I’m adding a timber-framed screen porch onto the gable end of my house, where there is an exposed fieldstone chimney. Roofing material on the new porch will be a synthetic slate material (“Eco-Star”). I need to insert step flashing where the new roof pitch crosses up and around the projecting stone chimney. I’m assuming lead is the best flashing material for this use, as it will conform readily to the stone. My questions:
1). Should I run the flashing up to the nearest mortar joint, chipping out mortar to insert flashing (as one would do on a brick chimney), or use a diamond blade to cut a shallow kerf in a straight line across the chimney face?
2). How deep should the flashing be inserted in the chimney?
3). What material should I pack into the mortar recess or saw kerf to hold/seal the flashing?
Thanks, as always, for any advice.
George
Troy, NH
Replies
George
I dont know flashing for a stone fireplace but I know that you do it differnt then brick.
Mike Smith was typing about it not to long ago, I think its something called "through flashing", check into it, or get Mikes attention and get the real info from him.
I think Slateman could also help you with this question.
Doug
Edited 9/3/2006 10:16 am ET by DougU
Through flashing from a couple of years ago...http://forums.taunton.com/tp-breaktime/messages?msg=51239.1There's a graphic buried somewhere in that thread.Mongo
It's a whole different thing when doing pre-existing than doing a new one. Part of the answer would depend - for me - on the condition of the mortar and the type of stone. Either way, I would not try chipping mortar, I would use a grinder or diamond saw blade. Unless rounded river rocks, probably cut the straight kerf. Half inch deep.
Lead is the right flashing. Get the kerf cut good and clean.
To seal it in, I like Geocell caulk.
One way to seat it is to dry fit the flashing, then squirt the caulk, then fit the flashing permanently, then use a mortar to point the joint on top.
be patient, this will be a slow job on rock. You might have to use props and wedges to hold pieces in place overnight one at a time while things kick.
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I am a retired career roofer. I've flashed a few natural stone chimneys with sheet lead very successfully. I would use a 4-1/2" diamond masonry cutting wheel on my angle grinder, NOT an abrasive wheel. Cleaner cut and less dust. Carefully cut kerfs in the mortar joints. I think cutting the stone might create a stress point and lead to the stone cracking at that point. On the sides, over-cut the joints to provide about a 3" lap, about an inch deep. On the bottom flashing, try to follow a continuous horizontal mortar joint, so the flashing is one piece without laps. Form the bottom flashing up the sides, and interweave the side flashings with the shingles. To keep the sheet lead in the kerf, install bits of sheet lead above the flashing every couple of inches. I do this with a small piece of 3/16" x 1" flat steel bar, hammering in the lead pieces into the kerf beyond the surface of the mortar to create compression. Clean as much of the dust out of the kerf and neatly tool a bead of good quality urethane caulk at the flashing/stone joint. I usually use NP-1 caulk's "stone" color, it's a close match to most mortars. If the bottom flashing needs to be face-nailed to secure it to the roof, use copper roofing nails and seal the heads with Alcoa Aluminum Pigmented Gutterseal. It will weather-down to match the lead. Be sure to use appropriate personal protection against the mortar dust and lead contamination. Remember the first Rule of Roofing..."Be the raindrop!" Good luck!
I agree with almost all of roofguy's techniques that he's advised you on.
Definately use a grinder to cut out mortar joints-not a straight kerf thru the stone.Same with pointing in lead ounterflashings-his way is great.
I would differ in that I'd use copper or lead coated copper for the flashings in the roof and just use the lead as counterflashing-as per roofguys excellent description.Lead makes a lesser quality flashing in the roof since its so much softer and susceptible to "erosion corrosion" or the wearing away of the product due to constant water flowing .
Thru flashing would be an option if you were building the stonework from scratch,but since it's already in place this way will be sufficient.
Can you describe through-flashing for those of us who like to stay on the ground?
Through flashing would have to be done while the chimney is under construction,not if it's already built.
The metal man is enlisted to flash the inside of the unit after it exits the roof,including wrapping each flue for the first full tile above the roof.
The theory is that any water migrating thru the stone or brick top must be caught by the thru flashing and exit thru weep holes instead of finding its way into the structure.
I had a photo thread this Spring on one I did-you might find it by searching,I can't remember the numbers of the post
Thanks much to Roofguy and Slateman--this approach makes excellent sense to me, and was described in sufficient detail that I'm quite clear about how to do it. The mortar in this chimney is in fine condition (it's only about 30 years old), so it should hold up to this treatment well.
Best,
George
I'm gonna bring up one minor quibble with your excellent description. There is no reason to face nail anything on a roof except maybe the last ridge shingle. For your apron flashing, First nail down several pieces of the metal (1"x3-4") you're using for the apron so it protrudes past the front of the apron an inch or so, but so the nails will be covered by the apron. Install the apron and bend the tabs back up over the apron. In high wind areas pop rivet through the tab into the apron.
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