Chimney crowns, often referred to as chimney caps or top plates, are a crucial element in chimney construction. They play a vital role in preventing rainwater from entering the chimney, which can cause damage and reduce the chimney’s lifespan. Constructing an effective chimney crown is essential for anyone in the construction industry, as it ensures the chimney’s durability and functionality. In this comprehensive guide, we will delve into the construction procedures for concrete chimney crowns, offering detailed insights into both cast-in-place and precast chimney crown construction methods. By understanding and implementing these techniques, you can create chimney crowns that stand the test of time and protect chimneys from the elements.
BIA Recommendations for Chimney Crowns
Before we delve into the construction procedures, it’s crucial to understand the key recommendations provided by the Brick Institute of America (BIA) for chimney crowns. These recommendations serve as a foundation for building durable and effective chimney crowns:
- Size and Overhang: The chimney crown should extend a minimum of 2.5 inches beyond the chimney wall, with a drip edge provided at the overhang’s bottom. This extension is essential to ensure that rainwater is directed away from the chimney structure.
- Thickness: The thinnest part of precast concrete chimney crowns must be a minimum of 2 inches. This thickness provides the necessary structural strength.
- Material: Chimney crowns should be constructed from concrete only, and mortar should not be used. Concrete is a robust and durable material for chimney crowns.
- Reinforcement: In the case of cast-in-place concrete chimney crowns, reinforcement is necessary to prevent cracking and enhance structural integrity.
- Slope: The top of the chimney crown should be sloped downward from the flue liner towards its edge. This slope allows rainwater to flow away from the flue liner.
- Separation: The chimney crown should not come into direct contact with the flue liner. A sealant must be applied between them to prevent water infiltration.
- Bond Break: To accommodate the expansion and contraction of both materials, a bond break must be provided between the chimney crown and the masonry chase.
- Height Difference: Maintain a distance of 2 to 6 inches between the top of the flue liner and the top of the chimney crown. The flue liner should be higher than the chimney crown to prevent water from seeping in.
Construction of Concrete Chimney Crowns
Now, let’s explore the construction procedures for both cast-in-place and precast concrete chimney crowns in more detail. Understanding these methods is crucial for constructing effective chimney crowns that withstand the test of time.
1. Construction of Cast-in-place Concrete Chimney Crown:
a. Form Assembly: The process of constructing a cast-in-place chimney crown begins with assembling forms made of wood or steel. These forms should align with the required crown dimensions.
b. Drip Edge: It is highly recommended to use forms with built-in drip edge formation arrangements. If the form doesn’t include drip edges, a 2-inch lumber stick should be provided in the form, or a drip edge can be created using a masonry saw after form removal. The drip edge is crucial for preventing water from entering the chimney chase.
c. Flue Liner Protection: To protect the flue liner, cover it with compressible materials such as cardboard, fiberglass, or metal sleeves. These materials create a gap between the crown and the flue liner, allowing for expansion.
d. Metal Plate Installation: Cover the top of the chimney masonry chase with flat metal plates of the required thickness. These metal plates serve as a bond break between the masonry and crown and provide support for the concrete to be poured.
e. Reinforcement: Incorporate steel bars or welded wire mesh as reinforcement to prevent concrete from cracking. This reinforcement ensures the structural integrity of the chimney crown.
f. Concrete Pouring: Once the forms are in place, pour the concrete and compact it thoroughly. Use hammer blows on the sides of the form to ensure proper compaction, and smooth the surface using a trowel.
g. Curing and Sealant: After pouring the concrete, allow it to cure for at least 2 days. During the curing process, it’s essential to protect the concrete surface from the evaporation of water. After curing, seal the gap between the flue liner and the crown using silicone or butyl caulk to prevent water infiltration.
2. Construction of Precast Concrete Chimney Crown:
a. Hoisting: Precast chimney crowns are hoisted to the required height using suitable equipment. Careful handling during this process is crucial to prevent any damage to the precast crown.
b. Installation: When installing the precast crown, ensure that it does not come into direct contact with the masonry or chimney chase. This is a key principle in preventing potential issues related to expansion and contraction.
c. Bond Break and Mortar: Provide a bond break between the chimney crown and the chimney chase to accommodate expansion and contraction. Use mortar to create a stable bed for the precast crown, ensuring it remains in place.
d. Gap and Insulation: The gap between the flue lining and chimney crown should be filled with compressible materials, such as fiberglass. These materials serve as both an expansion joint and insulation for the flue lining, which is essential for maintaining proper temperature conditions inside the chimney.
e. Sealant: After filling the gap between the flue lining and chimney crown, seal it using silicone or butyl caulk. This step is critical in preventing water from infiltrating the chimney.
In-Depth Exploration of Cast-in-place and Precast Chimney Crown Construction:
Let’s take a closer look at each construction method and gain a deeper understanding of the procedures involved.
Cast-in-place Concrete Chimney Crown
Cast-in-place chimney crowns are a suitable choice for larger chimneys. They offer the advantage of customizability and can be adapted to different chimney sizes and shapes. Here is a more detailed breakdown of the steps involved in constructing a cast-in-place concrete chimney crown:
a. Form Assembly: As previously mentioned, assembling forms is the first step. The choice of forms depends on the specific requirements of the chimney, but both wood and steel forms are commonly used.
b. Drip Edge Importance: The presence of a drip edge in the form is crucial. It ensures that the overhang of the chimney crown is equipped to redirect rainwater away from the chimney structure. If the form lacks a built-in drip edge, a 2-inch lumber stick can be added, or a drip edge can be created after form removal using a masonry saw.
c. Flue Liner Protection: Protecting the flue liner is essential. To do this, cover it with compressible materials such as cardboard, fiberglass, or metal sleeves. These materials create a gap between the crown and the flue liner, allowing for the expansion and contraction of materials in response to temperature changes.
d. Metal Plate Support: The top of the chimney masonry chase should be covered with flat metal plates of the required thickness. These plates serve a dual purpose: they act as a bond break between the
masonry and the crown, and they provide a support system for the concrete that will be poured.
e. Reinforcement for Durability: To prevent cracking and ensure the structural integrity of the chimney crown, reinforcement is necessary. Steel bars or welded wire mesh are commonly used to provide the required reinforcement.
f. Proper Concrete Pouring: Pouring the concrete is a critical step. The concrete should be poured carefully and compacted thoroughly to eliminate voids and ensure a solid structure. To achieve this, use hammer blows on the sides of the form.
g. Surface Smoothing: After the concrete is poured, smooth the surface using a trowel. This step is essential to create a flat, even surface that is both functional and aesthetically pleasing.
h. Curing Process: The concrete must be allowed to cure for a minimum of 2 days. During this time, it’s essential to protect the concrete surface from the evaporation of water. Proper curing is crucial for the concrete’s strength and durability.
i. Sealing the Gap: After curing, seal the gap between the flue liner and the chimney crown using silicone or butyl caulk. This prevents water infiltration and ensures that the chimney remains watertight.
Precast Concrete Chimney Crown
Precast concrete chimney crowns are an excellent choice for smaller jobs and offer certain advantages, such as ease of installation. Let’s delve deeper into the steps involved in constructing a precast concrete chimney crown:
a. Hoisting the Precast Crown: Precast chimney crowns are typically cast off-site and then hoisted into position using suitable equipment. Careful handling during this process is crucial to prevent any damage to the precast crown.
b. Installation with Caution: When installing the precast crown, it’s essential to ensure that it does not come into direct contact with the masonry or chimney chase. This principle is crucial to prevent potential issues related to expansion and contraction, which can occur due to temperature changes.
c. Bond Break and Mortar Bed: Provide a bond break between the chimney crown and the chimney chase to accommodate the expansion and contraction of materials. Use mortar to create a stable bed for the precast crown, ensuring it remains securely in place.
d. Gap Filling and Insulation: The gap between the flue lining and the chimney crown should be filled with compressible materials, such as fiberglass. These materials serve a dual purpose: they act as an expansion joint, allowing for material movement, and they provide insulation for the flue lining, which is crucial for maintaining proper temperature conditions inside the chimney.
e. Sealant Application: After filling the gap between the flue lining and chimney crown with compressible materials, it’s important to seal it using silicone or butyl caulk. This final step ensures that the chimney crown remains water-resistant.
Understanding the Differences Between Cast-in-place and Precast Methods:
It’s important to note that the choice between cast-in-place and precast methods depends on several factors, including the size of the chimney and the specific requirements of the project. Let’s take a closer look at the differences between these two construction methods:
Advantages of Cast-in-place Concrete Chimney Crowns
a. Customizability: Cast-in-place crowns can be tailored to fit various chimney sizes and shapes. This level of customization ensures a perfect fit for each project.
b. Structural Integrity: The inclusion of steel reinforcement in cast-in-place crowns enhances their structural integrity and resistance to cracking.
c. Drip Edge Adaptation: The use of forms with built-in drip edges or the addition of drip edges after form removal ensures effective water diversion.
Advantages of Precast Concrete Chimney Crowns
a. Ease of Installation: Precast crowns are easier to install, making them suitable for smaller jobs. They require less on-site labor and can be hoisted into position with relative ease.
b. Uniformity: Precast crowns are manufactured off-site, ensuring a high level of consistency and uniformity in their construction.
c. Efficiency: The precast method is efficient and can save time on the construction site compared to the in-situ casting of a chimney crown.
d. Suitability for Small Jobs: Precast crowns are ideal for smaller chimney projects, where the advantages of easy installation and efficiency are particularly valuable.
e. Lower Labor Costs: The simplicity of installation means that precast crowns often require fewer labor hours, reducing labor costs.
Choosing the Right Method
When selecting the construction method for a chimney crown, it’s essential to consider the specific needs of the project. Larger chimneys with unique shapes and design requirements may benefit from cast-in-place crowns due to their customizability and added reinforcement. On the other hand, smaller jobs or projects with budget constraints may find precast crowns more efficient and cost-effective.
Both methods, when executed correctly, can result in highly functional and durable chimney crowns. The key is to adhere to the recommended guidelines and best practices while taking project-specific factors into account.
Conclusion
Constructing concrete chimney crowns is a critical aspect of chimney construction in the construction industry. Adhering to the recommendations provided by the Brick Institute of America (BIA) and choosing the appropriate construction method, whether cast-in-place or precast, ensures the chimney’s durability and functionality. A well-constructed chimney crown effectively protects chimneys from rainwater infiltration, enhancing their longevity and performance.
In this comprehensive guide, we have explored the BIA recommendations for chimney crowns and provided a detailed breakdown of the construction procedures for both cast-in-place and precast concrete chimney crowns. Understanding these methods and their advantages can help professionals in the construction industry make informed decisions when it comes to chimney crown construction.
By following these guidelines and best practices, you can ensure that the chimney crowns you construct are not only structurally sound but also capable of withstanding the elements for years to come. Whether you’re working on a large chimney with complex design requirements or a smaller, more straightforward project, the principles outlined in this guide will serve as a valuable resource for achieving success in chimney crown construction.