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Posted: Mon Dec 28, 2009 11:28am

Post Subject: Leaking cast iron chimney collar

Checked the boat yesterday and noticed that water had entered from the cast iron chimney collar on the curved roof. The fire and tiled hearth were wet. Can you recommed a suitable filler and sealant that will withstand the temperature when the fire is lit?


Posted: Mon Dec 28, 2009 12:24pm

Post Subject: Leaking cast iron chimney collar

Dear John, First of all you need to ensure that the chimney collar fits close to the roof and has not been fitted with some kind of packing piece that has deteriorated. You can get collars that are cast in a way to allow them to sit on a sloping (part of the curve) roof whilst the chimney remains more or less upright. Once you are sure the flange on the collar is as near a true fit as possible with minimal gaps you will stand a much better chance of making a long term seal. So I think you will need to remove the collar and remove all the existing sealant. The chimney hole in the collar is probably tapered outwards on both top and bottom away form the centre. This will allow you to pack the gap between pipe and collar with heat proof rope and finish with a heat resistant silicon sealer. The way I solved the problem of sealing the flange to the roof was to use a generous bead of heat resistant silicon around the collar boss and the n two more generous rings, further away form the boss of one of the paintable outdoor sealers. Once the collar was bolted down it only remained to clean the ooze of the paintable sealer from the joint and a few days later give it a coat or two of roof paint. Any chandlers or stove supplier should be able to supply the heat resistant silicon - I find it is usually red in colour. Many years of trying to stop leaks has convinced me that in this type of situation removal and re-bedding is the only sure way to go. If you want to try to just seal form the outside then as long as you are happy about paint flaking off I would use and ordinary silicon sealer. The edge of the collar flange rarely, if at all gets more than hand warm. Using a more sophisticated mastic like one of the Sika range will probably not be cost effective unless re-bedding. Tony Brooks

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