Levelling concrete floor for garage conversion

23 May 2008
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United Kingdom
We have a double garage (5.5m x 5.5m) attached to the side of the house.
It was built as part of the work to renovate the house about 3 years ago - we bought it 6 months ago.
The garage has a roller shutter door and with conversion to a room in due course planned by the previous owner. - the walls were built with a double skin and insulated cavity (with regs sign off).
The floor has insulation under the slab, and the surface is currently 100mm lower than the floor of the house.

We are considering converting the garage by installing a floor with underfloor heating, and insulated ceiling and replacing the roller shutter with patio doors.

Here is the problem.

The floor is not level and has a very rough finish to the concrete. It slopes by 40mm over 3 metres of the 5.5m width.
As we plan to lay a damp proof membrane and further 50/60mm insulation on top of the concrete before installing the u/f heating and screed, we were advised (by a local tradesman) that the easiest and cheapest way to level the base would be to use leveled coarse sand under the damp proof membrane.

I would appreciate any other views or suggestions to overcome the unevenness of the floor.
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