Need self-levelling screed?

[jpadie]

Hello all

 

I've nearly finished a garden room which has a sub floor in 18mm osb. 

 

It got wet during the build and the joints have swelled.  I've managed to plane them down but there is still about a 6mm tilt on a 1200mm level. 

 

I want to lay bamboo 14mm uniclic boards on 3mm foil underlay.  

 

My question is whether the unevenness in the subfloor is significant enough in your experience to warrant a self levelling screed?  And if so what do you recommend (my experience of self leveling mortars has been that they are rather guilty of misadvertising their self levelling nature....)

 

Thanks

Justin 

[oliwoodings]

What's underneath the osb? Could you just rip it up and replace it?

 

Alternatively do you have space to just overboard it with 12mm ply perpendicular to the current subfloor?

[jpadie]

1 hour ago, oliwoodings said:

What's underneath the osb? Could you just rip it up and replace it?

 

Alternatively do you have space to just overboard it with 12mm ply perpendicular to the current subfloor?

Under the osb are 2x6 joists and 140mm PIR.  The osb goes under the wall studs and sill plate.  So not easily removed.  It's also glued to the joists as well as screwed every 150mm. 

 

I could lay ply but with the bamboo and underlay i would lose 28mm of head height.  I had to stay under the 2.5m height rule so I'm very constrained and trying to get away with the absolute minimum head height loss possible.  Also would not ply just repeat the contours from the subfloor?

[stu w]

2 hours ago, jpadie said:

Hello all

 

I've nearly finished a garden room which has a sub floor in 18mm osb. 

 

It got wet during the build and the joints have swelled.  I've managed to plane them down but there is still about a 6mm tilt on a 1200mm level. 

 

I want to lay bamboo 14mm uniclic boards on 3mm foil underlay.  

 

My question is whether the unevenness in the subfloor is significant enough in your experience to warrant a self levelling screed?  And if so what do you recommend (my experience of self leveling mortars has been that they are rather guilty of misadvertising their self levelling nature....)

 

Thanks

Justin 

Take any fixing out in the high areas , plane the  6mm raised areas  and any bad bits so all the floor is totally flat , re-fix then overboard with 6mm ply glued and screwed to get the strength back into the floor 

Edited April 10 by stu w

[jpadie]

So what I'm hearing is that the underlay will not absorb any imperfections in the subfloor.  

 

Is planing and overboarding a preferred method to adding a self levelling screed? 

[stu w]

7 minutes ago, jpadie said:

So what I'm hearing is that the underlay will not absorb any imperfections in the subfloor.  

 

Is planing and overboarding a preferred method to adding a self levelling screed? 

When I have re-laid flooring in the past following on from someone else what has failed due to uneven sub floor I would say no the underlay will not absorb the deviations at around 6mm.

I personally have and never will use a levelling screed over a timber floor,  especially onto joists.

What area is the floor ? 

Edited April 10 by stu w

[jpadie]

Total area is 11 sqm. Joists at 400mm.  Spans of 2000mm max.  

 

The 6mm deviation is across 1200mm. Using a 600mm level everything seems more or less within a 1mm.  

 

 

[Bozza]

In a former home I had a downstairs loo with timber floorboards and low ceiling height. The floor was not level - out by quite a bit despite small meterage -  and it was going to be difficult wrong get it level via timber for several logistical reasons including boards going under walls.  I taped up the joints and poured a self levelling screed.  It was worth a punt and worked really well.  A click laminate went over it.

 

whether this would work for you I don’t know.  I’d imagine you’d need to prime the osb with pva to prevent it sucking in the the water from the compound.

 

if you’ve never used self levelling compound you do have to push it around a bit to find the low spots.

because of my previous success with compound if it were me in your scenario that’s what I’d do.  it wouldn’t have to be perfect as you’re putting in an underlay not like you’re gluing down an LVT onto it.

 

 

 

 

 

 

 

[jpadie]

On 11/04/2024 at 13:00, Bozza said:

In a former home I had a downstairs loo with timber floorboards and low ceiling height. The floor was not level - out by quite a bit despite small meterage -  and it was going to be difficult wrong get it level via timber for several logistical reasons including boards going under walls.  I taped up the joints and poured a self levelling screed.  It was worth a punt and worked really well.  A click laminate went over it.

 

whether this would work for you I don’t know.  I’d imagine you’d need to prime the osb with pva to prevent it sucking in the the water from the compound.

 

if you’ve never used self levelling compound you do have to push it around a bit to find the low spots.

because of my previous success with compound if it were me in your scenario that’s what I’d do.  it wouldn’t have to be perfect as you’re putting in an underlay not like you’re gluing down an LVT onto it.

 

 

 

 

 

 

 

Thanks Bozza.  This is what I was thinking too.  I will give it a go as it seems the thinnest method to create a flat surface for uniclic hard wood boards and if it all goes pearshaped, 12 sqm is not too traumatic to break out and remove.

 

Did you find PVA better than SBR?  There are so many conflicting opinions!

[Bozza]

5 hours ago, jpadie said:

Thanks Bozza.  This is what I was thinking too.  I will give it a go as it seems the thinnest method to create a flat surface for uniclic hard wood boards and if it all goes pearshaped, 12 sqm is not too traumatic to break out and remove.

 

Did you find PVA better than SBR?  There are so many conflicting opinions!

 

[Bozza]

I didn’t put anything down but if it were chipboard I would,  I don’t think it would make a lot of difference what you use just as long as it seals the chipboard & prevents the chipboard sucking in the moisture from the compound.

 

I’ve used the cheap no nonsense stuff from screwfix with success a few times,

 

ultimately worth a punt because sounds like plan B would be to rip it all up anyway.

[jpadie]

Coming back to this post, I have laid good quality screed down and it's taken pretty well with a good, solid adhesion to the sub floor.

 

Sadly, even with extra liquid the SLC has not created a level playing field and in some places I'm 4mm out.  Seems there is one relatively small area of about 40x40cm that's a couple of mm low and one large area of 2x 1.5 thats sloped to about 4mm low.

 

What's the optimum for releveling this please?  An ultra fine SLC?  In which case do I have to wait a week or so for a full cure before laying?  The thickness of the current screed is 3-8mm.

 

NB I have almost zero headroom allowance so overboarding is not a solution here.  Every mm counts and I'm looking to fill the low spots.

 

Thanks in advance 

[Alan Ambrose]

>>> even with extra liquid the SLC has not created a level playing field and in some places I'm 4mm out

 

Well I'm sympathetic 'cos I've been there before. I wonder whether a very low viscosity product will work? Or a careful re-application of the same product, just in the areas that are low, with some sort of tamping bar to get the level. Should adhere better if the original hasn't dried out I would think.

 

That's only my guess though...

[Bozza]

Adding too much compound is likely to create a lip between the new and old & create extra height where you don’t want it as the compound hasn’t got something to butt up to.  Make sure the new compound covers from 1mm thickness to minimise this eg

https://www.screwfix.com/p/no-nonsense-floor-levelling-compound-20kg/453hu. 

 

I’ve previously used floor tile adhesive as well for levelling small areas, which you can obviously level and feather with greater control maybes better option for your small 40x40 area. And sand thereafter if not perfect.

 

Don’t forget it doesn’t have to be 100% perfect if you’re laying foam underlay and laminate.

 

sounds like you’re close to resolution. 
 

if it were me I’d use some self leveller for the very deepest bit of the 4mm section only , being very careful not to overdo it as it would be easy to end up running over to where you don’t need it and thus creating extra height.

Pouring short will leave a low valley maybes 1-2mm deep maybes a few inches wide between the exiting pour and your new pour, and then once dry feather in some floor tile adhesive in that valley.  Then sand smooth.  Won’t look pretty but will be level & will work for your needs.

 

 

 

 

 

 

 

 

 

 

 

 

 

[Super_Paulie]

could you not batten it out to the tune of the highest dip, 4mm plywood and then use a dry screed and a flat edge to strike it off?

[Kelvin]

You can get bamboo look porcelain tiles so just fit those and level as you go. 

[FuerteStu]

I had a dip about the same size that I think developed from a small void where a doorway used to be. Evidently is soaked through where I'd sealed. 

 

What I did was mark out the low area, 'dink' a rim with a cold chisel so there wouldn't be a seam line, then top up with small amount of self leveling making sure it was smooth into the rim I'd formed with a sponge as it began to set. 

[jpadie]

Great answers people!

 

I've gone with a 1mm mapei SLC and marked out the low spots. I'll pour slowly and carefully so I don't overfill.  Will think about the rim idea overnight.

Am going to prime the areas that need a second layer although they are currently too damp to do so.  I could probably get away without it given the moisture in the floor but it's an extra 20 quid that I've already spent.  

The wood is already on site and has been for a few months now.  So no changing to tiles or a thinner board to get more headroom.   Another thing I'd do differently second time around!

[Kelvin]

I wouldn’t overthink the headroom if it’s literally mm. You really won’t notice it. 

[saveasteading]

43 minutes ago, Kelvin said:

headroom if it’s literally mm. You really won’t notice it. 

Ouch. Very low doors you see coming and duck under. Slightly low doors are installed to catch the unwary on an up-step or doing a Columbo U-turn..

[torre]

10 hours ago, Bozza said:

.

 

[jpadie]

14 hours ago, saveasteading said:

Ouch. Very low doors you see coming and duck under. Slightly low doors are installed to catch the unwary on an up-step or doing a Columbo U-turn..

 

 

I hear you!

 

I live in a 16th century cottage with beams traversing at 170cm.   I'm 178cm with a wood print impressed on my forehead.  

 

The head height of the garden room is low.  I can put my wrist against the ceiling and the floor is not on yet (another 18mm).  It's livable but the door height of the internal rooms is hard up against the limit that the door can be cut down.  I have a feeling that I will need to remove and trim the header down before fitting the liner to make it all work.  Which leads to redecorating that area and then also redoing another door that needs to match height for aesthetic reasons.   All a b***ock ache, truth be told 

[saveasteading]

34 minutes ago, jpadie said:

 I'm 178cm

add  shoes and walking must raise height by 3cm?

 

I lived in an old house and never caught my head on the ancient tiny doors because I developed a pace that allowed an easy ducking under. But I was often caught out when I turned back for any reason . 

Would I put in even lower doors to avoid this? that would be selfish and unfair on slightly smaller people.

Perhaps round off the door frame liners or fit a quadrant.

 

There is a pub near here where I can only stand between the oak floor joists in the ceiling, and my head skims the ceiling. I don't often go there.

 

[jpadie]

two days later and the moisture content of the screed is 30%.  fan on all the time.

question is whether i can still prime some patches before laying some more SLC or if it has to be dry.
the primer is mapei primer g - the info says it can be used between levels of SLC but the layer must be "perfectly dry".  does that mean down to 3% RH or just not puddling with water?  seems really odd since the primer needs diluting with water too!

also seems odd that a quick setting SLC poured at 5-9mm is still at 30% RH after two days when it's supposed to be ready for wood flooring after 2 days (which requires 4% moisture or lower).

[saveasteading]

1 hour ago, jpadie said:

screed is 30%.

That shouldn't be after this time. A puddle of 9mm would have evaporated in that time. 

either there is damp in the whole floor, or the measuring process is wrong. 

I once had this with a huge floor that 'stuck' at high humidity  despite heat and ventilation. it turned out I was using the gauge wrongly.