Roz

Do I need to sand my concrete slab before glueing engineered floor down

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Posted (edited)

Hi everyone

 

i’m about to lay my engineered oak floor to my concrete slab, sticking using sika 54 glue. 
 

a floor fitter I spoke to said we should sand the concrete slab to remove latency, at a very low grit. I haven’t found any other information about this online so can anyone share their experiences? The slab is pretty flat otherwise except for a few spots I can manage with a hand tool. If I don’t need to sand the slab I won’t as it doesn’t sound like a nice job and I would need to hire a floor sander

 

Is it generally recommended to prime the concrete? Or is that only if it’s dusty?

 

edit to add: the slab has a membrane, insulation and ufh pipes underneath it. It was a single pour

 

thanks! 

Edited by Roz

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You would nearly always use a self leveling compound to screed the floor Prior to laying your floor 

The polymer that you are sticking the boards down with is far more expensive than self leveling compound And would not only make the job easier It would probably save you money 

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8 minutes ago, nod said:

You would nearly always use a self leveling compound to screed the floor Prior to laying your floor 

The polymer that you are sticking the boards down with is far more expensive than self leveling compound And would not only make the job easier It would probably save you money 

Hi nod, the floor is pretty flat and level, would you use self levelling compound anyway? That seems like added expense . I see it could be a nicer job than sanding although I might mess it up! 

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+1 on compound, also known as latex. Quite cheap and easy to apply as it is self levelling. Remove any high spots and aim for 3mm coverage. We did this on our basement floor before laying karndean and it worked a treat.

 

 

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Sanding is a crap job to do, dust id horrendous, back breaking and you need to be checking levels etc as you go. self leveling compound anyday 

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Thanks @markc @Bitpipe and @nod . So you’d recommend compound even if floor is flat and level and latency not an issue? 
 

partly reluctant because all our floor is in there waiting but I’ll get over it and move it if necessary!

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If the floor is flat an even

 o tamp marks you be ok 

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Does the same apply if laying engineered wooden boards on sand and cement screed? The Wood Floor company we are going with also recommended Sika glue.

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39 minutes ago, hendriQ said:

Does the same apply if laying engineered wooden boards on sand and cement screed? The Wood Floor company we are going with also recommended Sika glue.

Depends how flat the screed is, if the screeders did a really good job it will be very close to level and a pretty smooth surface and would therefore be ok.

Concrete pours often have tamp marks or even power floated can have humps and puddles which the adhesive cant cope with

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If the only issue is a few screed marks, then they rub off easily. can use tools or just rub with a brick. 

I have known a floating engineered floor to make a slapping sound where there is a tiny hollow, but it  then relaxed to shape.

Wood bends, even laminated wood.

 

How thick is the glue?  Have you tried a straight edge?

 

Latency is another matter. Basically dust or a weak surface caused by too much water or too much trowelling, as that will be a weker bond than glue to wood.

 If it is ultra soft the it should be rubbed back to a clean hard surface.

If reasonably good, it might be worth a coat of PVA. perhaps a thin coat then a thicker one.

If hard, leave it.

Try scratching with finger nail then screwdriver.

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24 minutes ago, saveasteading said:

If the only issue is a few screed marks, then they rub off easily. can use tools or just rub with a brick. 

I have known a floating engineered floor to make a slapping sound where there is a tiny hollow, but it  then relaxed to shape.

Wood bends, even laminated wood.

 

How thick is the glue?  Have you tried a straight edge?

 

Latency is another matter. Basically dust or a weak surface caused by too much water or too much trowelling, as that will be a weker bond than glue to wood.

 If it is ultra soft the it should be rubbed back to a clean hard surface.

If reasonably good, it might be worth a coat of PVA. perhaps a thin coat then a thicker one.

If hard, leave it.

Try scratching with finger nail then screwdriver.

I’ve taken some images of a random patch - went over quickly with bottom range vacuum, and it obviously has some paint and plaster spots I should get up.

 

I tried rubbing with a crow bar and not much dust comes up, it seems pretty hard . Did video but don’t seem to be able to upload due to file size 

 

the glue you trowel on with a 6mm bead I think, which then compresses. I do need to triple check with a straight edge, I don’t have anything very long but will borrow sometching!

65EE2BD8-5229-4651-96FB-09295084ECD0.jpeg

10CC0864-8A64-4EBA-B9B5-86CA17E4CE21.jpeg

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Looks like a pretty good sound surface to me, if its reasonably flat then i would go for it

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yes thats a good surface for adhesive

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No need for a latex screed in my opinion. Floorers like it because they make quite easy extra money for it while also taking away some risk (there are some horrible efforts at flooring out there).

I have had lots of cases of floorers tutting about levels and moisture content and it has always been false: whether scam or ignorance who can say?

 

How long has your concrete been down?

Try putting a glass bowl upside down on it on a warm day, and if it doesn't steam up you are fine.

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27 minutes ago, saveasteading said:

No need for a latex screed in my opinion. Floorers like it because they make quite easy extra money for it while also taking away some risk (there are some horrible efforts at flooring out there).

I have had lots of cases of floorers tutting about levels and moisture content and it has always been false: whether scam or ignorance who can say?

 

How long has your concrete been down?

Try putting a glass bowl upside down on it on a warm day, and if it doesn't steam up you are fine.

Thanks! Glad to hear you think so 😄 I’ll double check with a long level later for flatness but otherwise I’m hoping that’s the general thought, I really didn’t want to sand it 😂

 

the concrete has been down over a year, but that’s a really good tip about the glass bowl! 

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2 minutes ago, saveasteading said:

A year, and no big cracks? Professional.

It’s a small barn to be fair, I don’t know if that helps. About 30 or so square metres for the slab downstairs. And the kudos for a job well done goes to our builder who did this but for us! ☺️

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