New screed meets old screeds

[Cranbrook]

Unusual (for me) job I've been to quote for this week where the customer has had a large extension added to a bungalow and has made a large open plan kitchen by utilising a hefty RSJ.. customer has also had another extension to the side to make a smaller utility room.. anyway, the thing is, the new screed in the large extension is a pumped AH and the smaller extending is a traditional sand cement screed. The existing screed is a sand cement that was previously tiled.

My question is, what would everyone do in this kind of instance?
P.s, tiles are 60 by 60 polished porcelain the customer tells me

 

[Flintstone]

Whats pumped ah?

 

[acaciaguy]

Is it pumped anhydrite ?

If so it’s a more complex / modern version of sand and cement - quicker setting and other inproved properties (apparently)

Is the floor level across the different area?

 

[Cranbrook]

I didn't have my laser to do a thorough inspection, but where tiles had been removed from the existing screed, there is definitely repair work to be done, so I'm leaning towards latexing the whole lot.. thoughts?

 

[Cranbrook]

Whats pumped ah?

Anhydrite mate

 

[LM]

Anhydrite mate

I didn't have my laser to do a thorough inspection, but where tiles had been removed from the existing screed, there is definitely repair work to be done, so I'm leaning towards latexing the whole lot.. thoughts?

Your’e gonna have to treat both sections as two separate floors, prepare and level accordingly then have a soft expansion joint between the two areas.

 

[Flintstone]

As Lee says, expansion joint in-between the two.

 

[acaciaguy]

Sounds a good plan to me

 

[acaciaguy]

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.

 

[Cranbrook]

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.

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

 

[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]

.

 

[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.

 

[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.