New screed meets old screeds - TilersForums.com

[LM]

You can use normal fibre slc’s on an anhydride screed with the correct priming after proper prep but the necessity for an expansion joint is that the two substrates are of a different composition and in turn will behave differently, ie: contract and expand at different rates. So an expansion joint in the correct place is a must or else your’e taking a chance.

 

[LM]

This was my train of thought.. I'd like to avoid an unsightly expansion joint as it will ruin the look of the kitchen diner imo

A colour matched silicone or the like the same width as the grout joint installed correctly isn’t really visible so it’s no big deal.

 

[Cranbrook]

You can use normal fibre slc’s on an anhydride screed with the correct priming after proper prep but the necessity for an expansion joint is that the two substrates are of a different composition and in turn will behave differently, ie: contract and expand at different rates. So an expansion joint in the correct place is a must or else your’e taking a chance.

Thanks for the information Lee that's very helpful, out of interest, if they were the same material screeds but laid years apart, would the same still apply?

 

[LM]

Lee Mac. Is there a slc or similar system that can be used on both surfaces to turn them in to “one” for tile fixing? Just a thought. I appreciate this may change heights of floors.

There are companies out there that reccomend strips of uncoupling membrane in localised areas instead of expansion joints but even at that it’s with the same substrate throughout. If you change the substrate then you will have movements at different rates.

 

[LM]

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[Cranbrook]

There are companies out there that reccomend strips of uncoupling membrane in localised areas instead of expansion joints but even at that it’s with the same substrate throughout. If you change the substrate then you will have movements at different rates.

Localised uncoupling was going to be my next suggestion!

 

[LM]

Thanks for the information Lee that's very helpful, out of interest, if they were the same material screeds but laid years apart, would the same still apply?

Strictly speaking you should have an expansion at every doorway etc. You also have to consider that the older substrate may have finished with settling whereas the new substrate won’t have but after time they should behave similaraly but two different slabs need to be treated independently. You will come across people telling you that it’ll be fine and there’s no need, but what are you to gain by taking the chance. A colour matched silicone isn’t notaceable if installed correctly.

 

[Cranbrook]

Strictly speaking you should have an expansion at every doorway etc. You also have to consider that the older substrate may have finished with settling whereas the new substrate won’t have but after time they should behave similaraly but two different slabs need to be treated independently. You will come across people telling you that it’ll be fine and there’s no need, but what are you to gain by taking the chance. A colour matched silicone isn’t notaceable if installed correctly.

Agreed, would you still use an uncoupling membrane in this instance?

 

[acaciaguy]

There are companies out there that reccomend strips of uncoupling membrane in localised areas instead of expansion joints but even at that it’s with the same substrate throughout. If you change the substrate then you will have movements at different rates.

That’s makes sense

 

[LM]

Agreed, would you still use an uncoupling membrane in this instance?

If it’s stone you should uncouple regardless, if it’s full bodied porcelain then you shouldn’t have to uncouple unless there is an issue with the screed. Uncoupling doesn't do away with the necessity of expansion joints, tile bay sizes and lengths of runs still have to be honoured and in some instances the amount of windows and colour of tile can also contribute to the limits of bays in respect to expansion. uncoupling though imo is never a bad thing.