Self levelling compound question

[jjramirez]

Hello all. I am planning to lay engineered floor in our bedroom but I need to prepare the existing sub-floor first. This is an old victorian house and existing floor is made of old pine floor-boards, which seem in good shape from a structural pov but does need flattening/leveling in places. My idea right now is to overboard the whole thing with 9 or 12mm plywood (glued and screwed) and level the low spots with self-levelling compound.

The question that I have is that it is only some parts of the floor that need levelling, especially a large-ish circular dip in the middle of the room of about 10mm depth at its deepest. Is it ok to just pour SLC in these low areas and feather out the edges? I will be gluing the engineered wood on top of this. Is this acceptable practice (gluing the final covering over a patch of SLC)?

Another idea that I had was to use caulk to block all joints between floorboards and pour the SLC directly over the floorboards and then overboard with plywood to then glue the final layer on top. In this case I would screw the plywood through the SLC.

What do you guys recommend?

Thanks.
Jacob

 

[scotcruz]

Think that idea works as long as existing floor doesn't bounce and is solid.

You could put down a 4-6mm ply before putting the SLC as it could cracked due to movement between the joints. Once it's all level you can put a 9mm ply to finish it all off

 

[Deleted member 174758]

If you lay SLC straight onto the floor boards you run the risk of the joints in the boards telegraphing through the SLC and cracking it, possibly causing the SLC to break up. And how do you know if the caulk you propose to use is compatible with the SLC? So not good.

Also, if you lay the SLC on the boards, then try to fix the plywood through it, you run the risk of breaking up the SLC with the screws, you will find it lot harder to actually drive the screws (SLC is pretty hard) and because it is self levelling, just what would the point be in doing this?

The way this stuff works you are supposed to screw the plywood to the floor boards at 150mm or less centres. This actually levels out a lot of the unevenness in the floor as well as covering any holes and joints in the boards. You then lay the SLC on top of the plywood. And that's it. I've been involved with enough commercial flooring over the years to have done loads of sub floor prep for the floorers and I've NEVER once seen plywood laid over SLC, probably because it would be pretty pointless

 

[opps]

I would be concerned about the SLC cracking. You may find that you hear crunching noises when you walk across the floor in a few months.

 

[jjramirez]

Ok cool. Thanks for the opinion guys, pretty much reinforced what I was thinking. So you guys think its ok to pour SLC over low spots only and feather the edges as opposed to filling the entire floor surface (I would try and find a SLC that mentions thin layers)?

 

[Deleted member 174758]

When I've seen SLC used it is laid in a full skim of about 8 to 10mm thickness to avoid cracking and normally isn't feather edged. On one job the floorers gave me half a bag of Ardex Feather Finish to taper the floor edges into the door openings in my living room and it seemed to do what you are looking for, although hopefully someone with a lot more experience like Dazlight will be along shortly to shoot me down on this one, or not, as the case may be

 

[dazlight]

We do this a lot. There is a few ways to do it but our main way.

Plywood whole area with 9mm ply. We use SP101 but as long as you get a decent grade you will be ok.
We then work out the levels. If bad we use strips of 6mm , 9mm ply to make out where we need to screed.
Sometimes I put screws in the floor every 600mm to the height I want the smoothing compound.

then we use ardex compounds but loads out there. At the moment we mainly use ardex K40HB or ardex NA.
Make sure you prime the plywood.

 

[jjramirez]

Great. Thanks for advise guys.