I want to reconstruct old Manhole chamber in Porch (Ed.)

[omits]

My upstairs BRoom soil goes down 12 feet then into a clay pipe in the foundation slab turns 90deg once there (in the concrete) then 8 feet to an old (1970s) 600 x 400 chamber. This chamber is brick and is in the floor of a small porch. Because the porch is old build and uninsulated I want to take the clay tiled floor up and the slab below and fit insulation and new wood floor. The chamber is the problem (about 50 - 70 cm deep and serves just the one pipe before the public drain about another 8 feet further on (outside drain serving two neighbours along the back of the properties) and I have an idea of reducing its dia. using a modern riser say 35cm dia. with a sealed lid (nice and small and round). To do this I propose to insert the riser sleeve so it rests on the bottom of existing chamber over the single pipe below (just from my BRoom) and filling in the sides with mortar (dressing the lower end of course so it can't trap stuff).

Any advice on this idea welcome as not sure what is the best solution. I'd rather not dismantle completely the old one and its never given any trouble and I don't think I can just 'fill it in'. Many thanks for your time.

 

[noseall]

My upstairs BRoom soil goes down 12 feet then into a clay pipe in the foundation slab turns 90deg once there (in the concrete) then 8 feet to an old (1970s) 600 x 400 chamber. This chamber is brick and is in the floor of a small porch. Because the porch is old build and uninsulated I want to take the clay tiled floor up and the slab below and fit insulation and new wood floor. The chamber is the problem (about 50 - 70 cm deep and serves just the one pipe before the public drain about another 8 feet further on (outside drain serving two neighbours along the back of the properties) and I have an idea of reducing its dia. using a modern riser say 35cm dia. with a sealed lid (nice and small and round). To do this I propose to insert the riser sleeve so it rests on the bottom of existing chamber over the single pipe below (just from my BRoom) and filling in the sides with mortar (dressing the lower end of course so it can't trap stuff).

Any advice on this idea welcome as not sure what is the best solution. I'd rather not dismantle completely the old one and its never given any trouble and I don't think I can just 'fill it in'. Many thanks for your time.

We ALWAYS renew in plastic when working on drains. An old IC would get dug out and replaced with a modern chamber with multiple inlets.