Lowering Floor, solutions for DPC

[sgh]

 

Hi everyone,

 

We are doing a major renovation of a 1930s semi and extension to the side wrapping round to the rear (open plan kitchen diner at the rear of the property) 

 

In the kitchen we currently have a suspended timber floor which has a 1.6m void underneath (we are on a slight slope hence the void). 

 

Because we are on a slope we are going to lower the floor of the rear and side extension by 30-40cm to gain more ceiling height. Architect has spec'ed block and beam as we are hoping to go with wet UFH/tiles.

 

We want to lower the floor of the existing kitchen room of the old part of the house (not the extension) to gain more ceiling height in the new open plan kitchen - diner. This will then match the extension floor level. This does mean our floor level in the existing kitchen will go below the 1930 DPC. Of course the new extension will have its own new DPC.

 

What are the possible solutions to this? I've come across the following reading online and speak to builders;

 

1. Because we've ripped everything out and have access, knock out 1 brick out at a time at the new floor level and place a new DPC and replace the bricks.

 

2. The block and beam joists, cover the ends going into the walls with DPM or something similar and then when placing DPM over the block and beam floor overlap it over onto the walls up till the old DPC and then plasterboard etc over it.

 

4. Go for a hard ground bearing hard floor and do the same a step 3, place DPM over the floor and wrap it up on the walls up till the old DPM.

 

5. Inject a new DPC into the bricks at the new lower DPC level.

 

6. Leave the bottom 30cm of bricks on the two walls where the new floor level is below the 1930s DPC, exposed (we were thinking of having some of them exposed as part of our design anyway). That should allow them to breath (?). Maybe paint a water repellant paint over the exposed brickwork.

 

Sorry if these questions are silly - we have no experience and haven't broken ground yet! Just been reading around the subject and watching videos.

 

Any advice would be very much appreciated!

[markc]

No such thing as a silly question, answer depends a lot on what is outside and level of surrounding ground compared to the new (loler) level

[Temp]

23 minutes ago, sgh said:

The block and beam joists, cover the ends going into the walls with DPM or something similar and then when placing DPM over the block and beam floor overlap it over onto the walls up till the old DPC and then plasterboard etc over it.

 

The above is probably the easiest.

 

[Adsibob]

Your situation sounds very similar to mine. We were converting and extending a 1930s semi, but council limited the height of our full width extension and so to make it “nicer” to live in we dropped the internal floor level of the whole house (not just the extension).

But in our case we took the opportunity to install a slab floor, instead of the ventilated timber subfloor that was there previously. To do this, we had a DP membrane cover the whole floor (over the slab) and then come up the sides, past the house’s original DPC. The build up looked something like this (there is some info missing, like sand blinding, which in think was in a separate drawing):

 

And before anybody tells me 100mm insulation isn’t enough, this wasn’t a passive build and I was starting with a 1930s house and a council that wanted to limit parts of our extension to only 2.4m high!

Edited May 4, 2023 by Adsibob