Installing Chimney Flashing
Leak-proof your chimney using this traditional, time-tested flashing method.
A full day
IntroductionThis traditional, time-tested flashing method guarantees a leakproof fix. Follow this step-by-step process and you can fix the leak and save $100s.
- Diamond blade (dry-cutting)
- Extension ladder
- roof brackets
- Roof harness
- straight cutting tin snips
- 26-gauge galvanized sheet steel (for prefabricated pieces)
- Drive-in expanding anchors
- Galvanized coil stock
- Galvanized front base flashing
- Galvanized step flashing
- Polyurethane caulk
- Self-adhering roofing membrane (ice and water barrier)
Chimney leaking? Look for flashing problems
Figure A: Chimney flashing details
Chimneys are notorious for leaking, and the culprit is almost always the sheet metal flashings. Just ask any roofer. But it doesn’t take an expert to spot problems—simply look for water-stained ceilings or other telltale signs of leaking in the vicinity of your chimney. So what is placed around a chimney to prevent leaks?
You’ll want to install new chimney flashing if it’s missing, rusted through, falling out or completely covered with roofing tar (a typical short-term fix that’s sure to be hiding bigger problems). It’s also a good time to install new flashing when you put on new shingles because you’ll want it to last as long as the new roofing (25 to 40 years).
In this article, we’ll tell you how to order new flashing and install it on a brick chimney. Our roof has asphalt shingles, but the procedure is the same for wood shingles. Slate and tile roofs require special flashing techniques that are best left to the pros. Flashing a chimney is an advanced DIY project. For starters, you have to be comfortable working on a roof. Then you have to measure, cut and bend sheet metal to fit precisely around the chimney and layer all the parts so they shed water. Even so, if you’re handy with tools and carefully follow our instructions and diagrams, you’ll be able to flash your chimney in a day. And as far as chimney flashing repair cost goes, by doing it yourself, you’ll save hundreds of dollars.
Project step-by-step (12)
Gather all materials first, then remove old flashing
Pry, chisel and scrape off old metal flashings and roofing cement.
Before you do anything, get your ladder, roof brackets and safety harness set up so you can work safely and efficiently on the roof. Then measure the chimney and order the saddle and flashing parts (see “How to Buy Chimney Flashing,” below).
We ordered all our 26-gauge galvanized metal flashing bent to the correct angles and dimensions from a sheet metal shop specializing in roofing. The shop did a great job. I dropped off the dimensions and three days later the stuff was ready to pick up. Even with the custom-made saddle, the flashing was inexpensive. In addition to the flashing and shingles, buy a roll of self-adhering roofing membrane (also called ice-and-water barrier), two tubes of polyurethane caulk, and a package of 25 drive-in expanding anchors from a home center or roofing store.
Besides basic hand tools like a hammer, tape measure, 2-ft. level and square, you’ll need a few special tools for working with sheet metal and cutting and drilling masonry. Buy a good pair of straight-cutting tin snips, a 3/16-in. masonry bit and a grinder (a great excuse to buy this useful tool) or a circular saw fitted with a drycut diamond blade for grooving the mortar (Photo 7).
Don’t forget safety equipment. You’ll need a sturdy extension ladder to get on the roof, and roof brackets and top-quality 2×10 planks to work safely once you get there. For maximum safety, especially if the roof is steep or high, buy a personal fall arrest system consisting of a safety harness, lanyard, rope-grab, rope and roof anchor.
How to Find Your Roof Slope
Find the slope of the roof. Label one edge of a 2-ft. square scrap of plywood or cardboard “roof” and lay the labeled edge against the roof. Draw a level line at least 12 in. long on the plywood and label the line “run.”
Measure 12 in. along the “run” line and make a mark. Use a framing square to draw a square line from this mark down to the edge of the plywood labeled “roof.” Label this line “rise.” Measure the length of the line labeled “rise” to determine the slope, expressed as inches of rise for every 12 in. of run. In our case, the roof slope is 6 in. of rise for every 12 in. of run.
Cut base flashing, then attach the base flashing
Cut a piece of prebent base flashing to fit across the front of the chimney. Cut a 45-degree angle to bend around one side.
Install front base flashing
Snug this bent end against one side of the chimney front. Mark the edge of the chimney on the opposite end and snip and bend this end. Pound four evenly spaced 1-in. galvanized roofing nails through the front edge of the flashing to hold it in place.
Once you complete the tearout and clean up, shingle up to the bottom of the chimney (Photo 2) and seal around it with 12-in. wide strips of ice-and-water barrier. Lap the strips 4 in. up onto the bricks and overlap adjacent strips to shed water. Run a full 36-in.wide strip across the top of the chimney. Stick this piece to the roof boards and up 8 in. onto the back of the chimney. Later you’ll seal the new metal saddle to this piece with additional strips of the membrane and then install roofing paper and shingles over the metal saddle.
Start the flashing job by installing the first piece of base flashing across the bottom of the chimney and over the top of the shingles (Photo 3).
Add step flashing and the chimney saddle: Install flashing around the corners
Snip and bend a piece of prebent 8-in. square step flashing to fit 1-1/2 in. around the front corner as shown. Apply a jelly bean–size dollop of caulk at the corner where the chimney meets the roof. Embed the corner step flashing in the caulk and nail it to the roof with two roofing nails.
Install flashing around the chimney
Nail the next shingle over the first step flashing. Then nail another step flashing over this shingle. Continue until you get to the top corner of the chimney. Cut and bend the last step flashing around the top.
Install the chimney saddle
Slide the prebent and soldered chimney saddle into place behind the chimney. Nail it to the roof along the top edge. Space 1-in. galvanized roofing nails about every 6 in. along the edges or closer if necessary to flatten the metal.
Seal the sides of the chimney with prebent pieces of step flashing under each course of shingles (Photos 4 and 5). Slightly flatten the step flashing before you nail it in to ensure a tighter, spring-tensioned fit against the chimney. Pay close attention to overlapping each piece of flashing onto the preceding one in such a way that water is diverted over the top of the shingles. This is the key to a successful flashing job.
Complete the base flashing by nailing the saddle to the roof and sealing it with additional strips of membrane (Photo 6). Shingle over the portion of the saddle that rests on the roof and cut the shingles neatly along the valley, leaving about 2 in. of metal exposed.
How to Buy Chimney Flashing
Making sharp, straight bends in sheet metal is tricky unless you have access to a tool called a sheet metal bending brake. Even then, you need experience to bend complicated shapes. That’s why we’re recommending that you order your chimney flashing from a sheet metal fabricating shop. Large roofing companies often have their own sheet metal shops and are the best place to buy the chimney saddle and flashing; they’ll know exactly what you need. Otherwise, check the Yellow Pages under “Sheet Metal.” We used 26-gauge galvanized sheet steel for our flashing and saddle, but prefinished steel or copper are other possibilities.
Most chimneys have a small rooflike structure behind them, called saddle (Photo 6), that diverts snow and water to both sides of the chimney. Whether your chimney has a wooden saddle covered with shingles, a sheet metal saddle or no saddle at all (like the chimney in this story), we recommend installing a new custom-made sheet metal saddle (Photo 6).
Custom-made saddles are soldered together at critical spots that are hard to seal on saddles that are built in place. Soldered sheet metal saddles less than 3 ft. wide are self-supporting and easier to install if you just tear out any existing metal or wood saddle. Most old saddles were built right over the top of the roof boards and need no patching. Otherwise, fill in the missing boards before installing the new saddle. Saddles wider than 3 ft. may need additional support. Ask your sheet metal fabricator for advice.
Bending and soldering a saddle is complicated, but sheet metal shops need only two pieces of information from you to bend up a saddle: the width of the chimney and the slope of the roof (see “How to Find Your Roof Slope,”).
In addition to the saddle, you’ll need the following sheet metal parts (see Figure A):
- Front base flashing. Order about 12 in. more than the width of the chimney. Some lumberyards stock a “dormer flashing” that will work in this location. You may have to adjust the angle by bending it to match your roof slope.
- Prebent step flashing, 8 x 8 in. Roofers call these “shingle tins”. You’ll need one for each shingle along the edge of the chimney plus a few extra. Most lumberyards stock these.
- Front cap flashing and side cap flashing. Some sheet metal shops stock a flashing for this purpose, but you’ll probably have to have it made. Measure at the front of the chimney from the roof to the third mortar joint and add about an inch to determine the height you’ll need. The shape is relatively simple, so you could buy flat sheet metal, called “coil stock,” and bend these yourself.
- Back cap flashing. This flashing is identical to the front and side cap flashing.
Cover the base flashing with cap flashing: Saw grooves in the mortar
Saw 1-in. deep grooves in the mortar joints to accept the L-shaped cap flashing. See Figure A for help in locating the grooves. Use a dry-cut diamond blade mounted in a grinder or a circular saw. Wear goggles, hearing protection and a good two-strap dust mask.
Install the front cap flashing
Cut the front cap flashing 8 in. longer than the width of the chimney, then bend it around the front corners of the chimney.
Drive in the flange
Drive the flange on the cap flashing into the groove in the mortar joint. Use a block of wood to keep from denting the flashing.
Secure the flashing
Drill a 3/16-in. x 1-1/2 in.-deep hole through the metal and into the bricks on each corner of the chimney. Then press the expanding plastic anchors into the holes and pound in the protruding nail to expand the anchor and secure the flashing.
Continue to install cap flashing
Cut the side cap flashing to fit the angle of the roof so it overlaps each lower cap flashing at least 2 in. Pound the flashing into the grooves and fasten with the drive-in expanding anchors. Bend a piece of cap flashing to fit around the top corner of the chimney and add the cap flashing across the back of the chimney.
Seal the flashing with caulk
Now it’s time for chimney flashing sealant. Seal the cap flashing by caulking along the mortar joint with polyurethane caulk.
The next step is to mark and cut the grooves in the chimney’s mortar joints to accept the cap flashing. Start at the front of the chimney and mark the third mortar joint up from the roof. Extend this mark around both front corners about 6 in. Now mark the mortar joints on the sides of the chimney so the flashing steps up until you reach the back (Photo 11). Depending on the slope of the roof, each step will jump up one or two mortar joints and should lap over the previous piece of flashing at least 4 in. Mark straight across the back of the chimney to cover the saddle (Photo 12).
Cutting the groove in the mortar is easiest with a grinder and a diamond blade (Photo 7). A circular saw with an inexpensive abrasive masonry blade will also work. Wear goggles, a good dust mask and hearing protection. Cut close to the brick on the lower edge of the mortar joint. This way, the final caulk joint will be less visible. Cut the slots about 1 in. deep and 1/8 in. wide. Make them a few inches longer than you think is necessary, keeping in mind that each piece of cap flashing has to overlap the one below it by about 2 in.
The little backward bend on the short leg of the cap flashing acts as a barb to hold the flashing in until you’re ready to caulk. If the flashing is still loose, wedge it tight with a small plastic shim (sold as toilet shims in hardware stores and home centers) or secure it with expanding anchors as shown in Photo 10. Once all the cap flashings are in place, complete the job by filling the groove with polyurethane caulk (Photo 12).
Your new flashing will last longer with a coat of paint. But don’t paint the bare galvanized metal without cleaning it first with lacquer thinner. Another option is to order the slightly more expensive prepainted metal flashing.