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How to Repair Damaged Chimney Flashing

What is Chimney Flashing?

The chimney’s flashing is a metal barrier wrapped around the base and the roofline that seals the point where they meet.

The flashing serves many purposes, but the primary one is to prevent anything from being able to leak between the chimney stack and the roof boards.

What Are the Dangers of Damaged Chimney Flashing?

Damaged chimney flashing allows water to get into sensitive areas of your chimney and your house.

Water that seeps between the external stack and the roofline will leak into your walls and the ceilings in your home, compromising the structural integrity.

Water can also weaken the mortar bind the chimney’s bricks, increasing the risk of a collapse.

Damaged flashing can also provide a spot of entry from pests, dirt, and other debris that should be kept outside the house.

How to Maintain Chimney Flashing

  • Notice the signs of faulty flashing. Compromised flashing leads to water damage. If you see any visible signs of water damage to the roof around the chimney, such as stains or peeling paint, you need to fix your flashing.
  • Regularly inspect the flashing for signs of corrosion and rust. Use a wire brush and solvent to scrape off rust and flaking paint. Be careful not to expose asphalt roof shingles to any solvent.
  • Recaulk pieces of the flashing that have separated from the chimney, roof, surrounding shingles, or each other.
  • Spray the metal with a zinc-based primer and then spray on a few coats of a rust resistant paint of your choice.

How to Repair Damaged Chimney Flashing

  • What You Will Need
    • Caulk
    • Sheet metal and a saw to cut it
    • Sandpaper and/or a wire brush
    • Roofing cement
    • Ladder
    • Roofing harness
  • Steps
    • Find any small, pin-sized holes in the chimney flashing. Fill these in with roofing cement.
    • Holes up to ¾ inches in size need to be patched. Cut out a patch from a material matching the flashing. 
    • Using sandpaper or a wire brush, roughen the area around the hole and wipe away the burrs. Use roofing cement to affix the patch over the hole, and then cover the whole thing with another layer of roofing cement.
    • Spray the metal with a zinc-based primer and then with a few layers of a rust resistant paint of your choice.

How To Replace Chimney Flashing

  • What You Will Need
    • Chisel
    • Hammer
    • Nails
    • Wooden board, a level, and a pencil
    • Caulk
    • Ice-and-water barrier strips
    • Sheet metal and a power saw to cut it
    • A diamond saw
    • Drive-in expanding anchors
    • Ladder
    • Roofing harness
  • Steps
    • Using a chisel and hammer, remove the shingles surrounding the old flashing, pry up the flashing itself, and remove any concrete or other materials holding onto the flashing. You should expose the roof boards in your roof. Be very careful not to accidentally dig your chisel into the wooden boards.
    • Measure the slope of your roof. In order to make sure the flashing fits, especially the side flashing, you must know the slope of your roof.
      • Hold a wooden board on its edge against your roof. Take the level and put one end where the top edge of the board meets the roof. If you lower the level until it is straight, you should be able to draw a straight line on the wooden board while it is on the roof. This is your run.
      • Measure 12 inches (or another easy to calculate number) along the run and draw another straight line down. This should form a right triangle. This new line is the rise.
      • The slope is the rise over the run. If you measure a rise of 9” and a run of 12”, then your roof extends up 9 inches for every 12 inches it extends out; or, your roof rises by 3 inch for every 4 inches of length.
    • Cut new flashing from your sheet metal.
      • The front flashing will be a solid piece that must be strategically bent to fit the front of your chimney.
      • The side flashing, or step flashing, must be cut out as several pieces. They will be placed side-by side, covering each side of the chimney. Cut the side flashing to match the slope of your roof; if you measured a slope of 9”/12”, then you can cut each piece of flashing to be three inches tall and four inches wide. The result will be a series of small flashing panels that create a step-like pattern with the slope of the roof.
      • The back flashing, or saddle flashing, is also a solid piece of metal but must be folded to create a three dimensional saddle structure for the back of the chimney. This shape helps direct water away from the roof.
    • Lay down ice-and-water barrier strips around the area where the flashing will go, on the roof boards. Take special care to seal the area around the chimney stack.
    • Nail the shingles back onto the roof, up to the chimney.
    • Install the base flashing first. The base flashing sits on the roof and extends partly up the stack. The base flashing consists of front, side, and saddle pieces.
    • Nail the flashing into the chimney with the drive-in anchors. Nail the flashing to the roof boards with standard nails.
    • Using your diamond saw, carve a groove into a mortar joint in the chimney stack. The groove will serve as the anchor for your cap flashing. Don’t cut too deep or you risk damaging the mortar.
    • Install the cap flashing around the chimney. The cap flashing rests in the groove you just carved and extends down the chimney stack. The cap flashing consists of front, side, and saddle pieces.
    • Caulk the gaps.
    • Spray the metal with a zinc-based primer and then with a few layers of a rust resistant paint of your choice.

When to Call a Professional

If your chimney flashing is suffering from damage any worse than a small hole or two, then it will need repairs that require power tools, sheet metal, and advanced masonry skills.

If you don’t have the tools, time, or skills required, then you should hire a professional contractor to come help fix your chimney flashing.

It is also a good idea to call a contractor if your damaged flashing has led to any other damage. For example, if the faulty flashing has softened the bricks in the stack, a contractor will be needed to assess the situation and recommend the safest course of action.

Also call a contractor if you see water damage on the inside of your house, as it means that there could be water causing wood rot within the building.

Lastly, bear in mind that even the simplest repairs to your chimney flashing will have to take place all the way up on your roof.

You will need a ladder just to get up, and you will need a harness to prevent yourself from falling. Even if your roof has a low pitch, you must be sure to take proper precautions.

If your roof has a sharp pitch, safety precautions will be even more essential.

Cost to Repair Chimney Flashing

DIY

The biggest costs in a DIY chimney flashing repair will be any tools that you lack. Buying a diamond saw, for example, will be so expensive that it won’t be worth it to continue doing it yourself.

If you don’t have the tools, it will be thousands of dollars more to do it yourself than to hire a professional. That’s not to mention the inherent safety risks of working on your roof, or even just the challenge of actually doing physical work while standing on a sharply inclined surface.

If you do already have the tools, you are only looking at the cost of the caulk, ice-and-water strips, sheet metal, and anchors. These things can be purchased at a hardware store for a few cents.

Chimney Repair Contractors

The cost of having a professional contractor fix your chimney flashing will be between around $100 and $500.

The main benefit to hiring a contractor even if you have the tools is that the contractor can provide a guarantee of the quality of their work.

If your contract has been written well, it will also be the contractor’s responsibility to take care of any hiccups that occur while repairing your chimney, saving you from having to take on the extra challenges yourself.

Contractors also come prepared with all of the necessary tools, safety equipment, and the knowledge of how to do the job right.

Time to Replace Chimney Flashing

Making minor repairs to your chimney flashing should only take a few seconds of caulking to complete.

Replacing the flashing will take at least the full day, provided you don’t waste any time while measuring and cutting the flashing. A team of contractors can work a little faster with the additional workforce.

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