Screeding Garage floor

[barneybodge]

Hi all,

My double Garage disaster project is almost coming to an end at last, I may even be lucky to get it finished by xmas.

Unfortunately the large concrete base was not levelled properly and I now have to lay a 2" screed all over, yes the base is out by up to 2 inches

Ive ordered enough Sharp sand and cement to do the job but im unsure as to what preparation needs to be done. So far I have been told 3 things:

1: Lay a builders polythene dpc everywhere before sreeding.
2: Use a 'blackjack' style bitumen paint everywhere instead.
3: Don't bother with anything and just screed straight on to existing concrete base.

Also, I have seen builders using thick insulation boards called 'cellotex' ? on a base and then screeding over the top. Im sure they know what they are doing but it seems like a really weird way of laying a very important screed. What if the cellotex moves or crushes, surely the screed on top will eventually crack ?

Cheers

 

[TexMex]

Before giving advise as to how to continue with this job, could you please tell us:

What thickness is the concrete base that you have already?
What kind of construction is the garage? (precast concrete? brick? wood?)
Does it include a cavity?
Is there a DPM in the base already? (did the bulders put a plastic sheet in).
Is the garage attached to the house?
if so, how far below the DPC is the top of the base already?
Are you expecting to use the garage as a workshop too?

I know I must sound really nosey, but there are garages, and then there are garages. I'm just trying to get a feel for the cat we are trying to skin here.

 

[JerryD]

If the floor is currently out by 2" and you want to put a 2" screed over it then surely it will STILL be out by 2"?

What you probably mean is that you want to screed it level by making the screed 2" thick where the old floor is low and screed to nothing at the higher points.

If that's the case then you've got a problem, screed doesn't like feathering to nothing, in fact screed doesn't like being less than 2" thick anywhere. This means putting 4" of screed over the low points in order to get 2" over the high points.

Can you sensibly do this? What about floor levels in relation to doors and driveway?

Whatever you do DON'T lay down a sheet of polythene! Any screed laid direct on that will crack and fall apart as it can't adhere to the old concrete floor. If you must damp proof the old floor use Synthaprufe, minimum 2 coats and (and here's the trick) when the final coat is painted on (while it is still wet) throw damp sharp sand over it (as if feeding the chickens) and this will dry into the Synthaprufe and create a rough, grippy surface for the screed to key into.

 

[barneybodge]

Hi all,

Sorry I didnt explain everything properly.


The garage is a Timber framed building sitting on a soleplate, the soleplate had to be level so it was packed up on slates and then backfilled with a sharp and cement mix. My screed will be roughly 2" all over and levelled off to the bottom of the soleplate. All walls and ceiling are insulated with rockwool and cellotex, foil membrane and then plasterboard. The outside is cladded with some sort of cement fibre cladding that looks like shiplap, looks fantastic.

Garage is not attached to house, but I have to say that it looks like another house with its dominating height and 2 dormers ! Its the same size as a 2 bed barratt home. Downstairs will be used as a typical garage and upstairs will be used for storage.


Soleplate sits on a sharp / cement bed with a dpm sandwiched between them (black stuff on a roll) and then the frames were bolted down on top of the soleplate, I assume I cannot let my screed go above dpm level ?

The existing base is a monster with blinding / wirework and over a metre deep, it took 8 readymix lorries to fill it. I cannot remember if the builders used a membrane or not.

I need to start screeding today as I have a plsterer coming monday and want to be out of his way why he works on the walls and ceiling.

Any further advice would be great

 

[JerryD]

First bit of advice, don't screed until the plastering is done! Why do you want to have the floor finished before the messy plasterwork is done?

To be honest, a sharp sand typical screed does not have much strength and will wear badly. For heavy use you need a grano screed where granite chippings are used instead of (or as well as) sharp sand.

 

[barneybodge]

Thanks Jerry,

Bit late to change the mix because I have 2 metres of sharp outside. Ive had 4-1 sharp mix laid before for much heavier use and its been ok. Went to jewsons and got Synthaprufe, great stuff and it reminds me of the primer you get with flashband, all ready and keyed up for the morning.

I just hope I can get my levelling sorted ok, going to use timber screeding guides and do it in sections.

Plastering is upstairs only, so no problem doing my downstairs screed first. Main garage will just be painted because we have totally blown our budget and cant afford plastering downstairs too.

If my back gives up on me tomorrow then I may be posting a 'gutted' message on here !

Another question if I may, 1 metre is equal to 1000 litres and my belle mixer will hold up to 90 litres so I make that 22 full mixes for 2 metres ? Which doesn't sound anywhere near enough but if its true then its a right result because I thought I would be loading the mixer until my arms fell off.

 

[masona]

Another question if I may, 1 metre is equal to 1000 litres and my belle mixer will hold up to 90 litres so I make that 22 full mixes for 2 metres ?

Correct but if your mixer drum capacity is 90 litres then it will be more than 22 mixes because it cannot mixes a full load.

 

[JerryD]

I think 90 litres is the mixing capacity, the actual drum is nearly 140 litres. However when mixing drier mixes like screed, you'll find it quicker to do lots of smaller mixes rather than try to get a 'full' mix out each time. This is because the dry stuff doesn't mix as well as a sloppy wet mix so it needs more room to tumble. If you're doing 4:1 mix then I'd recommend 8 sand then 4 cement then another 8 sand (water in first!). This will make about a barrow full but only take about 5 minutes to fill and mix. Tip this into a barrow but before taking the barrowful anywhere, chuck in another mix. This can then be mixing while you are barrowing. You'll do it in half the time.

I wish I could come over and help you but i've got to er... um ........


btw, 2 cubic metres of sharp sand won't make 2 cubic metres of screed I'm afraid, more like 1.75 due to bulking of sand. Hope you have enough.