Extension / Garage conversion - Screeding questions

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Hi, I'm looking for some guidance with a screeding task I have to do in the next couple of days.

I'm self-building a corner extension to the side of my kitchen and rear of my garage and also converting the rear 2m of the garage to a utility room.

The existing kitchen floor level is around 40/50mm higher than my concrete slab in the extension part so I need to make this up with screed. Additionally, the floor in the utility part in the garage needs raising 110 to 130mm and this will be done with 50mm celotex on a dpm then 60 to 80mm screed.

screed.jpg


The extension area is approx 2.5m x 3m and the utility is approx 2.5m x 1.8m. I'm mixing the screed with my son and have sand/cement arriving tomorrow. I've also ordered the fibres that you can add to strengthen the mix.

1. The concrete slab is a couple of months old and has a fairly rough texture. It was simply tamped level with a wooden joist and had light rain on it just afterwards. I'm thinking it is rough enough to give the screed a good enough key to bond to. Other than cleaning and vacuuming do you think it will need any more in the way of preparation so the screed gets a good bond?

2. I'm unsure what to do about the strip where the new screed will cover over the old cavity wall which has been removed down to dpc. Clearly the screed is going to go from being bonded to the concrete slab transitioning to a strip approx 300m wide where it will be sitting on the dpm from under the slab + the old dpc from the original wall. I can't imagine it will bond to this. Should this be reinforced with chicken wire or similar? Or will the fibres that I'm adding be sufficient?

3. Similar to above. At the back of the old garage wall, the bonded screed of the extension will transition over the old single skin dpc of the rear garage wall into a floating screed over the celotex. Should this area be reinforced or will the fibres be enough?

4. I'm also a bit green when it comes to mixing the screed. I have the use of an electric mixer which sits on the floor. I'm unsure of the best process for converting the sand/cement which will be at the front of the house, into screed that is needed in the extension. I'm thinking that it will be impossible to get the mixed screed from the mixer into a wheelbarrow as the mixer is too low, therefore it would be best to have the mixer in situ. How long is it recommended to mix the screed for before it is fully mixed? Is it best to add the ingredients in a particular order?

5. I'm thinking of laying the screed in one big strip with a baton on one side for levels and the existing screed on the other. This will be approx 2.5m wide. Is this feasible or will it be much harder? It avoids the need for a central rail and the hassle that entails. It doesn't need to have a perfectly smooth finish as I'll be tiling over it.

Many thanks for any help that is offered.
 
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Does your new extension have insulation below the screed?

Im a bit surprised 50mm insulation will satisfy building regs, assuming your garage conversion is to a habitable room.
 
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Thanks for the reply. The new extension has 100mm polystyrene under the slab. The garage part is over the existing garage floor and I only have 110mm at the narrowest end (the floor slopes to the front of the garage). BCO seemed happy with 50mm insulation + 60mm screed. I had planned to do 80mm insulation but 30mm screed seems too thin for a floating screed (especially once a washing machine is bouncing around on top of it!)
 
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When laying screed generally some blobs are put down around the perimeter and these are levelled by checking with a long level and straight timber.

Then the blobs are filled in between to form strips around the perimeter and a straight edge to do the centre bit. Big areas have strips put down the middle.

If you have a laser, you can use that -you just need a stick with a mark for the laser line and the bottom of the stick is top of screed.

Im not too sure about transition from bonded to unbonded.
 
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Have you ever screeded before? What size area are you screeding? It’s at the “pretty hard” end of the DIY building work scale.
I’d brush a slurry coat of cement and water over the existing slab first.
Re. Mixing it, last time I did it I think I did 8 shovels sharp sand, 4 (1 bag) cement, another 8 sand. You mix it semi dry, so that you can form a ball of it in your hand, but no wetter. Note whether your sand as delivered, is already wet (likely in this weather) it may need no water at all.
can you not get a mixer stand? You need to do anything to make the job easier.
+ Don’t forget knee protection - cement burns
 
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Thanks 23vc, I'm a screeding virgin! The area is approx 2.5m x 3m bonded running into 2.5m x 1.8m floating.

What ratio is the cement slurry mix? I'll do that.

Mixer came from a mate but I think I can mix it in situ. Good call re the knee protection - visit to screwfix in the morning!

What do you think about doing it in one big 2.5m strip - better to divide into two? Any thoughts on reinforcement and the area over the old DPC?

Cheers
 
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One tip I learned from bitter experience - don't leave the mix going for too long in the drum - it'll form pellets of cement/sand and not mix properly:unsure:
 
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What ratio is the cement slurry mix? I'll do that.

What do you think about doing it in one big 2.5m strip - better to divide into two? Any thoughts on reinforcement and the area over the old DPC?

How deep is the bit you’re planning on doing as a bonded screed? The cement slurry thing was more a general thing for going over concrete, for a proper fully bonded screed there’s some info here, I’d use SBR / follow the guidelines at http://www.screeding.org/html/body_traditional_screed.html

I’d try and get the whole lot done in one go, it doesn’t go off on you like with plastering so you can just keep going as you mix, move, screed, rule off etc

Area over old dpc, either put a dpm trough in it or if it’s really shallow I’ve blackjacked it in the past
 
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One tip I learned from bitter experience - don't leave the mix going for too long in the drum - it'll form pellets of cement/sand and not mix properly:unsure:

apparently concrete/belle type mixers aren’t great for mixing s&c screeds anyway. I’ve always used em tho, too much to mix by hand, not tried the fancy mixers you can get. I’d consider getting it delivered off a truck tho
 

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