poured chimney cap new build.

davidhick77

hi, just looking for a bit of advice on pouring my own chimney caps. 4" inch concrete block construction with 2 flues. just need advice on correct method of finishing off the job.
flues and filling currently to a level of about a foot below blockwork at the moment. timber frame with drip strip allready made. need advice on depth of concrete below blockwork level, expansion material around flue...use of d.p.c.??. sealing around flue at finish level.
thanks to anyone who can help

paddym3

fill the lime sand filling to top place on another flue and pour 4 inch concrete with agood slop out. wouldn worry about sealing the concrete to pot. if the concrete is lean ie lots cement (3 sand to one cement) the concrete will seal to pot. if the slop of the cap is good the water will run off easily cut the last flu about 6 inch from top of concrete at a later date

mickdw

You MUST leave a movement joint between flue and concrete cap or else you will get cracks.

the keen edge

I've formed and poured a good few chimney cappings and none have leaked or fallen down. I'm on the west coast so you can be satisfied that this method of construction has been well tested.

I'm of the mind that pre-cast cappings are an effective(cost and structural) option where a specific design is not required; however you have your formwork already made so the merits of pre-cast vs. in-situ can wait for another day. As will discussion of release oils, plywood linings, etc.

Ok, well I'd firstly point out that with some relative care and attention to details forming a chimney cap that performs as it should, is at this stage of your construction process easily achieved. However repairing a leaking, crumbling or otherwise faulty capping is an absolute nightmare and costly too.
You don't want to be up on the roof in next November, hanging off a slating ladder scratching your head wondering how can so much water ingress through a seemingly well formed chimney capping.

Firstly preparation.
Fill the void space with the sand/lime mix, compressed to within an inch or so of the finished block height.
The final flue should be left full size; adjust the overall flue projection by cutting the penultimate flue to the required length, ensuring that the collar of the final flue is covered by the finished capping.
Wrap the final flue, from its collar to above the finished height of concrete in DPC. Use sellotape/electrical tape to keep it in place.
Form an additional upstand that sits on top of your main form and around the flues. 25mm height by 300mm*300mm for a single flue, so maybe 300mm*900mm for your double breast. This additional upstand will further isolate the capping-flue joint, from the larger surface area of the capping proper.

At each corner, and mid-way on the two wider sides of the chimney stack, nail 2' lengths of 4" or 6" timber material vertically to the stack. Refrain from nailing to the final top course blockwork to avoid loosening blocks.
When set the capping should be ~40mm below the finished top course of blockwork.

The mix.
Getting the mix correct is definitely the most important aspect of the job. Personally I never make the concrete mix myself for this job. I would have the formwork constructed and in place at the earliest opportunity and then whenever concrete was being order form the ready mix suppliers for footings, foundations, etc: I would let the suppliers know that an extra gauge would be required for a capping and ask them to add a waterproof additive to the mix. Alternatively I'd pick up a trailer load myself if need be.
The bottom line is that the concrete mix is, obviously, the integral component. And regardless of how big or small the quantity of concrete is needed, getting it from a dedicated concrete batching plant will ensure its integrity.

Filling the form.
With the form set level in place you are right to start filling. The main thing to ensure during filling is to apply a even tamp all around the perimeter of the form, basically by striking with a hammer. The capping surface should fall from the flue towards the edge of the capping, and the concrete should be finished with care to a smooth, dense glassy appearance with a steel trowel. Cover the wet capping with polyethene if rain is expected to fall during the curing period.

Striking and finishing.
Allow a couple of days to past before striking the form. When striking disassemble the form piece by piece to avoid damaging edges.
The exposed dpc wrapped around the top flue should now be carefully trimmed to the top surface of the capping. After this is done, seal the around capping-flue joint over the trimmed dpc with Tec7. Allow the Tec7 time to cure and then liberally apply a waterproof concrete sealant paint to the top surfaces of the capping. Woodies have a decent range of these type of paints.

And finally.
Stand in your driveway looking upon your capping, give clap yourself on the back for a job well done confident that wherever a leak may occur in your house in definitely will not be from that chimney capping.