulk
Polyurethane caulks can also be painted. It’s flexible and weather resistant. Although it is more expensive than the more common caulk types, it is a standard of the construction industry for outdoor use.
The reason being is that they have superior bonding ability to the extent that they can be used as an adhesive in certain applications.
Because of this, polyurethane caulk works well in joints between dissimilar materials, for example, window sill plate joints of wood to concrete, joints between flashing and masonry around chimneys, and joints in driveways and concrete slabs between different masonry types.
Furthermore, polyurethane caulks are the most expensive and more difficult to apply than latex, butyl, or elastomeric type caulking. Yet, on the other hand, polyurethane lasts longer than latex and butyl caulking. It can cover a wider gap (up to 3/4 inch) and will stretch farther. It takes about 2 days for the outside to skin over, about 1 month to cure fully, and it can last more than 20 years.
Another issue is that not only must the weather be good (not rainy or snowy), but the wood surfaces must be completely dry, with a maximum of 18% surface moisture content. If rain or other inclement weather is imminent, the caulking products should not be applied because to become effective, they will need several days to cure and bond. Further notes on caulking a leaking chimney are:
- Apply caulks when the temperature is between 40°F and 100°F.
- Clean the surface: vacuum or wipe dust and dirt, then remove any oils or surface deposits with a suitable solvent.
- Use a backer rod to limit the joint depth to half the joint width.
- A tool with a spoon or other rounded instrument to completely fill the joint and force the caulk against the sides.
The general rule of thumb is if you have a crack or joint that exceeds one-half-inch by one-half-inch, you should use a foam backer rod.
