Thread: Lowering a concrete floor

Surprised this one hasnt been mentioned before but nothing found in a search.

We have a highset house, single skin brick and pier below, with an inset slab. We're thinking of fitting out the lower floor, which is currently just a couple of garages, a bathroom and a lot of empty space. The council seems to have no great objections, as long as we conform to the std building codes. The stumbling block is the internal height. After fitting a floating floor and a ceiling, the finished internal height should be about 2300mm. The council says we should be able to get permission to put a kitchen, bathroom, laundry and rumpus in this, which is actually a pretty good result if we can slip a dining room into the 'kitchen' area. To put a proper loungeroom or bedroom in instead of the rumpus it would have to be 2400mm. I would much rather leave the option open to put anything we want in, and anyway I dont like the feel of low ceilings - I'd like it to be at least 2400mm just from an aesthetic point of view. The upstairs is 2700mm so I dont want an underground dungeon effect.

I am vaguely aware that there are two ways we can deal with this. One is to remove the current slab and replace with a lower one. Actually, this has already been done over about a quarter of the area because the previous owners had a large caravan, so some of the floor has already been taken down about 200mm (remember its an inset slab, so I presume it just needs to be broken up and carted out - maybe ????). The other option is to lower the top of the slab by cutting and grinding the top off. I dont know the thickness of the slab, but I think its very thick judging by the aforementioned lowered part, and also because the previous owner seemed to have been in the habit of doing things right (at least when other people were doing the work), if not overbuilding.

My question is does anyone know about this type of work. Is it practical? a common solution? What issues are involved? Are there alternatives? Anyone had it done??? Costs???? Any information at all would be appreciated.

Standard caveat applies - I'm intending to get professional advice when the time comes, and someone else to do the work - I just want to start building up my knowledge before getting people in. I realise work like this is not cheap, but in the scheme of things not worth skimping on.

Cheers
Arron

Can`t really comment on the floor,but would like to know how much you allowed for ceiling instalation? as there are a few ways to do this.

I doubt you will be able to grind the existing slab by 100mm (if I'm reading that right) and it would be expensive, time consuming and very messy... and you probably won't have a slab left at the end of it.

You would be better off breaking up and removing the existing slab and then excavating down the required depth for the new slab.

Originally Posted by PlasterPro

Can`t really comment on the floor,but would like to know how much you allowed for ceiling instalation? as there are a few ways to do this.

about 30mm from the bottom of the rsj's. Just a guess as it is something I havent investigated. I allowed 20mm for floating floor and underlay, and the existing height is about 2350, which is how I came up with 2300 finished. I have attached a picture of the downstairs which I posted before when asking about windows. Any advice about installing ceilings to maximize height will be gratefully accepted.

I have been wondering about just boxing in the RSJ's and fixing the ceiling to the bottom of the floor joists (or battens therein, or whatever). However I'm wondering if this would look daggy, or overly constrain where we can put the internal walls. I would at least give me about 2600mm over the majority of the floorspace

thanks
Arron

What about boxing in the RSJ's as bulkheads?

I don't recall what the rules are (in regards to bulkheads and their effect on habitable ceiling height), but it might give you the required head height.

Originally Posted by Vernonv

I doubt you will be able to grind the existing slab by 100mm (if I'm reading that right) and it would be expensive, time consuming and very messy... and you probably won't have a slab left at the end of it.

You would be better off breaking up and removing the existing slab and then excavating down the required depth for the new slab.

Yes, you are reading it right - 100mm to make it legal for bedroom and loungeroom. In hindsight, I can see that is not going to work.

50mm is the compromise position, but then the cost becomes an issue because all I am buying is a bit of feelgood airspace, not an alternative floor plan.

Tending towards ripping it all out.

Arron

Vernon's suggestion is a good one - if putting the lining between those RSJs gives to sufficient height then that's the way to go. Boxing in the beams can be made to emphasise them or to minimise their appearance (a little).

If all you are talking about is 100mm leave the floor alone - design build to the BCA that allows 'non-habitable' rooms to have the lower average height as advised by council then how you use the rooms is your business . . .

When you go to sell you would need to be clear about the nature of the approval area (as it will be on file at the council anyway), but few people would see that as an issue - and if they do then you just revert it to the designed use.

If you still want to 'lower the floor' then your only effective choice will be to remove the existing one and put in a new one to the right height.

Originally Posted by Vernonv

What about boxing in the RSJ's as bulkheads?

I don't recall what the rules are (in regards to bulkheads and their effect on habitable ceiling height), but it might give you the required head height.

Box out your rsj's

Arron

Vern, Bloss and Bricks are talking good sense.

Last doctors appointment I was reading a glossy that featured a house with boxed RSJ's that had down lights set into their side so they shone across the ceiling. Looked absolutely great in their photos but probably grossly inefficient.

If you cannot easily lower the floor, can you raise the roof. Back in the eighties (much less regulated then now) I helped a friend raise an entire Queenslander (about 50 squares including verandahs - 500 sq metres). We used service station jacks and railway sleepers. It turned out to be so easy that the design expectations were changed from a planned 8 ft ceilings in lower level to 12 feet.

Cheers

Graeme

Hope you don't me imposing upon your thread Arron, but can someone please advise, when the ceiling height states 2400 for a bedroom, is this measured from the RSJ or the bottom of the floor joists? We are in a similar situation to Arron, and boxing around the RSJs would save a lot of money and work if the measurement is taken from the floor joists

Thanks,

Alex

Originally Posted by AlexJ66

Hope you don't me imposing upon your thread Arron, but can someone please advise, when the ceiling height states 2400 for a bedroom, is this measured from the RSJ or the bottom of the floor joists? We are in a similar situation to Arron, and boxing around the RSJs would save a lot of money and work if the measurement is taken from the floor joists

Thanks,

Alex

2400mm is from top of floor to bottom of finished ceiling in a 'habitable room' such as a bedroom - and averaged across the room (this allows for dormer windows attic rooms and so on). In practice no-one is likely to actually measure unless it is very obviously a low ceiling. In a bedroom on a lower floor for example you'd have to get a narky certifier to not allow some leeway on boxed in RSJs if the rest of the ceiling was near the top of them or was the bottom of the floorboards above - unless the RSJs were so low as to be a danger - ie: lower than 2m to 2.1m.