Page 4 - 2018-03-CFR Volume 114 - Old Versus New Concrete Slabs - Staying Out Of Trouble With Both - March 2018

The slab may also be affected by grading that is March
pitched toward the building or drainage that intrudes
at the perimeters of the slab keeping the concrete
perpetually hydrated. If old slabs are covered by a
new non permeable flooring material such as any
sheet vinyl, rubber or linoleum, moisture vapor, which
may not have affected the old flooring can spoil an
otherwise good new installation. Thinking that mois-
ture is not an issue with an old slab on grade is play-
ing with fire. The chances that moisture is present
are bankable and anyone who thinks otherwise
doesn’t understand the dynamics at work with an old
slab on grade. Just because the old flooring stayed
in place unaffected by the moisture that was always
present doesn’t mean it won’t affect the new stuff.
Once you cap the surface with what essentially is a
topical moisture vapor retarder, that being the new
flooring, the chances of it being adversely affected
are very likely inevitable. This would be the same for
using a topical sheet moisture vapor retarding mem-
brane. There’s always a gamble when installing
flooring on an old slab and you may not see the fail-
ure for several months or even years.

Hill and Grading Towards Building
We are currently involved in a sheet flooring failure
that was installed in a new building over lightweight
concrete in 2012 that didn’t start to fail until 2016 –
four years after installation. This floor is sitting on a
simmering slab.
On an old slab it’s good to know or find out what
flooring was down before, how it was removed and
who and when the moisture testing was done, if it
was at all. This information will help you understand
the conditions you face that may challenge or even
prevent you from commencing your installation. With
old slabs always assume, and I hate saying that be-
cause you should never assume anything with floor-
ing but in this case go with it – always assume an old
slab has no vapor retarder or an substandard one.

4 Commercial Flooring Report