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My living room has a concrete floor and adjoins my dining room. The dining room used to be a kitchen in a pre-extension life and has a screed floor. The screed floor is about 20mm higher than the concrete floor of the lounge.

I have some existing hard wood flooring (maybe 20mm or so deep) to go over the screed and will be laying carpet in the lounge.

My question is twofold.

  1. How close in height does the concrete and screed need to be in order to get a smooth transition?
  2. How would I go about raising the concrete floor? I was hoping to lay some ply.

Background:

The house is 1930s. The concrete floor was added at some point later to replace a cavity floor. There is a channel, 100mm wide, around two edges of the concrete floor (including the edge with the transistion) where old heating pipes are laid.

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If you choose to use plywood, go with the 3/4" T&G for the layer under the carpet. The trough were the heat pipes used to be can be spanned by the 3/4" plywood too. Also since the carpet compresses under foot, keep the plywood below the hardwood layer about 3/8" or so lower. That way when you step there, the transition will feel level.

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Thank you so much for the detailed answer (and picture), really helped me out trying to decide how to tackle this issue. Just out of curiosity what is the reason for the two layers of ply below the carpet? Is it just to get the height right? –  Matt Apr 7 at 7:58
    
That is correct, the first layer are strips of plywood to act as a spacer to get the subfloor to the right height for the carpet. –  Jack Apr 7 at 12:24
    
Perfect, thanks. One last question, is ply the correct choice for this? Would Chipboard be ok for the subfloor as it's much cheaper, but not as strong, especially given the trough around two edges of the room. Thanks again. –  Matt Apr 7 at 15:30
    
I would still use the ply. If costs are a concern, you could go with a 5/8" T&G, I would decrease the spacing of the spacers, say to 12" OC. This would reduces the amount of deflection between the spacers. The trough at the edge would still be covered by this too. The possibility of somebody stepping in that area is nil, and to have a piece of furniture that would apply a lot of pressure there would also be slight. Then again, you could add a little pressure treated framing in there to support the edge too. I have assumed there are no water issues with your floor... –  Jack Apr 7 at 15:39
    
Regarding water: the concrete appears very dry. There is treatment for rising damp in the brick around the edges below which is a little damp. I'll just not fit the wood all the way to the edge and/or fit damp proof membrane all the way around the edge of the wood. Hopefully that will work. I'll follow your first suggestion about the thicker ply, seems more stable. If I can't get t&g would 4'x8' boards be ok to lay or would they come loose? –  Matt Apr 7 at 16:47
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