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I am doing a remodel of a 1970s built upstairs condo (WA state)

Once we took off the old carpeting and vinyl flooring, it turned out that the subfloor is made from concrete. Under the subfloor is wood (it's a wooden construction).

This is in all rooms including previously carpeted areas and including the bathroom. It's also cracked in many places given the age.

Can someone tell me what is the purpose of this concrete? It seems to me like it would add unnecessary weight to a wooden structure.

Photo of concrete in living room and bathroom

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    This is/was very common to "float" floors when tile or some other "solid" floor covering is/was planned. The two pictures you posted appear to be a bathroom (floor drain in picture) and a dining area/breakfast nook/kitchen or some similar type space, all of which would be quite likely to have a tile floor. To see it "throughout" would be unusual, as bedrooms and common living areas are often carpeted, but not too strange. – Jimmy Fix-it Jan 6 '17 at 8:31
  • Thanks! Yeah this is throughout the unit (even in the pretty large surface area that was carpeted, e.g. bedroom/living room.) Sounds like I'll just repair it and let it be. – user7374453 Jan 6 '17 at 10:58
  • @jimmy Fix-it I agree with your comment but it should be an answer.+ – Ed Beal Jan 6 '17 at 14:12
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That type of subflooring, often built with "gyp-crete", a self-leveling gypsum slurry that's poured into place, is used as a sound-deadening measure. Its primary function is to dampen foot traffic and other sounds of living for the benefit of residents below. It's a common practice in budget-built hotels, apartments, etc. where pre-cast or other concrete floors aren't used.

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There are several concrete-like substances which actually weigh a fair bit less than concrete, and even less than an alternative substrate in some cases. They are used where the easy fitment and easy leveling of concrete is desired, but not the weight.

I have even seen it used in floors of subway cars, where construction is burly yet weight is still a consideration. (The electric bill being proportional to the weight of the train).

In an apartment, of course, you have the most appreciated benefit of sound deadening between tenants, which from a purely mercenay PoV, means the landlord is less often thrust into tenant-tenant disputes.

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