Chimney Crown Replacement
Chimney crown replacement may be necessary if your chimney crown, also known as the top plate or a chimney cap, is cracked or damaged.
The crown plays a vital role in the protection of your chimney by sealing off the top portion of your chimney and shedding water away from the masonry construction.
Why do a lot of chimney crowns fail over time? Many crowns are poorly constructed right from the start.
Crowns built out of thin mortar without proper reinforcement will eventually crack over time due to temperature changes and weather conditions.
As the chimney crown deteriorates it causes two things. First, it allows moisture to run down into the masonry structure of the chimney causing spalling on the exterior bricks. This moisture also causes the bricks to separate and eventually fall apart.
Second, water can run between the crown and the chimney flue causing damage to the clay tile or even worse, causing damage to the chimney damper, smoke chamber, or the fireplace itself.
Building The Perfect Chimney Crown
Proper chimney crown replacement requires a few important steps which will substantially increase the life of the crown. Here are a few details to consider:
Do not make the crown flush with the outside chimney wall. The crown should extend at least 2” past the outside wall and contain a drip edge underneath.
Use concrete for construction not mortar. Concrete is stronger and can withstand temperature fluctuations and weather exposure better than mortar. For extra strength, 1”x 1” wire mesh should be placed in the concrete during the pour. Lay the wire mesh half way through the pour so it’s in the middle of the concrete. Cut the mesh slightly smaller than the crown so it’s not exposed to moisture which will lead to rust, possibly causing the crown to crack.
The crown should be at least 4” thick and have a slight outward slope to shed water away from the chimney flue.
The concrete crown should not come into contact with the flue. During construction the flue should be wrapped with a compression material to allow a 1/4” separation between the concrete crown and the flue tile.
Form a bond break. Use steel support bars to form a grid pattern from one side of the masonry wall to the other. Then lay sheet metal or cement board across the metal supports to form a thin bond break between the chimney crown and the masonry walls.
This serves two purposes. First, without the barrier the cement will just run down the chimney structure filling the airspace between the flue and the masonry walls. Second, it allows the masonry walls and the concrete crown to expand and contract at different rates, reducing the possibly of them cracking.
When building the bond break don’t use plywood or other similar materials. Wood is not only flammable, but it will soak up moisture which will expand when it freezes causing the crown to crack.
The top of the flue liner should extend at least 2” past the top of the crown but no further than 6”. Check your local building codes to see what’s recommended in your area.
Chimney Crown Replacement – Forms
Since you’ll be pouring concrete to construct the crown you’ll either have to build your own form out of wood or purchase one.
Building a form is often time consuming and dependent on your carpentry skills. To make things easier you can purchase adjustable steel forms or foam forms that can be cut or adjusted to fit your chimney.
Plus, they’re reusable and should last many years if properly cared for.
The downfall of using steel forms is the cost.
Many forms are sold in pairs meaning you’ll need to purchase 2 pairs to create a 4 sided form.
Many homeowners will only use the form once so the cost of a steel form may not outweigh the convenience they offer.
Plus, they usually have a built in drip edge and aesthetic shape.
Foam forms offer convenience without spending a lot of money.
To use one, the foam must be cut and shaped to the dimensions of your chimney.
Overall – Chimney Crown Replacement
If done properly, chimney crown replacement is a project that most homeowners can complete on their own saving you hundreds of dollars in labor.
Plus, your new chimney crown will effectively shed water and protect your chimney and fireplace from serious damage for many years.