FFL slab

[23vc]

Hi
Quick question for anyone who has done an extension without screed IE. Pouring the slab up to ffl. I am doing this as it saves cost of breaking up a lot of sound thick concrete below and Building inspector is happy with the approach, but there is no room for screed. (I prefer screeding and always have done before)
Anyway question is, can you only pour the slab once the walls are up and it's watertight? Because if you do it when your at dpc height, I'm worried it would just flood in through the existing external doorway when it rains as there's no step down.
Anyway hope the question makes sense, sorry it's a bit of a ramble!
Cheers
John

 

[vinn]

Does the present slab have a membrane (DPM)?
How thick will the new pour be?
You will need to think about tieing-in to the house DPC and using the house FFL as your benchmark.
Pouring slabs on slabs is done all the time.
Any gaps in the raised perimeter can simply be shuttered off with timber or ply.

We prefer to eliminate screeding - so do many large contractors: its simpler and cheaper.

 

[23vc]

Thanks for the reply vinn
No dpm currently the plan is a bit of blinding in places where it's rough then dpm, insulation 80mm slab 140mm ish.
So basically not enough room for screed although I guess I could use a pumped one that can go down to 40mm ish.
So if I pour the new slab to ffl then if it chucks it down before the roof is on then surely the water would come in under the existing kitchen door? If that makes sense?!

Do you Power float it then (not confident to do that myself) or do you put self leveler on top which is what I was planning to do
Thanks again for the reply the info is v much appreciated

 

[vinn]

You pour by nine in the morning on a dry day.
SLC is a form of screed.
Finished floor is the top of the floor coverings such as tile or whatever - not the actual top of the pour unless that suits you eg. using sheet vinyl.

You can hire manual floats, no need for power floating unless you have a large extension footprint.

Use tape to seal from water damage during the build - but you should have figured out by design by now how you intend to detail all door openings and knock throughs at floor level.

 

[23vc]

Thanks, I was planning on pouring the slab to within about half an inch of the existing floor slab and making the rest up if needed with some slc. Yeah sorry when I said ffl I meant the level of the new finished slab. Not sure if I'm missing something when you say worked out detail of door openings, was planning on just pouring up to and including the door step stopping just before the threshold (hence my worry about rain getting in) appreciate I may be off the mark here and really appreciate the advice

 

[tomfe]

Why are you using a 140mm slab and only 80mm of insulation?

You can just use a bull float to get a finish good enough for a floor finish.

Why have you got an internal door?

 

[23vc]

Sorry bad description on my part and good point about the insulation, basically I've got 220mm to make up which doesn't allow for a screed or at least not the dry mix screed I've done before. So yeah obviously I should up the insulation to 120 thanks
No internal door, just the existing kitchen door and patio doors which will become openings into the extension.
Thanks for the replies guys I've done a lot of fit out work including screeding but the slab pour and lack of screed I'm out of my comfort zone

 

[vinn]

"including the door step" sounds like you are building in trouble - the idea is to have a mono slab with its DPM tight to the house wall. Any step(s) should have been removed.

Thresholds in cavity walls esp from old to new are a constant source of difficulty for DIY'ers - how do we bridge it, how do we DPC it, where does the join between finished floors come etc?
Thats what worked out detail would have answered before any build.

 

[23vc]

OK thanks, I've not started yet so I've not messed up yet. I'll give the bco a shout about how the threshold should work, appreciate the advice as ever

 

[garyo]

I've done it like this before and yes, you might risk a bit of a flood under your back patio doors (say) if it chucks it down during your build. However, you should have left enough DPM lapped up the wall of the existing house to stop it flooding in to your house. Then when you knock through you have 40cm of DPM to play with when bridging over your cavity. The difficulty is in the two DPM corners at the house where you really need to cut and tape it to get it under your new walls correctly - if not done right you can end up with leakage.

 

[23vc]

Thanks mate that makes sense. Yeah I haven't got my head round the cavity bridging bit yet as vinn alluded to !

 

[23vc]

PS I may pour to say 40mm below ffl as I'll still have 100 insulation+100 slab which gives me the option of either a pumped screed or I could use ply/chipboard over the concrete to get it level after the knock through, please tell me if that's bonkers though
Cheers
John