Drying of screed

[Jude1234]

The screed on the ground floor over the UFH is liquid screed (i.e. Calcium Sulphate).  We have been waiting weeks for it to dry so that the wooden floor can be laid (needs a moisture level of 3.5-4%).  In desperation the builder is rushing to get the UFH on to dry it out.  However both the bamboo supplier and a potential floor layer have said that the top layer needs to be sanded/scraped off to get a true moisture reading.  Is this true? And if it is why isn’t my builder suggesting/doing this?  Am I missing something?

[jack]

I don't know about the need for moisture readings, but the stuff on the top is called laitance, and it should be removed (by sanding) before any form of adhered flooring goes down:

 

https://www.easyflow.org.uk/additional-services/laitance-removal-screed-sanding/

 

There are some newer screeds that allegedly don't develop laitance, but the possible costs of rectifying the floor if it fails in the future makes me think that you should sand it anyway.

[joe90]

I had one of these new screeds, very wet time of year and house just been plastered. I got two industrial de humidifiers to help but I scraped the surface anyway to make it super flat (small bubbles on the surface). HOWEVER, as some will know from another thread I have some tiles (not all) that are sounding hollow, yet to find out what the problem is but we don’t believe it’s the screed causing it. I have read somewhere the screed should not be dried by heating it with the UFH.

Edited September 19, 2018 by joe90

[Ian]

advice from BAL is here:

https://www.bal-adhesives.com/tiling-onto-calcium-sulfate-anhydrite-screeds/

 

Edit: Also this advice from Sika:

"Calcium Sulphate screeds will not normally start to dry properly until the laitance have been removed. The sooner this process is done the easier it is to eradicate. This is normally the responsibility of the screeding contractor who is best placed to know when the screed has cured sufficiently to proceed. The conditions on site and the thickness of surface laitance play a large part in the drying times that will be required. As a general rule of thumb if the laitance are removed at the end of the 1st week of placing a light sanding action is all that will be needed. In the 2nd week a courser aggregate maybe required. In weeks 3 and 4 you may well require a rotary carborundum stone or a heavy weight diamond grinder. In short, the longer the laitance are left the denser they become and more difficult they are to remove. Once the laitance are removed Calcium Sulphate screeds dry similarly to that of sand/cement, which given the optimum conditions is 1mm per day up to 40mm thick and an additional 2 days per 1mm over this thickness."

Edited September 19, 2018 by Ian

[Jude1234]

Thanks so much Ian.  I will discuss this with the builder, looks like this should have been done ages ago

[Jude1234]

Does the scraping need a special machine or is it done by hand? As I am struggling with my builder he might not do it. 

[Ferdinand]

6 hours ago, Ian said:

advice from BAL is here:

https://www.bal-adhesives.com/tiling-onto-calcium-sulfate-anhydrite-screeds/

 

Edit: Also this advice from Sika:

"Calcium Sulphate screeds will not normally start to dry properly until the laitance have been removed. The sooner this process is done the easier it is to eradicate. This is normally the responsibility of the screeding contractor who is best placed to know when the screed has cured sufficiently to proceed. The conditions on site and the thickness of surface laitance play a large part in the drying times that will be required. As a general rule of thumb if the laitance are removed at the end of the 1st week of placing a light sanding action is all that will be needed. In the 2nd week a courser aggregate maybe required. In weeks 3 and 4 you may well require a rotary carborundum stone or a heavy weight diamond grinder. In short, the longer the laitance are left the denser they become and more difficult they are to remove. Once the laitance are removed Calcium Sulphate screeds dry similarly to that of sand/cement, which given the optimum conditions is 1mm per day up to 40mm thick and an additional 2 days per 1mm over this thickness."

 

Does it make a difference whether one is having click fit or attached flooring?

 

Ferdinand

[Jude1234]

The floor company said that a damp proof layer could be applied on top of the screed if necessary but I don't really want to do that due to the extra time and cost.

 

Anyone scraped the top layer of before?  Is it a simple job for a newbie and and industrial sander?

[vivienz]

Can i butt in with a slightly off topic query? My slab was powerfloated - does this still need sanding or can i lay stuff straight onto the powerfloated surface?

[Onoff]

Slightly off topic...

 

When I've floor painted garages and utility areas I've always used a hand wire brush to abrade the surface and sometimes a cup wheel in a small grinder. Then vacuumed and vacuum again. Paint sticks/lasts so much better.

[Ian]

12 hours ago, Ferdinand said:

 

Does it make a difference whether one is having click fit or attached flooring?

 

Ferdinand

With a click fit type floor you wouldn't need to use adhesive to stick the floor to the screed which makes the while process much simpler however you still need to ensure the screed is properly dry, or alternatively, use one of the special loose underlays that allow any residual moisture to escape to the edges of the floor.

[jack]

1 hour ago, vivienz said:

Can i butt in with a slightly off topic query? My slab was powerfloated - does this still need sanding or can i lay stuff straight onto the powerfloated surface?

 

I don't believe any sanding is needed. You may want to confirm whether a sealer/primer is recommended for whatever floor covering you plan to lay.

[Ian]

1 hour ago, vivienz said:

Can i butt in with a slightly off topic query? My slab was powerfloated - does this still need sanding or can i lay stuff straight onto the powerfloated surface?

sand cement screeds and concrete slabs are much more forgiving than anhydrite type screeds and typically need less work to remove laitance prior to tiling. You still need to remove any laitance however there will be a lot less of it compared to anhydrite screeds. The reason for this is that anhydrite screeds contain far more water which is why the laitance is more prevalent.

 

The main issue with power floated slabs is that the power floating process seals the surface of the concrete making the drying process much longer. In fact I would say that 12 months drying time would not be unusual for a power floated slab. The good news is that there are plenty of liquid dpm products that can be applied to the top of the slab to get around this problem however they are not cheap.

 

 

[Ferdinand]

16 minutes ago, Ian said:

With a click fit type floor you wouldn't need to use adhesive to stick the floor to the screed which makes the while process much simpler however you still need to ensure the screed is properly dry, or alternatively, use one of the special loose underlays that allow any residual moisture to escape to the edges of the floor.

 

Thanks Ian.

 

So that would presumably mean that the laitance would still need to be removed for a click-fit?

 

(Self-educating here as I have not done an on-screed floor myself for some time as main protagonist.)

 

F

[Ian]

1 hour ago, Jude1234 said:

The floor company said that a damp proof layer could be applied on top of the screed if necessary but I don't really want to do that due to the extra time and cost.

 

Anyone scraped the top layer of before?  Is it a simple job for a newbie and and industrial sander?

You've got 2 main issues that need resolving:

• ensuring the screed is dry enough (it needs to get down to 75% RH)
• ensuring the screed is properly prepared ready for the final floor finishes. (If you are planning to stick anything down onto the screed like the bamboo the screed will need the surface laitance removing)

[Ian]

12 minutes ago, Ferdinand said:

 

Thanks Ian.

 

So that would presumably mean that the laitance would still need to be removed for a click-fit?

 

(Self-educating here as I have not done an on-screed floor myself for some time as main protagonist.)

 

F

If you are not trying to stick anything down onto the surface of the screed then getting rid of the laitance is not really an issue although you would still want the surface to be clean and dust free. Your main issue is making sure that the screed is dry enough (it needs to be at 75% RH). There are liquid surface dpm products that can get around the drying issue but they are quite expensive. Alternatively I've occasionally used specialist profiled underlays which allow any free moisture to escape to the edges of the floor.

[MarkyP]

my first post here !

 

I did a fair bit of research into anhydrite when our 60mm screed was poured. Laitance removal is required where there is a need to bond a floor finish or intermediate product to the screed. The laitance skin can debond from the screed causing failure of flooring systems. I read conflicting views on whether laitance removal speeds up the drying process, I found a study (which I will try to relocate) which found it made no difference. I found most suppliers recommended removal of laitance as speeding up drying but none quantified it, all required it where a bonded floor finish was planned. All of the suppliers I considered permitted the use of UFH to accelerate the drying time. Ask your builder for the contract documentation or at least the supplier's name so you can ask them for the aftercare documentation, they all provide this with the contract as a backside covering exercise. In this you should find guidance on drying and whether you can use UFH. Our documentation went into some detail about UFH temperature control and increasing from ambient temp in 5 degree intervals over several days. We ran ours at a low 35c degrees supply temp for a week and it was bone dry (following a period of 6 weeks of natural drying). I don't think you need to remove the laitance if you are laying a floating floor (not glued down). I nearly forgot to mention that our screed supplier stipulated 48hrs of wind tight drying (all windows and doors shut or openings poly sheeted and taped). It then required 7 days of natural drying/curing before use of force drying with UFH was permitted.

Edited September 20, 2018 by MarkyP

[Jude1234]

Just following up on this issue.  We have had the UFH on for over a week. Then it was off for over 48hrs and the moisture levels are still high. The bamboo flooring is meant to be laid on Monday so the fitter has suggested painting a damp proof membrane on prior to fitting.  I have 2 concerns 1. Cost with materials and labour it will add on  £400. 2. Could there be an underlying problem that is preventing the floor from drying out.   I would appreciate any advice on whether this is likely or not.  Whilst not ideal to not have flooring laid prior to moving in I would prefer it to having to rip it all up if there is a problem later on.

 

Just to add the builder did confirm that the laitance was scraped off soon after the screed was laid.

Edited October 19, 2018 by Jude1234

[Ian]

25 minutes ago, Jude1234 said:

Just following up on this issue.  We have had the UFH on for over a week. Then it was off for over 48hrs and the moisture levels are still high. The bamboo flooring is meant to be laid on Monday so the fitter has suggested painting a damp proof membrane on prior to fitting.  I have 2 concerns 1. Cost with materials and labour it will add on  £400. 2. Could there be an underlying problem that is preventing the floor from drying out.   I would appreciate any advice on whether this is likely or not.  Whilst not ideal to not have flooring laid prior to moving in I would prefer it to having to rip it all up if there is a problem later on.

I don't think it's a good idea to use a liquid dpm on top of a still-damp Calcium Suphate screed like yours. It would be okay with a sand cement screed or concrete but calcium suphate screeds have a high gypsum content. If you use a liquid dpm before the screed has dried out properly it will trap any residual moisture and there is the potential for the gypsum to swell up and become soft leading to your floor finish debonding. http://www.contractflooringjournal.co.uk/blog/anhydrite-screeds-handle-with-care/

 

Edit: I've read that some manufaturers of liquid DPM claim they have products suitable for use with calcium sulphate screeds but it's not something I have any personal experience with. EG: http://www.uzin.co.uk/products/calcium-sulphate-screeds-need-uzin/

Edited October 19, 2018 by Ian

[Ian]

@Jude1234

In order to get your screed dry you need a differential between the relative humidity of the screed and the relative humidity of the air in the house. For example, if the air in the room is warm with high humidity because all the windows are shut while the UFH is on then the screed is not going to dry out.

 

Have you tried running dehumidifiers at the same time as the UFH?

Edited October 19, 2018 by Ian

[PeterW]

I would get a big industrial dehumidifier in there ASAP. 

 

They will take a lot of moisture out very quickly. 

[Ian]

@Jude1234

What method are you using to test the relative humidity levels in the screed?

 

Edit - the reason for asking is that some of the commonly used test methods can be unreliable, particularly when the screed is calcium sulphate. If your builder has been testing using a Tramex CME meter (see photo) they can sometimes give false high readings. It would be worth checking using a different method. It doesnt sound like you have the time to get a hygrometer test done as they take 72 hours but you could get a calcium carbide test done quickly if you can find someone to do it for you.

 

 

 

 

Edited October 19, 2018 by Ian

[sam]

Many anhydrite screed manufacturers provide information. We have used a Gypsol screed and their post instalation guide can be found at: http://gypsol.co.uk/media/1497/6-gypsol-post-installation.pdf

 

As soon as the screed was hard enough to walk on I found that much of the laitance could be removed with a stiff brush but I still plan to remove the remainder by sanding before laying tiles.

 

According to the manufacturer's documentation it is fine to force dry with UFH after 7 days. Our installer insists that it is better to do this.

[Jude1234]

We have decided to delay laying of the floor rather than invalidating the guarantee and paying  £600 for the DPM. Hopefully when the UFH is on and also the MVHR it will dry out. Currently value is 5.8% using a Tramex 

[Ian]

5 minutes ago, Jude1234 said:

Currently value is 5.8% using a Tramex 

Just to re-state the point I made earlier: you can easily get a false positive reading from a Tramex meter, especially as they are calibrated for concrete and not calcium sulphate. It's worth getting a hygrometer or calcium carbide test done to double-check that the Tramex readings are correct. Using the Tramex the floor might be bone dry and your meter is telling you it's wet.