Hi beatrice1711, The first thing to try is to identify which joints are leaking in the loft and to remake them. If they are solvent joints use pvc glue, if they are ring seal use silicone mastic. The apron around the pipe at roof level may be leaking but if this is happening you may be able to see daylight when you look inside to confirm this. I have never seen a cap fitted on a soil vent pipe - don't waste your money. Regards S
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