Screed Drying for Engineered Flooring

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diy42354

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Hi - could I source your thoughts on concrete drying times suitable for flooring?

We've replaced a ground froor slab (44m2) with a DPM, 6" concrete, 100mm kingspan, and 65mm Anhidrite Screed containing a wet loop underfloor heating system.

The Screed pour went in 33 days ago, followed by ongoing dehumidification and taking the Underfloor Heating through a ramp up and down cycle, and now operating consistently 24/7 at 20 degrees.

Testing the moisture levels gives very variable results from patch to patch and it's hard to gauge a reliable figure. The colour looks dry but in some areas patches still appear where items have been standing, but these are diminshing arpidly.

The plan is to lay a vapourproof underlay with 14/3 real wood engineered floor when the surface is acceptably dry. Naturally it would be good to get that done before Christmas but not at the risk of damp or warped floors.

Can I ask your thoughts on what you would consider a safe point to proceed with laying the floor?
 
Thanks Dazlight - I have only just discovered this forum but, having just looked over there, they do have a similar question at http://theflooringforum.com/threads/anhydrite-screed-and-engineered-wood-floor.14448/#post-103233 which is very helpful.

The moisture readings I'm getting vary between 0% and 18% or more which seem too inconsistent to draw any conclusions. Leaving a bucket (for instance) on the floor would still pull some patchy moisture to the surface in places.

At present I'm inclined to just leave it for a few more weeks and just have a Christmas with concrete floors. :(
 
Just to add that my moisture readings are taken using a Brennenstuhl 1298680 Moisture Detector MD (http://goo.gl/r06b99) which gives % moisture content where low is 1.5 to 16.9%, Middle 17 to 19.9% and High 20-33%.

Another thread suggested taping plastic bags to the floor to see the moisture that gathers - which seems like a good idea.
 
Wasting your time with electrical moisture meters, you won't get an accurate reading.

Typical screed drying times are 1mm per day for the first 50mm and a more vague rate over 50mm thickness. Likely with a dehumidifier and running the heating you can significantly reduce that time, but you can see with your 65mm screed it is possible that it is too high in MC% after 33 days.

You want to use a hygrometer box (sealed box goes over the screed, gets left 1/2 days and you measure the RH within that box), wants to be about 55% RH or less for underfloor heating which equates to approx 1% moisture content (check that, I am not a concrete person).

Once the screed is dry, then there is no point using a vapour proof underlay?

Such underlays are used where people want to lay the floor before the screed is dry, which personally makes me nervous, but it has been done.

Also, why float and not glue? (nothing wrong with floating, but with underfloor heating and good engineered boards IMO glue'd will 'feel' better).
 
Thanks for the good points.

You are right that I didn't put much credence on the meter reading. I will see if I can source a hygometer box to take better readings. Either way I'm happy to wait until it is dry but still put down a vapour proof underlay just for reassurance (I don't want to be nervous) plus any sound properties.

I had thought that floating was the way to go with such a large area (running 44m2 through downstairs of house) and any maintenance issues, but I will look into it.

Incidentally you mention 'good engineered boards' which I know has been the source of many threads of discussion here and elsewhere. Given budget constraints, I had been looking at the Howdens engineered oak which includes 5D style (?) joins but seems to use a laminate type fibreboard base rather than ply. The finish and price are great but it has be concerned about its quality.
 
Fibreboard will be weaker and less stable than a good quality plywood base.

Both are suitable though, and you get what you pay for.
 
Thanks again. Here's a photo of the Howdens fibreboard version (bottom) and a the Woodandbeyond ply (top). The Howdens version seems a lot more flimsy and more susceptible to warping/damage.

kEaTO5S.jpg
 
The plywood base will be physically stronger and more able to resist cupping (not immune, just more resistant).

The fibreboard one I assume represents a significant cost saving, if you are happy you have good conditions you may be fine with it.

Although it has a click system which is much less hassle if you are floating it.
 
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