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Self levelling compound on a first floor room which has chipboard floor?

We have had an extension over our garage to create a larger bedroom with ensuite. The old floor and the extended floor are now part of the same room but I have noticed that the new floor is lower than the old floor. We think the floor was level to the old floor when first laid but it has been around 6 months now, carpet was laid around 4 months ago and we can now feel a step down to the new floor. The high point of the floor is above a steel beam which sits and then there seems to be a step down so we think the joists may have shrunk whereas the steel has not?
The project manager has finally agreed there is a step down and is offering to rectify the problem by using a self levelling compound. I do not like the sound of this as I have never heard of self levelling compound being used on a first floor only on a ground floor concrete floor. The product he has mentioned can be bought in wickes and does say can be used on rigid wood floors but I do not think it would be a good idea to use on a first floor chipboard as it could leak through to the ceiling below and also I do not like the sound of levelling compound going up to a skirting.
I have said that a friend advised to take up the floor and use packers then relay floor but the project manager does not seem very keen on this.
I am worried we are going to get a bodged job rather than a proper professional finish. Can anyone comment? Are my fears valid? Thanks

6 Answers from MyBuilder Carpenters & Joiners

Best Answer

Generally when cutting floor joists into an RSJ you should allow a minimum of 10mm above the RSJ to allow for shrinkage of the floor joists, which I suspect is what has happened here. The problem can be overcome with certain levelling compounds. However, it's not something that we would generally use for this problem as a first option. Obviously it would depend on the position of the RSJ and amount of shrinkage. If it is a case of the floor has a high point over the RSJ, you could consider cutting each side of the RSJ and bridging it with a Ply material of a narrower gauge to allow for the shrinkage. Allowing it to sit on the RSJ, forming a solid base on top of the RSJ. This will however depend on how much difference your talking about.


Answered 2nd May 2013

when you say step down how much do you mean? couple of millimeters / inches?
if it is 3 - 5 mm laytex is perfectly fine - timbers do shrink a little, and regards being on first floor laytex is quite solid form not like water!


Answered 25th Apr 2013

My answer assumes no major building issues or failures and just a 'normal' settling down of new extension.
The sinking down of the new section should be dealt with by overboarding it with plywood nailed at 6 inch centres (4 inch around the perimeter of each sheet). The ply should be the same (or as near as possible to the height difference) as is available in 4mm, 6mm, 8mm and so on. Any discrepancies can be then smooth on the joints with a wood screed such as arditex feather finish.
Less preferably, it is possible to screed the area instead but use a reputable flooring firm, as it must be a flexible wood screed and a suitable primer must be applied first. The screeds in the main DIY shops are inadequate in my opinion and this is usually what general builders use.


Answered 14th Dec 2015

There are 2 options which are less intrusive than lifting the floor. For starters you should not lift it and place packers on the joists.
Applying hardwood plywood (do not use softwood, it is cheaper for a reason) on top is your best option. Glue and screw it down and you will get no issues.
It can also be latexed (self leveling compound) but you will need to use a 2 part compound and not water based. I always use arditex na as it bonds to any surface, has a built in primer, and is waterproof. It is approx £25 -£30 per bag/bottle and will work a treat.


Answered 31st Jan 2019

Is this so called project manager have any idea what he is saying. The floor needs to be taken up and rectified from underneath chipboard flooring. he is trying to cut corners ( cheap option) which will not work


Answered 20th Apr 2013

I would avoid the self levelling compound option. I would insist the floor is lifted and rectified. if the floor moves any more the latex will blow and break up, it is not recommended to latex onto chipboard unless a ply has first been put down to counteract shrinkage or expansion. and if you do I would only use a flexible latex not like what others have suggested but a product such as stopgap 700 superflex


Answered 30th Oct 2019

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