Chimney repairs

[ath007]

Does anyone know the correct cement/ aggregate mix for a chimney capping. The current one is cracked and lose and is best replaced. Additives - unibond? waterproofer?

[MustafaTinkl]

So that the right advice can be given please say exactly what it is you are proposing to do i.e. flaunching up existing chimney pots that are still there, replace chimney pots which have been removed, seal off unwanted flues, whatever.

[ath007]

Will remove the metal 'pot' from the flaunching- have a gas fire fitted, remove remainder of flaunching, remove cracked capping'stone' casting.Install new shuttering to cast a new capping'stone', refit 'pot' and flaunch once more. Thanks.

Arthur

[MustafaTinkl]

Understood. Presumably a brick stack in stretcher bond extending a few courses above the roof and whilst the existing capstone is cracked you have examined the brickwork beneath and found it sound. The new capstone should be not less than 75mm thick, be wider than the brickwork by not more than 20mm, and have a 5mm half round or square drip. It should be laid on a DPC of polythene sheet. The mix should be 1 of cement 2 of sand and 4 of aggregate (the agg should be something like 12mm clean stones, no fines or sand). You presumably cannot get a vibrator up there, so it must be well rammed by hand. If you can introduce a mesh of 6mm steel rods in the middle so much the better. Trowel the top of the concrete to fall away from the flue. The flaunching should 1 of cement to 3 of sand with only sufficient water to produce a stiff mix. It is better to bullnose the flaunching rather than come to a feather edge. No additives of any sort are necessary.

[ath007]

thanks Mustafathink, the existing cap stone is 2 inches thick and extends out an inch and three quarters. Did not know about the dpc!, was going to use waterproofer in the cap and flaunching mix. Cheers

[MustafaTinkl]

2" thick is alright only if the cap is pre-cast in a factory. Doing it in-situ as you are 3" is the minimum. The projection of 1 3/4" is far to much and contravenes the recommendations: 20mm is the recommended maximum.