weather sealing old chimney

a sheet metal "Chaise" top, and paint/ seal exterior of brick sides

mortar a 2 inch thick piece of concrete to the top of the opening not used.

or

make a 2 inch thick piece of concrete the same width as the chimney (both openings) , mortar 6 bricks on edge (1 at each corner and two in the middle) and set the concrete on top of them. Like this : http://www.sas-sa.com/imagenes/gat.jpeg

Wind driven can still get in , though, with the concrete rain hat.

If it's never going to be used again, take it down and save the cost of "temporary" weatherization. Moisture is penetrating the joints from inside/out as seen by the leaching which means it will eventually come down unless it is retucked and proper seal made. If the chimney gives the house "curb appeal" then cap it with a chase cover and plan on making repairs. JMO though.

Wood Heat Stoves said:

a sheet metal "Chaise" top, and paint/ seal exterior of brick sides

Click to expand...

Good idea; Stainless Steel sheet metal will work best.

I am with the take down. It will look better than capping it. For the same reasons Retreadsme said I think it will save you headaches down the road.

I would power wash it,scrape out the mortar joints and repoint .Knock the loose mortar off the crown and recrown it with two pieces of flagstone,shale,or last choice ,slate. After its dry, I would give it a coat of Sil-Act(commercial masonary silicone based sealant). Much easier then taking the chimney down below the roof line and reworking rafters,sheathing,and shingles.
I do a few every year.

You may want a regular stainless steel chimney cap with spark arrestor/screen. Try doing some research...thought I've read someplace that completely sealing the top without ventilation can cause condensation problems.
Or, try the stainless steel sheet metal idea, with edges turned down 2 inches at the perimeter. Hold metal off from the masonry with spacers about an inch to allow some ventilation. Use mechanical fasteners and probably incorporate an insect screen to keep out bats & bees.

In the unused fireplaces, make sure the damper is closed or block flue off with unfaced fiberglass. Perhaps add a written reminder note somewhere in the firebox that the chimney is blocked.

I've got a similar situation with the chimney in my shop. It's a block chimney with clay liner. Most of the chimney runs on the inside, but the part that sticks up thru the roof is starting to flake/delaminate. It's i no danger of becoming unstable, etc. but what's the best way to stop the deterioration?

If the terra cotta flue is deteriorating,chisel it out and set a new piece with mortar.Clean out bottom.
If the block is deteriorating,wire and two coat stucco. You can buy different colors for the top coat to make it look pretty.

Try not to decommission your chimney in a less than durable way. It will
erode the future value of your home. Curb appeal counts too!

Choose wisely.

We had an old chimney that was used as a gas vent when we moved in. I put a big concrete paver on it, supported by four concrete garden edging blocks (the scalloped type), then put hardware cloth to screen the openings and covered it all with roofing tar. Cost under $10, took less than an hour, and looked great from the ground. A few years later we took out that old freestanding propane furnace, and the chimney was no longer necessary. The builder wanted to cover it with siding, which I thought would be ugly. I went up on the roof and hit a brick with a 4 lb. hammer. The mortar had completely failed. The only thing holding up the chimney was the stucco on the outside. In an hour or two I had taken the bricks down to ceiling level in the house and then it was just for the roofers to seal it up.

So that's kind of a long story, but I guess the moral is that if bricks pop loose with a light tap of a hammer, then it's time to take it down.

Put a piece of tin over it now to block the majority of the rain temporarily. Then, decide either to tear it down below the roof line and roof it over or get it rebuilt correctly. Looks like water has been doing damage for a long time and I don't think there is any middle ground that is going to keep the inside of the house dry.