Best practice for a large (120sqm) tiling job

[Qwerty89]

Hi all

I’m so glad to have stumbled upon this forum, which seems to be a treasure trove of advice. I wanted to put my own situation up, and ask for some advice from the specialists on here!

I’ve added the details, as hopefully this helps others too…

Situation

• Ground floor renovation job
• We have had wet underfloor heating pipes installed, and air pressure tested
• We have 7.5mm of a new sand and cement fast drying screed (Tarmac Truscreed C30) installed, and it has been down for 2 weeks
• There are expansion gaps around most of the perimeter, but there are no control of movement joints in the screed (yet)
• One of the rooms is quite large (60sqm) so I think the builder needs to make movement joints in the screed to split it into two bays (25 and 35 sqm)
• The screed manufacturer’s instructions say it should be dry enough to accept final floor finish in 15-21 days (5-7 days per 2.5mm of screed)
• We plan to install porcelain tiles throughout the ground floor - the total are is around 100 sqm so I’m very keen to ensure it’s done properly
• The tiles are 10mm thick, and 20cm x 120cm wood effect style tiles.
• The builder tells me he usually waits 30 days after screed installation, then installs tiles with flexible adhesive, and then commissions UFH after. He says he’s never had any problems with cracking.
• After reading this forum and other websites, it seems to me that the correct order should be:
  • Install movement joints
  • Wait until screed is dry (hygrometer test, 75% RH)
  • THEN commission underfloor heating over 7 days (5 degree increments, 3 days at full heat, and then cool down)
  • THEN repair any cracks that emerge in the screed
  • THEN install tiles
  • Wait 14 days for adhesive and grout to dry without turning UFH on again

My questions:

1. Is the order of events above correct?
2. Do we really need decoupling matts? There is mixed advice online, and this would also seem to add another 10mm at least to the floor height (which we don’t have!), as well as a few thousand pounds too.
3. How thick does the adhesive need to be? Builder says 8mm adhesive + 10mm tile = 18mm build up. If we need to install decoupling matts, can we reduce the build up somehow?
4. When putting movement joints in the screed and tiles, how wide do these need to be? Ideally I want something 3mm wide only with flexible Silicon so it is indistinguishable from normal grout lines, but not sure if this is enough, or better to buy movement strips.
5. Would it be okay to install the kitchen units + island on top of these tiles (the UFH pipes won’t be underneath) or should we tile around the kitchen?
6. Is there anything else I’ve missed?

 

[Slippery]

I'm not suitably qualified to answer most of this but ;
3. 8mm adhesive? maybe he means an 8mm trowel which will give a 4mm bed!

 

[Nick Curelaru]

Very important to remember. Between the concrete screed and the tiles you need to have an anti fracture membrane installed

 

[Ajax123]

Underfloor heating should never be air pressure tested. It should always be tested when water filled. I assume you mean 75mm and not 7.5mm. Tiles should never be installed until AFTER the underfloor heating has been commissioned and run. It is not essential to uncouple porcelain but given the size it would offer some benefit. Screed should be split into maximum 40m2 bays with full depth movement joints in all door thresholds and between independently controlled zones. Movement joints are impossible to form after the sceeed isinstalled. If he says he has never had problems with cracking he's either never done it before or he's lying. Movement joints in screeds should be minimum 5mm and up to 12mm THEY should extend through the full depth of the screed. If they do not they cannot take account of expansion. These should reflect through the tile face. Minimum grout joints (these are not movement joints) should be 3mm. You can place kitchen units on top of tiles but obviously you will use more tiles. The advantage being if you change the kitchen at a later date you don't need retiling.

 

[Lifetime Tile Expert]

Very important to remember. Between the concrete screed and the tiles you need to have an anti fracture membrane installed

This is not true. Anti-fracture membranes are not crack-proof intermediate surfaces to lay tile and stone over. It is dimpled plastic attached to a fiberglass cloth for which mortar attaches below and over this membrane. Over time the plastic degrades and is rendered useless. There are liquid applied and self-adhesive rubberized anti-fracture membranes available, too. All anti-fracture membranes have limitations, and all manufacturers blame the installation when a manufactured product fails. Schluter theorized a plastic anti-fracture membrane will mimic a sand strata which was used for thick stone over a mud bed hundreds of years ago. Common sense should kick in for most people who understand that was a marketing trick. In fact, Schluters own world headquarters in New York experienced complete systems failure's which you wont read about online.

 

[gamer1]

Order of Events: Your proposed order seems well-researched and generally aligns with best practices. Ensuring the screed is dry and stable before tiling is crucial to prevent issues like cracking. However, it's always a good idea to consult with a local expert, as climate and specific conditions can influence the timeline.

Decoupling Mats: The need for decoupling mats depends on various factors, including the condition of your substrate and the type of tiles you're using. In some cases, they can help prevent cracks and ensure a more stable surface. If floor height is a concern, look into thinner mat options or consult a professional for guidance.

Adhesive Thickness: The thickness of adhesive can vary based on the tile size and substrate condition. Your builder's estimate of 8mm adhesive is a reasonable starting point, but it's best to follow the adhesive manufacturer's recommendations, which can vary. Thinner adhesive may be suitable for smaller tiles.

Movement Joints: The width of movement joints in both the screed and tiles is crucial to accommodate expansion and contraction. While you may want a minimal look, it's essential to follow industry standards, which often recommend at least 5-10mm for movement joints. Flexible Silicon can help them blend in seamlessly.

Kitchen Units: Tiling under the kitchen units is generally recommended for a more cohesive look and ease of cleaning. However, it's essential to plan this with your kitchen installation to ensure proper support and leveling.

Consult a Professional: With such a complex project, it's wise to consult with a local tile and underfloor heating expert who can assess your specific situation, offer tailored recommendations, and ensure compliance with local building codes.