Jan 31 , 2019

Installing Chimney Flashing

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.

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.

Chimney Maintenance

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