Exposed DPM in cold wintery weather

[grahamA]

Hi Guys

 

So the temperatures in the UK for the next few weeks look like they are going to drop significantly just at the wrong time when I want to pour concrete for my extension slab! My question is that if we can't get the slab poured for a few weeks due to freezing temperatures, will it be ok to install the DPM and insulation etc and leave it exposed while we wait for slightly higher temperatures? Its a bit of chicken and egg situation where we want to be ready for the pour if temperatures do rise but don't want to leave things exposed that could deteriate or fill with water / ice! 

 

My floor build up will be as follows:

 

150mm Type 1 MOT

50mm binding sand

1200 gauge DPM

150mm Insulation 

1000 gauge DPM 

150mm concrete pour with A252 mesh.

 

We have currently nearly installed the MOT but not sure how much further to go at this point?

 

Any advise / recommendations welcome.  

 

Thanks 

Graham

[Russell griffiths]

If you put the dpm in and it rains you will have a big pond, dpm and insulation is only a half day job, do it the day before you pour. 

[grahamA]

Thanks Russel. I think i'm swaying towards agreeing with you. Its probably more hassle than its worth trying to keep water out of it. 

[Canski]

I agree with Russell, It could be a long process getting rid of leaves and snow if it comes down. You could always cut the DPM to length ready to throw in and fold it up somewhere warm.

[Mr Punter]

Or you can just do the RC slab on top of the MOT, then do the DPM and insulation, then screed / floor finish.

[Conor]

You can do a concrete slab pour no bother in freezing weather, just need some sort of insulating cover (Hessian, carpet, loose straw etc) while it cures. Curing cement generates heat, if you cover it, it'll stay warm enough to cure properly and not freeze. Sounds like you've insulation underneath which is much better than a pour directly on ground. Oh, you don't want two layers of dpm, or you'll just trap moisture permenantly.

[saveasteading]

2 hours ago, Conor said:

You can do a concrete slab pour no bother in freezing weather,

I don't agree, unless this is an enclosed space. There isn't a huge amount of heat off an enclosed slab. Ive even had to break out 1m2 plinths that the cold got into (in Kent, not Cairngorms).

The 3 degrees and rising is a decent guide, but not even that always.

 

 

[saveasteading]

5 hours ago, grahamA said:

50mm concrete pour with A252 mesh.

That is very heavy reinforcement for a normal extension slab. Why?

What area is the slab?

[grahamA]

On 03/12/2022 at 17:01, saveasteading said:

That is very heavy reinforcement for a normal extension slab. Why?

What area is the slab?

 

I'm not to sure thats just what the SE has specified! It does seem somewhat over kill now i've done some research. It not load bearing i don't think given the inner leaf walls will take the first floor and roo with their own footings and its just going to be used as an open plan kitchen and utility room. Kitchen is 25m2 and the utility is 12m2 (ajoining with an internal structural wall).

[grahamA]

On 03/12/2022 at 16:57, saveasteading said:

I don't agree, unless this is an enclosed space. There isn't a huge amount of heat off an enclosed slab. Ive even had to break out 1m2 plinths that the cold got into (in Kent, not Cairngorms).

The 3 degrees and rising is a decent guide, but not even that always.

 

 

 

Yeah i'm concerned about frezing over night temps this week. Its supposed to be -4 / -5 later in the week in the North West and thats the earliest we could pour. I do have access to some carpets etc to put over the top but not sure thats going to be sufficient given the weather forecast?? 

[grahamA]

On 03/12/2022 at 14:38, Conor said:

Oh, you don't want two layers of dpm, or you'll just trap moisture permenantly.

 

Why would there be mositure in there if they are installed at the same time? Its what the architect has specified and i assumed it was to stop the insulation getting damaged from the concrete pour?

[saveasteading]

So it is not worth thousands.

37m2, plus laps and cutting waste.

4.8 x 2.4m,,, so you need about 4 sheets at.£60

Change to A142 and it saves you half the  cost.

£120 is not to be sniffed at, and the whole principle of reducing materials should be in the SEs  policy.

 

Using handier , smaller sheets there are more laps.

 

Presumably only for crack control, but the SE can explain.

 

I would ask the SE if you can use lighter mesh.

Also why the slab is 150 thick?  it is sitting on insulation with a fraction of the strength.

This could be a 60mm screed.

150mm is enough for a light warehouse 

settle for 100mm and you save 2m3 concrete.

 

Why? good for your wallet, your back, and saves the planet steel and concrete.

If he says no, do please ask why? Because it is normal is not a good answer.

 

100mm concrete with A142 mesh please Mr SE.

If there is a good reason, please let us know.

 

There you are, now save  over £300 and a lot of labour.

[saveasteading]

I would also specify 2 layers of dpm.

The bottom one keeps the damp below the structure, but also sand out of the insulation.

The top one is essential to prevent the slab from leaking between the insulation slabs, and maybe floating it.

[Russell griffiths]

Why don’t you build the rest of the extension and pour the floor when the roof is on.