Hi all, this is about my third thread today but i have a few questions. I have never used a decoupling membrane before but i think i need to on this job, what do you guys think. Its 33m2 floor using 600x600 porcelain tiles. The customer is a mate of mine who's been in building trade for a few years so he's prepped the floor. The floor is about 2/3 wooden and 1/3 concrete, to get it all level he has overboarded the floor boards with 18mm ply and its solid. the rest of the floor hight has been brought level using SLC. So where you walk in the room its a porch (concrete) which has been levelled with slc, then its the wooden part of the floor then concrete levelled with slc again. (hope you can picture that). The questions;
1. Would you use a decoupling membrane?
2. What would happen if i dont? or is it a must?
3. How much would it cost roughly?
4. Where is best place to get it?
5. How do you lay it?
Sorry for all the questions but if we can avoid using it we will because he's trying to keep costs down plus with never using one before i want to be 100% on everything. Cheers.

is there any bounce on the wood floor even though he has put 18mm ply on top

sorry its a late reply, the floor is solid. The boards were solid before he overboarded.

you could tile it without but it would always be a gamble. Give wetdecs a ring and see what he can do for you

De-coupling membranes are mainly for lateral movement, not deflection, if you have two substrates . It is important to use expansion joints on the point where timber meets solid floor.

I would be more concerned about expansion jointing at the subtrate thresholds than decoupling.

as above 2 posts

De-coupling membranes are mainly for lateral movement, not deflection, if you have two substrates . It is important to use expansion joints on the point where timber meets solid floor.


What if the way the floor is set out i have a full tile across the two surfaces? hope you know what i mean.
What do you guys use for an expansion joint? can you use silicone instead of grout at the expansion point or do you use some kind of expansion profile/trim?
I dont want the joint to be visable if possible.

if both floor substrates are solid with zero deflection, I'd use a de coupling matting to take up the lateral stresses that will occur

Originally Posted by lcjohno

De-coupling membranes are mainly for lateral movement, not deflection, if you have two substrates . It is important to use expansion joints on the point where timber meets solid floor.


What if the way the floor is set out i have a full tile across the two surfaces? hope you know what i mean.
What do you guys use for an expansion joint? can you use silicone instead of grout at the expansion point or do you use some kind of expansion profile/trim?
I dont want the joint to be visable if possible.

If you mean straddle the two surfaces with tile then this is where your crack is likely to occur. By using an expansion/movement joint at this point directly above where the two meet this should prevent the tile cracking in future.


If the guy is your friend, and something happens to the floor, he may feel he fitted/laid the subfloor well and it’s your fault if it goes wrong. You are then left with fixing the problem (when you may not be able to get the same tiles again) and may end your friendship.


Movement joints are unwelcome because people want a nice consistent look to the floor, but in some cases they are a necessity and they look better than a cracked floor. Check out some of the expansion profiles against the tile you are using and see how they look.