Problem,leaking ispection hatch 4" Terrain soil stack

[Pssst]

Hi all, i normally hang around in the gas safe forum as thats my speciality. I am just investigating some damp skirting board in the house which is at the boxing in for the internal vertical soil stack. Anyway ive ripped the boarding off and discovered a 4" Terrain soil stack with an inspection cover at the base secured with a single screw in the centre. The gasket has clearly crumbled as when there is high drain flow, ie bath emptying,,is Soding out big time. The question is, can i get a new gasket? if i try and undo the screw is it likely to shear off internally? whats inside and how does the screw secure? if it shears off, what are my possible solutions? The pipe would be about 40 years old from new build.

Thanks all
[automerge]1589643326[/automerge]
Now sorted !...undid central screw and with a bit of wiggling the cover came loose but not away from the pipe as its retained by an internal flange. It slides down to uncover a vertical hole in the soil stack for rodding/inspection. The marks on the pipe indicated that whoever had originally fitted the cover had not centralised it so the top left hand corner was barely being covered by the gasket. Fast forward and it started lashing out at that point. I think it had been weeping for years and had just been tricking down and under the concrete raft into the earth ! .Slid it down,cleaned up the surfaces and marked the central points on the pipe. Recentered the cover and tightened screw,,job done,,hopefully this info will be useful for others.