Pug / Dry Screed for UFH

Discussion in 'Builders' Talk' started by Steve Rye, Oct 26, 2020.

  1. Steve Rye

    Steve Rye New Member

    Hello
    I'm currently in the process of installing under floor heating in an old semi with joisted floors.

    I placed suspended 100m celotex between the joists (held underneath by timbers going cross from the joists) and installed the 16mm pipework.

    Yesterday, being a nice day, I mixed the dry screed that most people seem to reccomend for covering the pipes.
    I mixed 1:7 cement with kiln dried sand, no added water. I completed about 30% of the floor area, approx 40 bags of sand.

    I can't get my head around this actually setting hard, next day it's still very sand like and shows no sign of hardening at all.
    Have I got this wrong? Do I need to get out my watering can over what I have already laid?

    Some advice would be very welcome before I do the other 70%!

    Cheers
    Steve
     
  2. I-Man

    I-Man Screwfix Select

    Kiln Dried Sand??? Are you sure about that one!?
     
  3. Steve Rye

    Steve Rye New Member

  4. I-Man

    I-Man Screwfix Select

    Hi Steve - couldn't find any mention of kiln dried sand in that link you just sent. Where else have you seen it mentioned?

    It's generally used for brushing in-between paving, never seen it used for something like floor screed.
     
  5. Steve Rye

    Steve Rye New Member

    Hi I-Man

    I should have posted the text rather than the link:

    Pug Screed The ‘Pug system’ (a thin biscuit of dry screed) is used when the heat source (heat pump) produces low flow temperatures of 35-45ºC. The Pug system can be used in timber suspended floors. The Pug system is installed between joists, usually set at 400m centers, allowing 135-200mm centres for the pipe layout design. A minimum of 25mm thick dry screed is then laid, ensuring that it penetrates fully around the pipe and sits level with the top of the batten (5a/b/c/d). As the screed is directly beneath the covering board, heat conductivity is increased. The timber floor finish can be screwed into the still exposed timber battens. It is important to remember that the screed is to be mixed and installed dry 8:1 Sand/cement mix and is for heat distribution only. Any cracking will not affect system performance. The Pug system is ideal for constructions where floor build up is restricted. Our preference is for timber boards to be fixed on battens between joists to support the system. An alternative is to use the insulation board to support the system. The Approximate weight loading for the pug mix when used as above, would be 20- 25 KG M². Check with structural engineer or other qualified person if in doubt about weight loading


    Other links
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=2558
     
  6. I-Man

    I-Man Screwfix Select

    Hi, I still suspect that Kiln dried is the wrong type of sand for this sort of application. Again in the article you've sent, it doesn't really specify Kiln dried, apart from one user's suggestion.

    You say you've not added any water, so then how do you expect the screed mix to set? Kiln dried sand has no moisture in at all, unlike building/sharp sand etc.

    I'm no expert in this particular area, ..but giving you my take on it based on past experience. I'm sure there are other's who are better placed to comment..hoping they come along soon :)
     
  7. Steve Rye

    Steve Rye New Member

    Ok, thanks.
    I'm thinking of getting my garden sprayer out to moisten the mix I've put down...
     
  8. stevie22

    stevie22 Screwfix Select

    About the most expensive way to buy sand! I would have just used normal sharp sand. The article suggests material doesn't set and unless your sharp sand is wet and well tamped down it's not going to set hard

    What depth have you put down? Will the joists take the extra weight?
     
  9. Steve Rye

    Steve Rye New Member

    The depth is only 30mm -25mm, so not a great weight. The joists are 6x2 over 3meters.


    Yes. In hindsight I should have bought "normal" sand, I was concentrating on the "dry" too much.

    Do you think that simply damping the area with a fine spray (garden spray) will enable the mix to set?
     
    Last edited: Oct 26, 2020

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