replacing chimney cap

[herm0016]

my chimney cap is in bad shape, missing some concrete, cracks. I want to remove the existing and pour a new cap, and then seal it.

I am planing on using some rebar and i have a couple 80 lb bags of concrete. I will build a form around the chimney and mix it a bit stiff.

I have seen recommendations that say to make a 1 or 2 inch overhang to protect the brick and some have not done this.

any advise? have you replaced a chimney cap before? the chimney is in pretty good shape still, so i do not plan on removing any of the bricks / stucco over the bricks.

 

[jmagg]

I've done more caps than I care to remember. Use a 2x4 to case the chimney even with the top of the brick then screw the form to that, this will give you the 1.5 inch overhang.Caps are generally 4-5 inches thick. Find the cubic footage of your form so youre sure to have enough material. Spray the inside of the form with dish soapy water, this will help it release and help it close up. Dump the material in and tap the sides of the form to vibrate and fill the corners, as well as closing up most voids at the form. . Put a minor wash from the flue to the edge to avoid puddling. Trowel smooth when thumbprint hard. The form will show separation when its dry enough to strip, or leave it overnight. If you catch the cure just right, you can rubber float the edge to close in any honeycombs (voids), or you can mix a little fat up after and close it up that way.

 

[herm0016]

thanks!!

should i seal it with something after it cures for a month or so? should i leave a gap to caulk around the clay flue? maybe a few wraps of plastic wrap or some thin packing foam or something?

 

[jmagg]

I've never caulked at the flue, just pack it tight and finish it tight.

 

[bigi]

jmagg,

I might do my crown repair. Which concrete material mix do you recommend? How long does it need to set before smoothing the top?

Thanks

 

[jmagg]

4000 or 5000 compressive strength. The higher the strength, the higher the Portland cement content. More Portland will set a little quicker and be easier to finish and close in. Set times vary widely depending on conditions. You'll want to place it, tap the sides and be sure the corners are full. Get the wash (top) in place where you want it, float it to bring the fat up and close it in (no stone showing). If you lost it at this point it should pass and be functional. Also at this point, I cut the edges of the form at the concrete with a trowel.
Water will begin to come to the top. You'll want to wait until the water evaporates, then flat trowel the top and fill in voids with fat thats already there. If you trowel off the water to speed up the process, youll take the fat with it. Concrete is usually troweled a couple times in the finish stage. More water in the initial mix weakens, not enough leaves voids. Try not to add water in the finish trowel process as this weakens. Youll need a flat trowel and a margin trowel. You'll need to babysit the set. The concrete is the boss and doesn't forgive. After initial set, stripping of forms and filling voids, slowing the cure is advised by misting or covering.
/concrete 101

 

[bigi]

^thanks!