How to Cap a Chimney

Three Methods:Round Chimney CapSingle Flue Chimney CapCrown (Top) Mount Chimney Cap

Chimney caps protect the inside of your chimney and the inside of your home from weather damage and outdoor pests. Most people prefer to hire a professional when capping their chimney, but you can technically install a cap on your own. The steps you need to take vary based on the type of chimney you have, so make sure that you select the right type of cap before proceeding.

Steps

Before You Begin: Measuring the Chimney

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    Identify your chimney type. You'll need to measure the dimensions of your chimney before buying a cap for it, and you'll need to know which type of chimney you have to do so. Most chimneys can be divided into two basic categories: square masonry chimneys and round chimneys.[1]
    • For square masonry chimneys, you will need to measure the flue—the pipe that carries the flame and smoke outside. Chimneys with one flue can be capped with a single flue chimney cap, but those with multiple flues will usually need a crown mount cap.
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    • Round chimneys are usually made of metal but could also be made with masonry. Either way, you will need to measure the diameter of the chimney pipe before installing a round chimney cap.
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    Measure for a square masonry chimney. There are three subcategories to consider: single flue chimneys with extended flues, single flue chimneys without extended flues, and multi-flue chimneys. The single flue chimney with an extended flue is the most common.
    • Single flue chimneys with extended flues have a flue that extend beyond the crown of the chimney, making it easy to attach the cap with screws. Measure the width and length of the flue from the outside of the chimney.
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    • Single flue chimneys without extended flues are flush against the chimney top, so you will need a cap with a bracket that reaches into the flue and grips into the side. Measure the width and length of the opening itself, not the outer portion of the flue.
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    • For a multi-flue chimney, measure the total outside width and length of the combined flues. You also need to measure the height of the tallest flue. The cap you choose will need to have a screen height at least 5 inches (12.7 cm) taller than the top of your highest flue.
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    Measure for a round chimney. Most round chimneys are made of metal, but there are two sub-categories you need to concern yourself with: solid-pack insulated chimney pipes and air-insulated chimney pipes. Note that the measurements you need for the cap will be the same regardless of the pipe type.
    • Look into the chimney. You should notice a pipe within a pipe (double-wall) or a pipe within two larger pipes (triple-wall). If you see a metal cap or insulation packed in between the pipes, you have a solid-pack chimney pipe. If you see nothing but air, you have an air-insulated pipe.
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    • Measure the diameter of the inner pipe and the diameter of the outer pipe. The inner diameter should be used when selecting the cap, but the bottom ring of the cap should be large enough to cover the diameter of the larger pipe.
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    • If you have a triple-wall pipe, ignore the middle pipe when taking the measurements.
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Method 1
Round Chimney Cap

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    Push the cap inside the flue. Position the cap so that it is centered directly over the inner pipe of the chimney. Push down on the cap, using your body weight to apply extra force, until the cap slides into the pipe.[2]
    • Continue pushing until the solid part of the pipe is completely hidden inside the chimney. The netted or cage-like portion should be elevated above the chimney top.
    • Technically, the installation can stop here. Round chimney caps are primarily held in place by friction and pressure.
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    Consider applying caulk to the seam. If your chimney is made of metal, you should not apply caulk. On the other hand, if the pipe is made from another material, like clay tile, you should seal the joint with weather-proof silicone caulk.
    • Apply a thin, even strip of caulk around the entire perimeter of the cap, filling in the seam.
    • The caulk helps to secure the cap and further prevents moisture from getting inside.
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    Use pressure screws, if needed. While not always necessary, some prefer to secure a chimney cap to a metal pipe using pressure screws. Do not use pressure screws when working with materials other than metal, though.
    • The manufacturer's instructions that come with your cap will usually specify whether or not you should use pressure screws.
    • Use pressure screws that are 1/4 inch (0.64 cm) in diameter and 1-1/2 to 1-3/4 inches (3.8 to 4.4 cm) long.
    • Twist these screws into the holes along the side of the cap until they stop turning.

Method 2
Single Flue Chimney Cap

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    Align the cap with the flue. Position the cap over the flue, centering it as well as you can, before sliding it in place over or inside the flue.
    • Remember that a single flue chimney with an extended flue will require a cap that slides over the top of the flue. A chimney without an extended flue will require a cap that slides inside the flue, instead.
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    Fasten the cap to the foundation. Place a sheet metal screw in each hole along the base of the cap and twist each one in place using a drill.
    • Match the screw size to the size of the holes in your chimney cap. Similarly, use the correct drill bit size for that screw.
    • Work carefully as you drill the screw in place. You will be drilling into the foundation of the chimney, and if you work too sloppily, you could end up damaging the masonry.
    • If you are not comfortable drilling into the chimney, use pressure screws or bolts instead of sheet metal screws.
    • Note that you will only fasten the cap from the outside when capping an extended flue. If capping a flush flue, you will need to fasten the cap to the inside of the chimney flue.
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    Apply silicone caulk. Apply a thin line of silicone caulk to the entire perimeter of the chimney cap, filling in the seam.
    • This caulk will help keep more moisture out.

Method 3
Crown (Top) Mount Chimney Cap

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    Position the cap. Place the cap over the crown of the chimney, keeping it fairly centered over the top flue.
    • Since this type of cap essentially "cages in" the chimney, you do not need to worry about a perfect fit. Just make sure that the entire chimney is covered.
    • Note that the "crown" of the chimney is the concrete area around the flues.
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    Bend the flange. Use heavy pliers to bend the flange around the bottom of the chimney cap. Keep bending the flange until it conforms to the outer perimeter of the chimney crown.
    • The "flange" is simply the collar surrounding the bottom of the cap.
    • You need this flange to lie snug against the sides of the crown.
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    Trace the perimeter. Take a dark marker and trace around the lower, bent edge of the flange, making the mark directly on the chimney crown.
    • After tracing the flange, temporarily remove the cap and set it aside.
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    Apply adhesive. Generously apply weather-proof caulk or a similar adhesive to the crown of the chimney, positioning the adhesive just to the inside of the line you traced.
    • You can slather this on with a trowel, but the easiest way to apply the adhesive is to use a caulking gun.
    • Make sure that the bead you apply is 3/8 inch (0.95 cm) thick. Move the caulking gun gently from side to side, creating a wavy line roughly 1 inch (2.5 cm) wide.
    • Leave 1/4 inch (0.64 cm) gap at each corner as you apply the adhesive.
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    Press the cap into the adhesive. Place the cap back over the chimney and firmly press the flange down into the adhesive. Let dry.
    • If desired, you could smooth any adhesive that oozes out using a putty knife, but doing so is not necessary.
    • Oftentimes, weather-proof adhesive is enough to hold this type of cap in place. Some caps should also be secured further with masonry screws, though. Check the manufacturer's instructions to determine if screws are necessary. If they aren't you can stop at this step.
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    Make pilot holes in the chimney. Place a screw through each hole in your flange. Use a power drill fitted with an appropriate drill bit to twist the screws roughly halfway in place.
    • Work carefully. You will be drilling directly into the chimney crown, and if you are too sloppy or hasty, you could damage the masonry.
    • Use a 3/16 inch (0.5 cm) masonry bit. The masonry screws need to be 1/4 inch (0.64 cm) in diameter and 1-1/2 to 1-3/4 inch (3.8 to 4.4 cm) long.
    • You need at least one screw per side. Aside from that, the screws should be spaced about 12 inches (30.5 cm) apart.
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    Tighten the screws. Go back around the flange and finish drilling the screws in place. The head of each screw should lie flat against the side of the flange.
    • You should still work carefully to prevent any accidental damage to the masonry as you drill the screws in.

Tips

  • Note the advantages that come with capping your chimney. Chimney caps prevent excess rain, snow, and wind from coming into your chimney. Caps can also keep out birds and other animals.[3]
  • Invest in a stainless steel cap. Galvanized steel caps rust and degrade after a few years, but a stainless steel cap can last the for decades and require less maintenance.

Warnings

  • Work carefully. Since you'll be installing the chimney cap on the roof of your home, it is essential that you take every precaution necessary to prevent yourself from falling off the roof or dropping any tools.
  • Check the chimney before you install the cap. Look for signs of corrosion, rust, or other damage. If the inside of your chimney is damaged, you need to treat the damage before installing the cap. Failing to do so will cause the damage to get worse.
  • Consider hiring a professional. Chimney cap installation may sound simple, but if you're even the least bit uncomfortable moving around on the roof of your house, you would be better off hiring a professional than attempting the task yourself. Moreover, a professional will offer a greater selection of caps than a hardware store will.

Things You'll Need

Measuring the Chimney

  • Ruler, yardstick, or tape measure
  • Ladder

Round Chimney Cap

  • Ladder
  • Round chimney cap
  • Silicone caulk (optional)
  • Pressure screws, 1/4 inch (0.64 cm) in diameter and 1-1/2 to 1-3/4 inches (3.8 to 4.4 cm) long (optional)
  • Screwdriver (optional)

Single Flue Chimney Cap

  • Ladder
  • Single flue chimney cap
  • Sheet metal screws or pressure screws
  • Power drill with drill bits, or screwdriver
  • Silicone caulk

Crown (Top) Mount Chimney Cap

  • Ladder
  • Crown mount chimney cap
  • Pliers
  • Dark marker
  • Weather-proof adhesive or caulk
  • Caulking gun or masonry trowel
  • Putty knife (optional)
  • Masonry screws, 1/4 inch (0.64 cm) in diameter and 1-1/2 to 1-3/4 inches (3.8 to 4.4 cm) long (optional)
  • Drill and 3/16 inch (0.5 cm) drill bit (optional)

Article Info

Categories: Exterior Walls and Roof