Our crappy 1980’s extension is built with a suspended floor that is 2cm higher than the rest of the house.(!)

Below the suspended floor is concrete with DPC below. The concrete however isn’t level. Across a 10m length the suspended floor is 18cm above at one end, 8cm at the other end.

As we look to change some rooms it is time to sort the floor issue.

I don’t think there is insulation in any of it. I’m thinking new DPC on top of existing concrete, new timber suspended floor joists with combination of foam insulation sheets and some rockwool or spray foam to take up the difference in heights.

Thoughts?

The concrete under a suspended floor level means nothing its there to keep weeds and beasties at bay normally same level as ground level outside. You need maybe 250mm below the joists to conc for air space

That’s my worry.

That said, it’s not that depth/cavity under any of the floor. By halfway across the room the joists must rest on it. By the far end, where I’ve done some plumbing, it’s about 8cm cavity/joist depth with planed to shape wood resting on concrete.

There should be insulation boards on top of or more likely between the joists (whether timber or pre cast concrete). If theres genuinely nothing I’d be far more interested in resolving that than lowering the floor. If you do need to lower the finished floor level your options are limited without compromising the ventilation space and potentially causing a build up of moisture in there, which will eventually rot the joists.

It should have been a floating floor even back then or concrete up to ffl.
Easiest way to rectify, rip it all out dig down maybe 300mm below floor level and start again dpm, kingspan below 100mm of concrete with a142 mesh, 2×2 with new flooring to correct height

That’s pretty expensive and time consuming work just to lose a step in the floor. I’d also add that the conventional minimum depth for the void below the suspended floor is only 150mm not 250mm so this may help. Provided air bricks are sufficiently sized and good cross ventilation is provided.

Edited for spelling.

There are no air bricks at all in this area of the house.

This is part of a move of kitchen, bathroom and bedroom downstairs, so all in all won’t be much on top and our only chance to do the floor leveling and insulation.

Every suspended floor needs ventilation to function properly or it will just trap moisture and condensation. The lack of insulation will be costing you money as you’re heating the ground up below but with no air bricks, it might actually be accidentally minimising condensation as you’re heating a space that normally wouldnt be heated.

I’d try and resolve both these issues and if you can lower the floor at the same time do so, if it were me I’d get a surveyor or architect involved but I may be biased 😉

Architect or whoever did the original drawing may have been at fault and a surveyor will just add to the cost by his fee and telling you to rip it out.
Building control surely couldn’t have passed it as it’s a suspended floor with no vents and that extra height is a trip hazard. The wood might not have been treated and has started to rot

I’m thinking new DPC on top of existing concrete, new timber suspended floor joists with combination of foam insulation sheets and some rockwool or spray foam to take up the difference in heights.

I don’t follow what your plan is regarding the level. If it’s 2cm too high now, and only 8cm joists, that reduces to only 6cm for the joists – are you taking out the concrete, or raising the floor across the whole house?

I’m using planed down wood supported by the concrete underneath…
This would create a ‘solid’ floor.

The whole extension is well dodgy – half is 1982, the other half 1992. 1982 was flat roof, 1992 one incorporated the flat roof under a new pitched roof, but without insulation or any vents. 1982 is all blockwork with narrow cavity. 1992 is timber frame, properly insulated in walls and roof but no underfloor insulation at all. Joint between them is awful and all floor levels are uneven. Neither have air bricks. Main house is 1970’s semi with air bricks and 2’+ void below the floor.

This would have been a great one for wrighty he would have it all sorted within a few short paragraphs for you and had the dodgy builders punished for you too

is the main house solid or suspended floor ?
If the extension sub-floor does have a good dpc under it( dont know how you can check without destroying its integrity) place new dpc over concrete then could you level up with insulation to give 150mm – . 100? conc over to match levels
are there any air bricks in the original wall under extension? if so install some ducting through new floor to give air flow through.
what we really need is a resident go to builder type guy!

is the main house solid or suspended floor ?

Suspended over (bone dry) sand. The house is on a slope, so the sand had been added it seems to level the under floor. There is about 50cm under most of main house.

are there any air bricks in the original wall under extension

I assume so. However two archways have been created where there would have been windows and likely air bricks, as access into these newer extensions.

If the extension sub-floor does have a good dpc under it( dont know how you can check without destroying its integrity) place new dpc over concrete then could you level up with insulation to give 150mm – . 100? conc over to match levels

That’s basically my plan. I would lead new dpc up to skirting height and tape it to a breather membrane that’s under the floorboards, use water resistant materials that are usually used sub-ground in a new build, topped with breathable membrane under the floorboards. This would create a completely solid floor, encased in dpc below and sides, breather on top.

The house has no damp, I’ve added new French drains all round.

Can’t you just level the existing sub floor with concrete / screed/ self levelling then membrane, insulation with floated floor over the top? I’d use Sterling board over chipboard too.

The concrete you refer to under a suspended floor is the solum, by the sound of the quality of the extension it will probably be the very weak mix slurry 40mm maybe 50mm deep possibly on vegetable matter, tap on it and it will probably be boss, definitely wont take weight. The dry sand under the house is your solum usually had bitumen poured on top. Quite a balls up when they built the extension must have forgot about the flooring when setting the wall plate datum that is the 20mm high

Quite a balls up when they built the extension

It’s a DIY job, although signed off and plans there, built poorly.

If you want to do it properly then, dig out the existing concrete site cover, and replace with hardcore/sand blinding/concrete/screed. A lot of work just to lower the level 60mm. You could probably get away with just chucking concrete on top of the existing and get away with it. I did this on a previous house, where the extension was floored with some screed on sand!! It worked out fine.

I’d pull some boards and check for rot, if it’s fine after 30 years t is probably fine even if not built to regs. Unless it’s really cold and you have to have the insulation, I’d be looking more to putting sheet down on adjoining floors to raise them 2cm and make flooring flush rather than all that effort to lower the extension by 2cm.