View Full Version : Caulking chimney step and counter flashing

05-03-2004, 03:52 PM
Can anyone provide a recommendation on what caulk to use to seal copper chimney flashing at the following two locations (if possible, please be specific as to the manufacturer and even the application type):
1. The joint where the roofing shingles meet the step flashing.
2. The seam where the copper counter flashing is inserted into the masonry of the chimney.

Thank you,

05-04-2004, 09:52 AM
I do not do much roofing, so if you get better advise from someone else, take it. However, I do not think you would use caulk in either case. If you use caulk or mastic on the flashing where it meets the shingle you may trap water above. That is the theory behind step flashing, that any water that gets under a shingle will be carried out by the flashing below. As far as the counter flashing, you may be able to get away with caulking it in the groove cut into the chimney mortar, but most masons will probably encourage you to use mortar. I hope someone else can give you more information on this.

05-04-2004, 06:22 PM
I would use a black roof cement such as Henry makes where the shingles meet the step flashing, assuming you have asphalt shingles. At the counterflashing, I would use a color-matched polyurethane meeting ASTM C920, which is made by lots of companies such as Sika and Vulkem.

05-09-2004, 12:00 PM

Some people recomend using an asphalt roof cement or flashing cement to seal the shingle to the step flashing below it. I DO NOT AGREE. I see many cases where by doing this people trap water and create a leek in stead of allowing the water to run out and down the roof.

The purpose of shingles is to shed water not "waterproof a roof". Step flashing dose the same thing it directs water down the roof and out on to the shingle directly below it. I start with a piece of roof felt or ice and water shield over the roof boards. then I interlace the step flashing with the shingles as I shingle up the roof. Typically the step flashing should extend 3" to 4" on to the roof and be 2" longer than the shingle is high. These are layered under the shingles so they can not be seen. The sise of flashing is determond by the type of shingle. You can check an instlation manual for your brand.

The counter flashing is bent in a V along the top edge and pushed in to an open morter joint so the V grabes the brick to hold it in place. If this is a retro fit you may need to grind out the morter joint first. after it is installed you can re morter the joint or use a ployurethane calk. I use a PL brand that I can find locally. do not seal the botom edge with calk it should remain open so any leakage from above can drip out. I will sometimes use the ployurethane calk to seal the leading edge to the brick so the wind dose not lift it.

Dick Seibert
05-09-2004, 06:07 PM
I agree with Jim except for one thing. I don't like the idea of the polyurethane caulk. If you do use caulk instead of mortaring it in (I don't know why you would), you better give the owner a letter that states that the caulk has to be periodically inspected and replaced, and have him sign your copy. Remember, sealants are temporary seals.

05-10-2004, 04:09 PM

05-10-2004, 04:12 PM
Thanks for your input guys. I've just partially finished my first job flashing a chimney and so far I'm not impressed with the way I've been told to do it. Moving forward I would like to do it right. Any other input is appreciated.

Roger P
05-10-2004, 08:53 PM
Ditto Jim and Dick's advice.

Dont run the shingles tight to the vertical flashing, and no caulk or (asphalt)tar at the shingle/flashing intersection. Always leave a space of 1 to 1-1/2 for the water to run freely; otherwise the water can wick back under the shingles. I also add a by about 15 degree lip to the leg of the flashing on the roof to insure that if water does get under the shingle its directed down before it can run off onto the roof sheathing. It only takes about 15 seconds to do a half dozen at a time on a brake.

Norm Yeager
05-10-2004, 09:03 PM
I wouldn't use any caulk at the intersection of shingles and step flashing. Correctly installed flashing in this instance does not warrant caulk.
Each piece of step flashing will shed water. The counterflashing should overlap the step flashing by appx. 2" or more but should be held up appx. 1" above the intersection of the stepflashing and the shingles. This will allow the corner to wash clean and not accumulate debris. I prefer silicone caulk if you cut a saw kerf into the masonry. In my experience polyurethane has tenacious adhesion but doesn't hold up under infrared sun nearly as well as the best silicones.Jims' technique of bending a V into the counter flash is a good one for fastening to the chimney.A couple good resources for good flashing technique are Architectural Graphic Standards and Building Construction Illustrated by Ching.Both books have excellent isometric drawings on this subject as well as hundreds of others.

05-11-2004, 09:41 PM
I think mortar might be more appropriate for new construction where the flashing is installed with each course and can bond with the rest of the assembly.In a retrofit where you just grind out the joint there is just a small amount of mortar trying to bond with the flashing and the old mortar.It's probably just as temporary as a caulk joint---especially here in the Midwest with the freeze/thaw cycle.
Do you think polyurethane perfomance would be improved with a coat of paint?Seems like it might help.

Dick Seibert
05-12-2004, 10:00 AM

Good point, I agree but the owner should still be advised in writing to periodically inspect and replace.

Norm Yeager
05-12-2004, 08:05 PM
I'm not an authority on caulking,coatings,etc. but your suggestion to caulk the polyurethane seems like an excellent idea to me. Logistically it could be a problem because typically the caulk needs to cure before being painted. I've done more reroofs than I can remember and the things that made it worthwhile were that the money was good,owners seldom climbed up on the roof and as long as it looked good from the ground or a window and didn't leak we did most houses in one day and were on to the next job.An idea along the same line might be to put a skim coat of silicone over the polyurethane.

05-18-2004, 08:56 PM
Check out some of the on line instructions from shingle manuf. for good clear details. In the meantime:
NEVER EVER use cement at the shingle/step flashing joint. It's a sure sign of a poor job.


06-23-2004, 10:30 PM
Typically we will grind out the mortar joint then seal with Vulkem. Before all this is done, we have ice sheild installed around and on the chimney. The part of the flashing that is ground into the mortar is only an outer shell. There is always what we call baby tins under that shell.

IMO roof cement/tar has no place on a new shingle roof.

06-28-2004, 06:45 AM
I normally use a diamond blade to cut my flags in. The resulting kerf is too small to remortar. I've had great success with this product.


07-04-2004, 01:08 PM
Check out some of the on line instructions from shingle manuf. for good clear details. In the meantime:
NEVER EVER use cement at the shingle/step flashing joint. It's a sure sign of a poor job.


Patty I can't get that link to work. It trys to send me to the "new Homepage? and then just loops.