Rebuild top of chimney

[23vc]

Hi
My chimney got damaged last week and now looks like this (see pic) I’ve got an existing wood burner so it’s got a flexible lining which I need to terminate and rebuild the top of the chimney.
My plan was to go as high as necessary with the brickwork to allow the flue to terminate 600mm above the ridge (which I’ve heard is the minimum height) then set a new chimney pot with mortar haunching, then cut the flexible lining to shorten it a bit, fit a new top clamp to it after sliding the new pot down it, then finish with a bird guard.
Never done this before so the above could be totally wrong, any advice appreciated

 

[stuart45]

As you are rebuilding some of the stack you may as well use a closure plate and clamp on top the the brickwork. You can use an adaptor for the top of the pipe and build the pot in a few courses rather than relying on the flaunching which is really for weathering.

 

[23vc]

Thanks Stuart does the closure plate need drilling and screwing to the brickwork or can you bed it?
Also the existing chimney has a dpc which is where I have taken it down to, but I am worried about the top half above the dpc coming adrift in high winds as it’ll have a tv aerial on it and its only a 18” square chimney. Not sure if that’s an issue or if there is a way of reinforcing it?

 

[stuart45]

Thanks Stuart does the closure plate need drilling and screwing to the brickwork or can you bed it?

Just sits on the brickwork, clamp goes on top round the liner.



Also the existing chimney has a dpc which is where I have taken it down to, but I am worried about the top half above the dpc coming adrift in high winds as it’ll have a tv aerial on it and its only a 18” square chimney. Not sure if that’s an issue or if there is a way of reinforcing it?

It's normal for modern stacks to have a DPC tray, the weight of the stack holds it on.

 

[23vc]

Thanks Stuart
So the closer plate sits a couple of courses below the top then the pot sits on it and flaunching to hold it in place and provide weather proofing?
I’ve attached a sketch of what I think I need to do, any advice on it really appreciated (the old chimney which has now been dismantled is shown on the left)

Sorry meant to add, I’ve not included a tray just a dpc, as the old one didn’t have one and I’ve never had any issues with damp coming down from it, is that reasonable or cutting corners?

 

[stuart45]

The way I normally do it is to put the plate and clamp 2 or 3 courses down and then sit the pot on with the liner coming up a bit higher and an adapter to the top of the pipe.
Use a oversailing course or 2 to build the pot in and then another course back in to the size of the stack below. Then the pot is securely built in and flaunch the top to weather it use a sharp/building sand mix.
Up to you if want a tray, some without it seem OK.

 

[23vc]

Like this?

Not sure if I need the adapter you refer to as I have already got a bird guard and another clamp which keeps the liner level with the top of the pot (hooks over the top of pot)

 

[stuart45]

That looks about right. You don't need the adapter for the top in your case.
After the 2 oversailing courses you could build another course back to the original size as the pot tapers in and allows for this. It gives a bit of weight and stability to the oversailing courses. Slope a mortar fillet from the top of the oversail to the course that steps back again.

 

[23vc]

That’s spot on advice Stuart thanks so much
Final question (for now) do you bed the plate the full width of the brick like you do a dpc, with a sliver of mortar on brick then plate then normal bed for next course?

 

[stuart45]

Bed the plate with a bit of mortar so the hole is central. If you buy or have one made up the edges should be back a bit from the edges of the brickwork.

 

[23vc]

Magic thanks. Can I just put a load of mortar between oversail course to fill around pot base or am I better with a lean concrete? (Sorry I knew there would be more questions)

 

[stuart45]

A lot of wet mortar tends to make it belly out a bit, so use some dryer mortar with maybe a few bits of broken brick or stone etc.

 

[23vc]

Great, thanks

 

[stuart45]

No prob.
Are you insulating the flue?

 

[23vc]

Not planning to, keeping it as like for like as possible and trying to keep it simple