You are probably worried about flashing your brick chimney. Chimneys come on like an 800-pound gorilla and are responsible for a large number of do-it-yourself roofing projects which were never started. The method we will use to flash and tie in a chimney is very straightforward: you will be pleased with the outstanding (no-leak) results. You will be shown the way up and around the chimney, and my figures will put you on the roof doing each step.
Someone wise once said, Worry is sadistic entertainment.” He was right. We’ve come this far together, so relax and let’s enjoy ourselves as we tie in a chimney.
There are several steps to flashing a chimney. It takes the time to trim and fit the various components. It’s not complicated, it just takes time. You can easily spend four to six hours on your chimney, so get rid of any idea that you must complete it in two hours. The test of your work is not how fast you do it, but whether it leaks and whether it lasts.
It is best to have the upper and lower bent metal flashing already bent to the angles you need before you tear the roof off. I used 24-inch (width) coil, 0.027 (27 gauge) aluminum for these pieces. Let the bottom and top metal extend 6 inches beyond both sides of the chimney. If the chimney is 2 feet wide, make the metal 2′ + 6″ + 6″ = 3′ long. If you want the bottom flashing to rise up the brick 6 inches under the skirt flashing, mark the coil and bend the metal at an angle slightly less than the one between the sheathing and the lower face of the chimney. Remember, you want the metal to tend to spring toward, not away from, the brick face and sheathing.
The metal above the chimney will be bent at greater than a right angle (90°). Don’t over bend this metal either. Snow and ice tend to accumulate above the chimney, so make sure you have a substantial leg (6inches plus) protecting the back face of the brick chimney.
How else do you make the job easier? Let’s say you are doing a tear-off. The far side of a chimney is even with the left rake, but the remaining three sides are out in the shingles. Go ahead and tear off the right side of the front roof, but stop the tear-off on a rough vertical line 3 feet before you get to the chimney. Just leave the left end of the old roof intact from the ridge all the way down to the fascia.
Felt in, mark, and shingle the right portion of the roof. Roll runs of felt out 2 feet over the edge of the old roof and weight the ends down with bundles. Stop the new shingles about 1 foot from the edge of the old shingles and lay the ends loose. If the forecast at the end of the day has suddenly changed to rain for that night, you can run a heavy bead of mastic down the edge of the felt, overlapping the old shingles. This will help prevent rain from blowing in under the side. Weight down the edge over the mastic bead. The next day when you are ready to concentrate on the chimney, cut off the excess felt overlapping the old shingles. Just make sure you leave at least 6 inches of felt sticking out from under the new shingles. Complete the tear-off to the left rake. Lap the ends of the roll of felt the required 6 inches over the felt you previously laid. Complete the runs of felt to the left rake or the chimney.
If the chimney is at the rake, you may only need to flash the bottom, one side, and the top. If three sides of the chimney are outside the wall, you may have to flash only one side of the chimney. For our example, we will complete a chimney that is midway across and midway up a roof. You will know how to flash it all the way around. Then you can eliminate the parts that don’t apply to the chimney on your roof.
The Building Code calls for No. 30 felt below and above the chimney. I prefer the additional protection of modified bitumen (or 90 lb. rolled roofing) all the way around the chimney. Starting and stopping No. 15 felt to insert short sections of No. 30 felt (or in our case, modified bitumen) at the chimney is a lot of extra trouble. It is easiest to do a normal felting in and then come over the No. 15 felt with the modified bitumen. So when a run of felt is interrupted by the base of the chimney, cut out a large rectangular “notch” for the chimney, so the cut sides and bottom of the run of felt will he flat on the sheathing. When you are past the chimney, continue the run on across the roof. Trim the next run or runs of felt into the sides of the chimney. Lay the run of felt across the top of the chimney too, notching out a rectangular pattern for the upper portion of the chimney.
Hint: If a sudden squall should come up at this point, seal the edges of the felt to the chimney with mastic. Weight the felt down, and it will do a fairly decent job of keeping the rain out.
Roll the modified bitumen out below the chimney and cut it so that at least 1 foot extends beyond each side of the chimney. Slide the modified bitumen up the roof until approximately 6 inches of it runs up the face of the brick chimney. Use the straight blade to cut straight down the modified bitumen at each comer of the chimney. You want the 1 foot on either side of the chimney to lie flat on the sheathing and the portion you cut free to lie up the face of the brick.
Don’t make the cuts exactly in line with the sides of the chimney. Shift the location of the cuts l U inch in toward the center of the chimney. Nail the bitumen in place, keeping the nails well away from the chimney itself. (You can cut off the 6-inch portion that climbs up the brick face of the chimney. Cutting off the 6inches up the face makes the finished metal look a little neater, but it also eliminates a protective feature across the base of the chimney.)
Trim two 1-foot lengths of bitumen and lay a length into each side of the chimney. The 1-foot lengths overlap the 1-foot legs of the first piece of bitumen with a 2-inch vertical lap. If the chimney requires additional 1-foot lengths of bitumen up the side, go ahead and lay them. If the sides of the chimney are relatively short, allow more than a 2-inch downhill overlap. It’s just that much added protection. Nail all the side pieces in place.
When you reach the top of the chimney, cut a piece of modified bitumen the same way you did the piece across the bottom of the chimney, and let the 1-foot wide legs drop down over the 1-foot side pieces.
Lay the courses of shingles on up to the chimney. If you used a 24-inch coil stock to shape the metal and you bent 6 inches to climb the lower face of the brick chimney, you are going to have 18 inches of metal showing over the shingles beneath the chimney.
This length of lower leg provides maximum protection against rain blowing back in under the lower bent flashing. However, 18 inches is a lot of metal to be showing on a highly visible roof.
You will probably decide to trim this lower leg, for aesthetic reasons. Let’s say the top of the course of shingles just butted into the lower face of the chimney. That puts the tops of the keys down 7 inches from the chimney. You want the bottom edge of the metal to come out onto the finished surface of the tabs 2 inches. A lower leg with 9 inches over the shingles will look better than one with 18 inches.
Mark the metal 9 inches back from the lower milled edge. Pop a chalk line at the 9-inch marks and cut 9 inches off the lower leg. (This is a finished cut, and it will show.)
I prefer to have the milled edges of the coil showing on all finished edges. I set up the edges I cut with shears so they were under skirt flashing or covered with caulk. My cuts were fine, but the mill finished edges were cleaner. However, this isn’t always possible to do when you form or bend the metal in advance.
Center the bent metal flashing on the bottom face of the chimney and mark the 6-inch upper leg slightly inside the comers of the chimney. Cut the 6-inch leg perpendicular from the marks to the bend. Set the metal back in position and push the 6″ x 10″ side legs you just cut at each end, flush with the surface of the modified bitumen. (As with the modified bitumen, you want the legs to fit snugly against the brick sides. It’s fine if the legs tend to “climb” the brick a little bit.) Lightly hammer the bends down straight on the 10-inch side legs so they will lie flat on the modified bitumen.
As you keep the 6-inch long upper leg that climbs the face of the brick chimney pulled up tight to the chimney, nail the 10-inch leg down. An aluminum nail should be in each 10-inch side leg at the top corner farthest from the chimney.
Run a bead of mastic under the lower bent flashing edge which is out over the exposed portion of the shingles. Nail this lower edge down, centering the nails over the tabs of the course it covers.
Bring the next course of shingles in. Cut the shingle into the side of the chimney the same way we did when tying into a wall. The bottom piece of step flashing should extend out over the bottom leg of the lower bent metal flashing by 2 inches. This bottom piece will also have to fit up under the skirt flashing you will install around the chimney. Remember to add a second piece of step directly on top of the shingle and above the trimmed piece of step. Keep the bottom edge of the second piece of step just above the self-sealing strip. This will give you the proper downhill overlap for the remainder of the step flashing in the courses on up the roof.
Don’t nail into the shingles if the key way is at the step. Let the nail in the step flashing hold the shingles down. Notch the 2-inch leg of the last piece of step flashing so that it follows the corner of the brick. Leave a small section (3/s inch) of the 2-inch vertical leg in place at the bottom of the area you notched out.
If the top of the final course of shingle up the side is at a higher elevation than the top comer of the chimney, trim out as much of the bottom of the shingle as you have to and carry the few inches of the top part of the shingle over the modified bitumen all the way across the top of the chimney. This will raise the upper bent metal flashing and keep it level with the rest of the roof.
Reset the base and offset lines on the far side of the chimney. Tie in the shingles on the far side of the chimney all the way to the top corner as you did on the left side. Place the upper bent metal flashing above the chimney with its 6-inch leg up the chimney. Mark the 6-inch leg XU inch outside the comers of the chimney.
Cut from the marks perpendicularly down to the bend. Making the cut for the leg down the chimney slightly long will cause the bent metal flashing to carry the runoff water slightly beyond the comer of the chimney and out onto the base of the trimmed step pieces at the top of each side. Push the 10-inch legs down over the base of the top pieces of step flashing on each side of the chimney.
Hammer the bends on the 10-inch legs down flat. Nail each 10-inch leg down flush, driving the nail in the comer farthest from the chimney. Caulk under the top edge of the upper bent metal flashing and bed it down to the felt. Nail the top edge of the upper bent metal flashing down to the felt and sheathing, keeping the nails as far from the chimney as practical.
You will notice that by carrying the 3-inch base of the first piece of trimmed step flashing a few inches out on top of the bent metal flashing at the bottom of the chimney, we have carried most of the water that will run along the side of the chimney out past the lower corner of the chimney. By extending the 3-inch leg of the top piece of step flashing under the bent metal flashing above the chimney, we have intercepted the water that will roll off the ends of this upper flashing.
Next, we shape the skirt flashing on the side of the chimney. We want it to overlap the front face of the bent metal flashing that goes up the face of the chimney. Cut off the coil a length of aluminum long enough to rest on the roof and bend around the comers of the chimney, with a surplus 3 inches overlapping the upper and lower faces. Bend the lower overlap to the shape of the comer of the chimney. You will have to cut up slightly into the coil to make the 3-inch overlap in front rise above the bend in the lower bent metal flashing.
Sight down behind the metal and see where the mortar joints are. You are going to trim the metal so it follows the horizontal and vertical mortar joints in the brick. Your metal will have a stair-step pattern along the top when you are finished. To make it fit the bottom, you will have to cut up into the coil slightly from the comer bend. To make it fit at the top, you will have to cut from the bend slightly downward toward the bottom of the coil. You will mark the beginning and end of the steps by dimpling the center of the horizontal and vertical joints with a nail and marking the lines between these dimples with a straightedge.
Once the left and right skirt flashing have been installed, you can shape and install the front skirt flashing. Cut the length so that it extends 3 inches beyond the comers of the chimney. Cut it to a height that tie in with the skirt flashing on both sides.
Make sure the milled finish edge is on the bottom where it will remain visible. You will have to cut very narrow notches at the bottom corners of this front piece to make room for the Vs-inch leg you left in place on the first piece of step flashing at each comer.
Note that you will have to cut a “saddle” up into the bottom of this piece to make it fit down flush with the bend in the upper bent flashing. Install the upper skirt flashing.
You are ready to carry the courses on above the chimney. I recommend you leave 2 to 3 inches of the roof leg of the upper bent metal flashing exposed.
The wind will have an easier job of blowing dirt and leaves off the smooth metal than it will blowing the coarse shingles clean. With the overlapping metal flashing, you have done an excellent job of mechanically turning the water away from the chimney. There are four possible trouble spots — one at each corner of the chimney.
Use your finger and carefully knead caulk back into the small openings at each comer. (Be careful not to rake your finger across the sharp edge of the metal.)
Now caulk the entire top of the skirt flashing all the way around the chimney. Pull the metal away from the brick and let the bead of caulk get down behind the metal too. Caulk over the heads of the mortar nails so they won’t must. When you have caulked the small openings at the base of each corner and the top of the skirt flashing, the chimney can’t leak. Caulk over any exposed nail heads when you finish this section of roof.
Apply fresh caulk when you do the overlay two decades from now, and this flashing will last beyond the twenty- to twenty-five-year life of the overlay too. Congratulations! I knew you could do it.
TYING IN A CHIMNEY ON AN OVERLAY
When you get up to the chimney with the courses of shingle, gently raise the roof leg of the lower bent metal flashing. You may have to pry up the whole first course beneath the roof leg to keep from bending the metal. Pull the nails out and slide the new course of shingles in underneath the lower bent metal flashing. This keeps all runoff water from above and around the chimney on top of the new roof. Now butt the courses into the sides of the chimney and on across the top.
Flashing with the new shingles, lap the first pieces of step flashing out over the roof leg of the lower bent flashing and tuck the final piece of step up under the roof leg of the upper bent metal flashing.
If you don’t have room to slide the 2-inch leg of the step up under the lower edge of the skirt flashing, you can completely redo the flashing system. It’s neater to tear the whole skirt flashing off, but you can completely redo the metalwork right over the original metal. The old flashing and old shingles underneath will provide some minimal additional protection.
Many, many roofers will overlay a roof around a chimney and won’t redo the flashing. It’s okay if the flashing is in good shape, if he chips off and redoes the caulking, and slides and caulks the new shingles in under the lower bent metal flashing.
Nail: Unfortunately, some roofers don’t raise the roof leg of the lower bent metal flashing and lay the new shingles in under that leg. They just cover the roof leg with the new shingles and cut the shingles around the chimney. When they do that, the complete flashing system is covered. Water that penetrates to the old original step flashing runs down, beside the chimney and continues down the roof underneath the new shingles.
Your roofer in effect has punched the old shingles down the roof from the chimney full of nail or staples from the new shingles and expected the old roof to remain in service another twenty to thirty-five years. The homeowner will soon experience mildew on the ceiling at or near the fireplace, and the smell of rot will become noticeable.
Incidentally, not tying in under the flashing saves the roofer an absolute maximum of ten minutes.
Skylights raised above the shingles of the roof on a 2×4 box with a sealed metal frame are flashed the same way as a chimney. The metal framing sealed to the glass of the skylight drapes down over the 2 x 4’s and performs the same function as the skirt flashing on a chimney. The upper and lower bent metal flashing are bent and trimmed to fit the same way you did it on the chimney, and the step flashing along the side ties in the same way. The only difference is, instead of making your own skirt flashing, you trim the legs up the side of the 2×4 box so they will fit under the “skirt” of the metal framing of the skylight.
Framed skylights are much more costly than the bubble skylights with the 2lh- to 3-inch lip molded around the outside of the bubble. These bubbles are tied in with mastic and more mastic between sheathing and felt, felt and molded edge, molded edge and shingles, and finally mastic between the edge of the shingles and the bubble of the skylight. The bubble is guaranteed against manufacturing defects for so many weeks or months. The only guarantee I can give you on the bubble type is that it’s designed to leak and that is exactly what it’s going to do. There is a significant price difference between the better
The flashing around the chimney may leak, and you may not find the problem. You can install new step quality raised metal-framed skylight and the cheap bubble type. If you install a bubble skylight and it never leaks, your kind of luck belongs in Las Vegas.