Porcelain slabs on dry sand and cement mix?

[Leonjpenwill]

So… dads worked in construction for 40 years - followed his guidance here but think I maybe shouldn’t have, you tell me.

Wanted to lay a porcelain slab base under the pergola.

Laid a sub base of 100mm type1.
Then laid 50mm of dry sand and cement mix and 5:1 ratio.

Laid the slabs on that bed (with a weird slurry mix thing on the back of the slab too - which he didn’t think was worth anything).

We did all of the sand and cement on the Saturday, laid half of the slabs and then laid the other half on the Sunday - but sand and cement had started to harden.

Basically 1. Should it have been a wet mix? 2. Have I f*cked up by letting him persuade me it didn’t all need to be laid in one day?

Would welcome any thoughts or advice where possible? The space is 4.8m by 3.6m.

 

[StephenOak]

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You are more likely to get help if you post in the appropriate section. The Moderators can move this for you. Click on 'Report' to ask for that.

 

[Leonjpenwill]

Thank you! Hadn’t realised I’d put it in the wrong section. Will ask for it to be moved.

 

[kjacko]

Hang on, you laid the bed down and simply left it? What overnight? Even if dry I’m not sure that’s a good idea.
I’m doing this very job with my step-son. Porcelain slabs, buttered(?) with slurry primer then laid on a fairly wet mix of 4:1 sand:cement mix to give us amateurs a fighting chance of getting it level. Even then it’s been tough going.
There are plenty of videos on YouTube showing how to lay sand/cement for porcelain slabs. That’s where we got our info from and we’ve successfully laid half of the 30sqm area so far with a nice solid bed, no cavities beneath.

 

[Leonjpenwill]

Hang on, you laid the bed down and simply left it? What overnight? Even if dry I’m not sure that’s a good idea.
I’m doing this very job with my step-son. Porcelain slabs, buttered(?) with slurry primer then laid on a fairly wet mix of 4:1 sand:cement mix to give us amateurs a fighting chance of getting it level. Even then it’s been tough going.
There are plenty of videos on YouTube showing how to lay sand/cement for porcelain slabs. That’s where we got our info from and we’ve successfully laid half of the 30sqm area so far with a nice solid bed, no cavities beneath.

Yes quite… with hindsight it was an odd idea from the outset… but dad was pretty convinced and convincing! Curious whether there’s anything I can do to rescue it now or whether it’s a case of taking back up and starting again… ‍

 

[kjacko]

Well, from my experience only last weekend, we left some leftover gear on the hardcore and it obviously dried, but then it was too high for the new bed to go over it, so we have to just hack it off. If yours is level but dry you’ve likely left very little depth for a new mortar bed to go down.

 

[noseall]

So… dads worked in construction for 40 years - followed his guidance here but think I maybe shouldn’t have, you tell me.

Wanted to lay a porcelain slab base under the pergola.

Laid a sub base of 100mm type1.
Then laid 50mm of dry sand and cement mix and 5:1 ratio.

Laid the slabs on that bed (with a weird slurry mix thing on the back of the slab too - which he didn’t think was worth anything).

We did all of the sand and cement on the Saturday, laid half of the slabs and then laid the other half on the Sunday - but sand and cement had started to harden.

Basically 1. Should it have been a wet mix? 2. Have I f*cked up by letting him persuade me it didn’t all need to be laid in one day?

Would welcome any thoughts or advice where possible? The space is 4.8m by 3.6m.

Whaaaaaa....?

Slabs should be bedded and rubber hammered into a wet mix. ALWAYS.

spot2.jpg

• noseall
• 20 Dec 2023

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• noseall
• 20 Oct 2023

 

[stuart45]

You can use a dryish mix when laying the really heavy council slabs The 3ft x 2ft ones weigh over 60kg, so once they're down they don't lift it easily.
It's probably the reason why people used to think that laying slabs with a dry mix was OK, but with the smaller, lighter slabs it doesn't work.

 

[Leonjpenwill]

Whaaaaaa....?

Slabs should be bedded and rubber hammered into a wet mix. ALWAYS.

spot2.jpg

• noseall
• 20 Dec 2023

digby1.jpg

• noseall
• 20 Oct 2023

Damn it! Would you suggest taking it all back up and re-laying on a wet bed? I presume the answer will be yes. Got a few days off coming up so could sort it then…

What a pain! But is what it is.

What mix ratio do you use?

 

[Leonjpenwill]

You can use a dryish mix when laying the really heavy council slabs The 3ft x 2ft ones weigh over 60kg, so once they're down they don't lift it easily.
It's probably the reason why people used to think that laying slabs with a dry mix was OK, but with the smaller, lighter slabs it doesn't work.

This makes a lot of sense now! Thank you. Think I’ll be taking back up to re-lay! Ergh

 

[stuart45]

There's a really good site called pavingexpert.com that gives comprehensive information about methods of slab laying.

 

[noseall]

Damn it! Would you suggest taking it all back up and re-laying on a wet bed? I

Yes.
5 parts sharp sand to 1 cement.
Always slurry the backs of stone and porcelain.
Always hammer into the beds. The latest trend of regularised paving units means that because the are only 20mm thick, they need to be fully bedded.

 

[Leonjpenwill]

Yes.
5 parts sharp sand to 1 cement.
Always slurry the backs of stone and porcelain.
Always hammer into the beds. The latest trend of regularised paving units means that because the are only 20mm thick, they need to be fully bedded.

May sound like a silly question… but how much water?

 

[noseall]

May sound like a silly question… but how much water?

We actually use plasticiser (a drop of fairy) in our muck, so less water is needed. A lot will depend how saturated by rain etc, the screed sand is.

You want it workable, without being runny (too wet), or crumbly (too dry). It need to be able to hold its shape.

 

[stuart45]

If it's too wet the slabs will 'float a bit'. You'll lay one, and when you lay one next to it and tap it down it will lift a bit.Getting the mix right comes with experience.
The only way you'll get that is doing the job. The next job is always easier.