Engineered wood flooring - concrete floor prep

[Baball]

Hello!

Only tangentially wood working related, but I will be installing a glued-down herringbone engineered wood floor in our hallway. I've taken the carpet up to check the concrete floor beneath and have some questions that I hope the clever people here can help with. The skirting boards and architraves will be removed before installation, and the floor appears level.

Q1: What is the best way to remove the leftover underlay? How much of it needs to be taken up? Scraping and brushing didn't get it all (point 2 in photo)
Q2: How clean does the concrete need to be? Does left-over adhesive need to be removed? If so, how?
Q3: What is going on with the concrete at (4)? It seems to be only this section which is at the threshold to the kitchen.
Q4: Removing the nailed down carpet strips left small holes a few mm deep. Do I have to fill these, or can I leave them?
Q5: The house is 6 years old. I would presume, but cannot confirm, that a DPM was installed in the concrete floor. Do I need to use a DPM (like Sika Primer WB?) on the concrete before installation?
Q6: I've looked at the Bona, Sika and Osmo options for adhesive. Any recommendations?


Below is a photo of the concrete floor, with the points of interest marked.

1. I presume the carpet underlay was glued down to the concrete around the perimeter of each section of underlay. The red stuff is what is left over, before trying to remove it.
2. This is what remains after scraping and brushing. If you rub it enough with a finger then more can still come up.
3. I think this is the adhesive, but where underlay has come up (mostly) cleanly.
4. What is this?!
5. Minor breaking up of top surface from carpet strip removal

Thanks!

 

[HappyHacker]

I would read the advice from the adhesive manufacturer and the floor manufacturer. It is reasonable to assume there is a DPC but the adhesive may also act a a DPC. They usually have limits on floor humidity.

On 4) It looks as if the concrete is thin over something else. Is it the DPC? I would dig all that cracked stuff out and possibly what is under it, lay a screed mix of concrete, which would need to be allowed to dry out or one of the two mix floor levelling compounds which do not use water and dry quickly. Some of them are also flexible but the fact the concrete has cracked would make me look at the area in more detail.

I would fill the areas where the concrete is lifted following carpet strip removal.

Depending on the manufactures instruction I would not be too worried about any discolouration that does not come off by scraping/wire brushing.

 

[Cabinetman]

I’ve used Elastilon in the past, I think it would deal with all your concerns. Also I had to remove a floor that was stuck to it (it doesn’t and isn’t meant to stick to the concrete) and I can say that Hell does it stick!
The floor I used was just tongue and groove floorboards without glue.
https://www.elastilon.com/en/elastilon

 

[HOJ]

Can only answer Q6, I have used the Osmo MS for a big M2 solid oak floor onto a (clean) screed, it is flexible/rubbery so things can still move, 6 years in, no issues.


The product Ian @Cabinetman linked to looks interesting, couldn't see a price though.

 

[Cabinetman]

Can only answer Q6, I have used the Osmo MS for a big M2 solid oak floor onto a (clean) screed, it is flexible/rubbery so things can still move, 6 years in, no issues.
View attachment 186179

The product Ian @Cabinetman linked to looks interesting, couldn't see a price though.

Not cheap, but there are less expensive versions around. I think Screwfix do one.

 

[Bingy man]

No 4 caution as this could be a pipe duct - gas water or heating - or electricity. is it hollow when tapped ? . If so just remove ghe loose stuff and then use an acrylic floor levelling compound to make good ( prime if need be 1st ) As for the underlay remove as much as possible, I would not be concerned with small holes as these are unlikely to cause any issues, most flooring manufacturers of this type of flooring usually recommend priming the subfloor prior to applying the adhesive so again anything left is effectively sealed in . As above follow the floor laying instructions and you should be fine ..the main job is to remove any high spots and fill any large low spots ( acrylic levelling compound) good luck

 

[Chubber]

Is not this sort of product laid on a form of thin dense green underlay with glued joints? That's how I did my 'study'. I used cork strips at the edges to absorb expansion.

Perhaps you are overthinking this, if the substrates were OK for carpet, I'd vacuum up loose dust, give a coat of PVA sealer and go down that route.

 

[Baball]

I’ve used Elastilon in the past, I think it would deal with all your concerns. Also I had to remove a floor that was stuck to it (it doesn’t and isn’t meant to stick to the concrete) and I can say that Hell does it stick!
The floor I used was just tongue and groove floorboards without glue.
https://www.elastilon.com/en/elastilon

Thanks Cabinetman. That does look like a great solution, although I'm laying smallish herringbone boards (350mm x 80mm), and I'm not sure how it would go trying to keep boards in place while peeling the film back?

 

[Cabinetman]

Thanks Cabinetman. That does look like a great solution, although I'm laying smallish herringbone boards (350mm x 80mm), and I'm not sure how it would go trying to keep boards in place while peeling the film back?

Yes I see your point, that would be tricky, the Elastilon is 1mtr wide and when laying a long plank on it prior to pulling the plastic sheet out, you have a wide plank to keep pushed in place, with lots of little bits like that I think it would be a nightmare, it is instant grab and sticks like the proverbial so if one small plank wasn’t in the right place? Problem, also sometimes the pull out plastic gets caught in the T and G and tears, not the end of the world with 2 mtr planks, but small bits might end up being unfixed.
So no sorry I really don’t think I can recommend you try it.
Ian

 

[Baball]

Is not this sort of product laid on a form of thin dense green underlay with glued joints? That's how I did my 'study'. I used cork strips at the edges to absorb expansion.

Perhaps you are overthinking this, if the substrates were OK for carpet, I'd vacuum up loose dust, give a coat of PVA sealer and go down that route.

Thanks Chubber, I am a frequent over-thinker! But I've been under the impression that a herringbone floor needed to be glued down.

 

[Baball]

I would read the advice from the adhesive manufacturer and the floor manufacturer. It is reasonable to assume there is a DPC but the adhesive may also act a a DPC. They usually have limits on floor humidity.

On 4) It looks as if the concrete is thin over something else. Is it the DPC? I would dig all that cracked stuff out and possibly what is under it, lay a screed mix of concrete, which would need to be allowed to dry out or one of the two mix floor levelling compounds which do not use water and dry quickly. Some of them are also flexible but the fact the concrete has cracked would make me look at the area in more detail.

I would fill the areas where the concrete is lifted following carpet strip removal.

Depending on the manufactures instruction I would not be too worried about any discolouration that does not come off by scraping/wire brushing.

Thanks HappyHacker. You're right, I've been looking at this the wrong way round, trying to determine the correct process and then selecting the products, but selecting the adhesive, and working back from what it recommends would be better.

 

[Baball]

No 4 caution as this could be a pipe duct - gas water or heating - or electricity. is it hollow when tapped ? . If so just remove ghe loose stuff and then use an acrylic floor levelling compound to make good ( prime if need be 1st ) As for the underlay remove as much as possible, I would not be concerned with small holes as these are unlikely to cause any issues, most flooring manufacturers of this type of flooring usually recommend priming the subfloor prior to applying the adhesive so again anything left is effectively sealed in . As above follow the floor laying instructions and you should be fine ..the main job is to remove any high spots and fill any large low spots ( acrylic levelling compound) good luck

Thanks Bingy man, great advice as I hadn't considered something might be concealed under that cracked section.

 

[Baball]

No 4 caution as this could be a pipe duct - gas water or heating - or electricity. is it hollow when tapped ? . If so just remove ghe loose stuff and then use an acrylic floor levelling compound to make good ( prime if need be 1st ) As for the underlay remove as much as possible, I would not be concerned with small holes as these are unlikely to cause any issues, most flooring manufacturers of this type of flooring usually recommend priming the subfloor prior to applying the adhesive so again anything left is effectively sealed in . As above follow the floor laying instructions and you should be fine ..the main job is to remove any high spots and fill any large low spots ( acrylic levelling compound) good luck

Hi Bingy man,

I removed a lot of the loose concrete. I can't tell what this is. The yellow plastic isn't tight fitting over whatever lays beneath, which seems hard and not hollow when tapped. The yellow plastic seems to go down vertically from the top surface, rather than horizontal. There is what seems like white tape at the end and the whole section is very straight and square. It seems to end at the white tape at one end, and goes under the door frame the other end.

I will look for some levelling compound, but I wonder if it too would end up cracking, as the section is very shallow, and the plastic doesn't sit tight to the surface underneath.

I may still be over-thinking, but this has got me intrigued!

 

[Chubber]

I'm equally intrigued!

 

[Bingy man]

Hi Bingy man,

I removed a lot of the loose concrete. I can't tell what this is. The yellow plastic isn't tight fitting over whatever lays beneath, which seems hard and not hollow when tapped. The yellow plastic seems to go down vertically from the top surface, rather than horizontal. There is what seems like white tape at the end and the whole section is very straight and square. It seems to end at the white tape at one end, and goes under the door frame the other end.

I will look for some levelling compound, but I wonder if it too would end up cracking, as the section is very shallow, and the plastic doesn't sit tight to the surface underneath.

I may still be over-thinking, but this has got me intrigued!

View attachment 187641

As I said previously the only reason I can think of leaving a void or duct is for pipes or cables . But the route under a door frame is not best practice. Are there any pipes or cables emerging either side of the door frame . Yellow is usually associated with gas but it could be anything. Does that duct extend either side and continue or is it just the doorway? Nothing surprises me these days so I think I’d be carefully exposing it to confirm just don’t use a drill . Try using a pipe / cable detector as it’s likely quite shallow. Has any alterations been done at any point eg new radiator in a conservatory or a downstairs toilet or shower .?? I’m very curious as to what purpose it serves .
EDIT. Levelling compound will crack if that plastic is flexible given it’s a doorway . Best way is to remove and confirm what lies beneath. If nothing then fill with concrete or screed . If pipes or cables replace with something stronger and firmly fix with plugs / screws.
Slightly different but went to repair some block paving recently that had sunk and found the problem was a bodged drain pipe.

 

[Chubber]

Carrollian "Curioser and curiouser..."

 

[Baball]

There are no obvious pipes or cables, or a reason for any. The detector beeped at the red and blue locations in the photo. Red is under the white tape, blue could be from fixings in the door frame, but can't tell. I don't know how to build houses, but it seems strange to have a service emerge through the foundations in the middle of a doorway!

Photo also shows context - in the direction of the duct is another doorway to the utility room where the boiler is situated against an exterior wall (direction of green arrow), the gas meter is on the opposite side of that exterior wall.

It's a 6 year old new build with no alterations. Cutting through the yellow plastic reveals this below. It is hard like concrete.

 

[Bingy man]

There are no obvious pipes or cables, or a reason for any. The detector beeped at the red and blue locations in the photo. Red is under the white tape, blue could be from fixings in the door frame, but can't tell. I don't know how to build houses, but it seems strange to have a service emerge through the foundations in the middle of a doorway!

View attachment 187726

Photo also shows context - in the direction of the duct is another doorway to the utility room where the boiler is situated against an exterior wall (direction of green arrow).

It's a 6 year old new build with no alterations. Cutting through the yellow plastic reveals this below. It is hard like concrete.

View attachment 187727

It’s quite strange but looking at your photos I’d strongly suspect there is something under that screed or concrete- my reasoning for this is why would the builder not of just screeded the entire floor instead of leaving that part covered in yellow plastic .. so I’d just remove it as it’s not serving any purpose and prime the hard surface with an acrylic primer and just record / photograph and make a note not to drill into that area at any point . Maybe if you could contact the builder and get a better picture of the design and layout of all services and if possible obtain the original plans .. but I don’t think I’d go any further than what you have already exposed. That hard surface looks quite solid and secure so prime / leveling compound and hsppy days with your new flooring.. let us know how you get on and don’t forget pics of the finished floors..

 

[Terry - Somerset]

As a complete non-expert I have observed that buried pipes and wires are frequently covered by a coloured tape to indicate something vulnerable lies below.

Not so much for your benefit, but the benefit of others who may follow, it may be worth gluing down (so it no longer moves) the cut yellow plastic as a warning rather than removing it.

 

[HOJ]

My 2 pence worth, the plastic looks like Radon membrane/barrier, and shouldn't be compromised, secondly as you are across a threshold it may well be the case that the wall is a supporting wall and as such there will a footing in that line, with block work of some sort bought up to the floor level and leaving the thickness of a mortar joint below the finished floor screed in the adjacent rooms.