Broken ic cover cover slab - Unable to get cooect size

[kenn]

Hi, the concrete cover on the inspection chamber in my driveway has broken, I have contacted a number of BS but am unable to get one the correct size. The internal dimensions are 706 x 906 the closest I can get locally (merseyside) is 111.5 x 810. This would give me 2.5 cm to rest on either side, would I be able to use concrete blocks or a concrete lintle placed so that they protrude 7.5 cm over the inside of the chamber theby giving me 10 cm to rest the slab on. I would only have to do this on two sides as the slab is long enough to extend over the other two sides. The house was built in 1960 and was part of a self build estate. I also want to lay a channel drain with the water flowing into the chamber (the sewer is a combined system) Any comments would be appreciated.

[Tony McC]

I think you're talking about "corbelling" which means laying masonry in a way such that it overhangs (correct term is "oversails") the existing chamber wall and so reduces the opening dimensions. This is relatively standard practice in the trade and as long as you don;t reduce the opening to less than 600mm minimum, there's no problem.

Generally speaking, the maximum oversail when using engineering brick is 50mm. If you did this all the way around the chamber opening, you could reduce it by 100mm - 50mm each side.