old concrete slab foundations query

[donna-j]

First post, so please be gentle!

We have just bought a house, built in the 40s. The floors are all concrete. On the back is a 'sun room', built about 35 years ago. It also has a concrete floor and we can see that it's built on a concrete slab.

We would like to replace this sun room (shack!) with preferably a new extension. The extension would be smaller than the existing shack, so the concrete slab would have to be either dug out completely or 'trimmed'.

Does it sound feasible that we could built an extension on top of the slab that is already there or is this a how long is a piece of string question?

 

[bikerm4]

fairly unlikely that building control will let you.Give them a ring or visit them they are usually very helpful

 

[donna-j]

Thanks for answering. Is that because it's unlikely to be deep enough?

 

[bikerm4]

Thanks for answering. Is that because it's unlikely to be deep enough?

Might not be thick enough or no dpm or inferior sub base. As you say the
the sunroom is just a "shack". Better to tear it down,break up the concrete dig some footings and start afresh.

 

[matthew2ts]

it would be better to get insulation under the floor

plus damp membrane under the insulation.

i ripped up a concrete floor yesterday under it there was no insulation most concrete floors dont the concept of heat sink is relatively new

and the damp membrane was torn bags (used sand bags)

you could put a floating wood floor above the concrete so long as its insluated and above the dpm. but when you have done this this finished floor level may be higher than the surrounding floors.
what about under floor heating? if you do have to rip the concrete out and you want a warm floor......

 

[donna-j]

Thanks for helping. There is a dpm I think but obviously pretty old. The floor is very slightly lower than the house floor anyway so it could be raised a little. We'd go for adding to the central heating rather than under floor heating.

We'll have to get hold of the original plans to have a look. We've been told that it was all done with the proper consents, but it was 35 years ago.

If the extension was to be 3 x 4 m approx, how much wouod we save if we could actually use the existing foundations?

Thanks a million.

 

[real1314]

I had an extension done end of last year - about 3.5m by 5m, but the foundations.......

hmm, builder dug down to solid earth....
or not..

got down about 4 / 5 foot and still loose.
got the buildings regs blokes in...

piling needed.

dug out for pile holes and found the 2 foot thick concrete floor of the cotton mill that was there long before my house.
asked the buildings inspector if it was ok to build onto that....

NO.

5 Piles, drilled through the concrete, approx £3000 to the piling company and another £1500 in preparation costs.

Moral - it all depends on why they've used floats previously and whether the building inspector will let you do it again.
Bear in mind though, that you don't want to tyr selling in 15 yrs or so only to find that mortgage companies aren't happy with the foundations of your extension.

 

[Nige F]

WOW...Nightmare...and I thought we had bother down here on our wealden clay...call the building inspector 3 meter Peter ...no wonder there`s Trouble@t` mill.

 

[donna-j]

Thanks for answering. The whole house is built on concrete foundations. We have been told that this is because of the age of the house (1940s). The 'extension' was added about 25 years later. The original owner did both I think, so I am hoping that the 'extension/porch' has the same level of foundations as the rest of the house.

One wall of the porch is a proper wall, one is the external wall of the house and the other two are basically a wooden lean-to. Does this make a difference, that one wall has been built in brick on top of the foundations?