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Floor tiling

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Posted (edited)
20 minutes ago, Onoff said:

 

Remember the wet room corner has only just been done this year, with my home brew, SBR laden mix!

It’s highly unlikely that you’d have a problem in a small area of screed like that. 

 

The risk areas are new floor slabs or screeds of a size of eg 6m x 12m where it’s highly likely that the slab would develop a crack in the middle of the 12m direction.

 

If you have a floor finish on top that can’t crack such as vinyl or timber (or bamboo) then it’s not a problem as the crack in the screed below will never be seen. With inflexible ceramic or stone tiles however the risk is that they crack along the line of the crack in the screed below. That’s why decoupling mats are recommended.

Edited by Ian

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30 minutes ago, Onoff said:

 

Remember the wet room corner has only just been done this year, with my home brew, SBR laden mix!

 

 

It's all about area and the scale of movement though. 

 

The coefficient of linear thermal expansion for concrete is around 0.014mm per metre per deg C.  So a 5m wide room would move around 0.07mm per deg C change in temperature.  The range of temperature that our UFH heated slab sees across our widest room (about 5m) is at most 5 deg C, and that's the variation from floor cooling (18 deg C) to maximum floor heating (23 deg C).  So our kitchen/diner floor moves about 0.35mm worst case, across it's entire longest dimension. 

 

The large format travertine we've laid is 400mm wide on that axis, so across each 400mm wide tile the worst case movement, ignoring the coefficient of linear thermal expansion of the travertine, is going to be 0.028mm.  To give a sense of proportion, a human hair is around 0.025mm, so we're talking about that much absolute maximum relative movement, IF the travertine has zero coefficient of linear thermal expansion.  Flexible tile adhesive has a maximum allowable "give" in shear of around 0.2mm to 0.4mm  per metre, so around 0.08mm to 0.16mm across a 400mm wide tile, far more than enough to take up the relative movement 0.08mm per metre per deg C IF the tile had a zero coefficient of linear thermal expansion.

 

However, no tiles have a coefficient of thermal expansion that's zero, they range from around 0.004mm per metre per deg C for porcelain to about 0.008mm per metre per deg C for travertine.  Taking our travertine as an example, the actual differential movement is given by the difference between the two coefficients of thermal expansion.  In our case, for the 5m long run of travertine, with 400mm wide tiles, the absolute maximum relative movement, between the floor being on maximum cooling to being on maximum heating will be about 0.016mm per tile, massively within the ability of the adhesive to flex, and far too small an amount to cause a crack.

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I'm trying to get my brain around this - it's really interesting and very relevant to my project.

 

I'm confused whether I should be worrying about the relative movement across a single tile or across a floor span. My thought was that it would be the floor span - I considered the substrate as one unit which expands at one rate and the tiles and grout together as another unit. This way the 'last tile' will have the accumulative impact of the expansion of all the other tiles and substrate (actually I thought half this as it would expand in both directions). What would be the justification for considering just the expansion over the length of a single tile? Does the grout between tiles take up this movement? I am aware that you can get grouts with differing flexibility in the title but I cannot (yet) find relevant technical information in the data sheets.

 

My initial thought was that I should compare the expansion of my limestone tiles (0.008mm/m/°K) to my Gypsol screed (0.012mm/m/°K) at their maximum temperature difference from when laid (maybe 10° for the tiles and 12° for the screed) over the largest span (7m). This equates to a relative movement of 0.45mm, or 0.22mm assuming it moves equally in both directions.

 

 

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