New screed meets old screeds

[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 Silicon 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 Silicon 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.

 

[Cranbrook]

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.

What's a tile bay forgive my ignorance?

 

[LM]

I have in the past completed projects under technical guidance from a manufactuer that where a first for me and which other competing manufactures simply wouldn’t stand over or give advice to do with their products and to this day the installations are sound after many years of heavy abuse. On site evaluations and experience can make all the difference with these decisions.

 

[LM]

What's a tile bay forgive my ignorance?

A tile bay is the size of an area that is tiled ie: the maximum reccomended tile bay on standard s&c screed is 40mts but with ufh it’s 25mts. The longest reccomended run is 8mts but the preferred is 6mts so a bay size of 6x6mts on s&c screed and 5x5mts with ufh.

 

[LM]

Why’s that @jcrtiling?

 

[LM]

What's a tile bay forgive my ignorance?

Don’t be shy on phoning the technical help line of your adhesive manufactuer anytime on any job for guidance. There’s no such thing as a stupid question.

 

[GoneGuy]

Also in some instances where there might be large expanses of glass could create solar gain so that area and substrate could warm up quicker than the surrounding area

 

[Dumbo]

Why’s that @jcrtiling?

It's optimistic that one day my fat thumbs won't press buttons they're not meant to

 

[Dumbo]

Also in some instances where there might be large expanses of glass could create solar gain so that area and substrate could warm up quicker than the surrounding area

Just did a job like that , that had previously failed over a timber substrate .

 

[Glynn]

Lee Mac is right. And don't forget to test the moisture content in the anhydrite screed. It can take monthes to dry out if the preparation is not correct.