Dangerous to dig up concrete floor?

[smith8347]

I am about to start digging up the concrete floor of my ground floor bathroom. I am putting in 2 new soil pipes and drainage for a wet room Given the amount of work required for this I decided that I might as well break up and excavate the floor of the entire room, and install a modern floor with insulation and underfloor heating pipes. The UFH pipes will be capped off somewhere for future use.

The building is a 1970s bungalow and although I havn't confirmed, is likely to have a raft or slab foundation. Is there a danger that in excavating all of the slab for one part of the house, it could compromise the structural integrity of the building?

I remember installing a soil pipe on another house , and using a diamond saw + SDS chisel to cut a long trench through the slab, finding it a bit scary to see the sand, hardcore and earth exposed. This time it will be an entire room.

 

[^woody^]

If it's a raft, the raft is the floor and should not be broken up.

 

[smith8347]

Clearly the concrete would be re-poured although it would be much thinner to accommodate insulation and so on.

If this isn't possible, that is basically akin to saying that wet underfloor heating is impossible for this type of building?

 

[Mike13]

A modern house would have insulation below any raft. Underfloor heating has water at ~40C going through pipes to keep a room at ~20C, so you now need twice the insulation under the floor...
It does not make economic sense in my mind, welcome to see some calculation proving me wrong. I would then consider it for my house.

 

[lostinthelight]

I was going to cap ufh pipes as I wasn't going to connect it straight away.
From the research I did, the advise was to pressurise the pipes before laying the slab to test for leaks and to leave it pressurised while the cement cured so I fitted a manifold and isolation valves.

 

[^woody^]

If this isn't possible, that is basically akin to saying that wet underfloor heating is impossible for this type of building?

It's akin to saying that a raft is the structural foundation and if you break it you have a major problem if the building then starts moving because of that.

 

[Notch7]

The building is a 1970s bungalow and although I havn't confirmed, is likely to have a raft or slab foundation

what makes you think that? - raft foundations are normally only used where there is unstable ground like shrinkable clay or silt.

a raft foundation for a bungalow would need a significant amount of concrete and steel reinforcement

 

[smith8347]

I was going to cap ufh pipes as I wasn't going to connect it straight away.
From the research I did, the advise was to pressurise the pipes before laying the slab to test for leaks and to leave it pressurised while the cement cured so I fitted a manifold and isolation valves.

Agree, although just filling with water and making sure it is sealed and there are no air pockets should do the trick, it wouldn't have to be pressurised.

what makes you think that? - raft foundations are normally only used where there is unstable ground like shrinkable clay or silt.

a raft foundation for a bungalow would need a significant amount of concrete and steel reinforcement

I thought that based on what a trademan told me, although they weren't a builder per se. What would be a reliable way to test? Drill a few holes and guage the thickness of the slab/presense of rebar?

 

[smith8347]

A modern house would have insulation below any raft. Underfloor heating has water at ~40C going through pipes to keep a room at ~20C, so you now need twice the insulation under the floor...
It does not make economic sense in my mind, welcome to see some calculation proving me wrong. I would then consider it for my house.

It's an opportunist thing, since putting in the new soil pipes and drains will require breaking in and doing a significant amount of excavation anyway. I also need to dig down at least part way so that the floor of the wet room can be re-made with a drainage slope. From my experience, once a trench has been made it is realtively easy to chip away the sides and break up a larger area.

I have no idea whether I will ever do the whole house (not planning to currently) but even having UFH in the bathroom by itself would be nice.

 

[lostinthelight]

Agree, although just filling with water and making sure it is sealed and there are no air pockets should do the trick, it wouldn't have to be pressurised.?

https://www.johnguest.com/gb/en/res...derfloor-heating-mistakes-during-installation

 

[martygturner]

err Dig trial pit by outside wall , but if you have to put a new soil in why bother, dig out at the exit point by the wall if the brick carry's on past the slab then you have what ? As for all that guff about oooh you cannot touch a slab its just a mechanism for transferring load to the ground, how much of the ground will you be taking out from under the walls? 6 inches for the new pipe, the rest of the perimeter will be happily supporting the walls. My house has no foundations just 6 inches of concreter direct on the earth....its 90 years old and still solid. The power of bonded brick to span a small opening is immense...see any bdridge.

 

[Nozzle]

I had my solid concrete floor dug up to have insulation an UFH. Big job bu well worth it. The foundation were trench rather than a raft. Don't do it if you have a raft.

Nozzle

 

[smith8347]

OK an update. I finally had the time to carry out a little survey.

• I drilled a hole vertically down into the floor somewhere in the middle of the building, far from any walls. The drill bit broke through the concrete at around 20 cm deep (the point at which there was no resistance).

• I repeated this next to one of the exterior walls where a raft should be thicker. Same result.

• I then dug a small trench near the wall outside to inspect the edge of the foundation, the sketch shows a cross section of what I observed. I don't know how deep the concrete pad goes.

I now reasonably confident that these are normal strip foundations (or at least, not a raft). Any thoughts?

 

[martygturner]

Yep...those sound like strip foundations, your internal floor is above the foundations as its 200mm thick, which is a standard floor,150mm concrete, 50mm screed. Ie The internal floor starts above the foundation top level, they are not connected.