How dry is your basement floor?

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How dry is your basement floor?

Strelka, C. S.

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R e f S e r THL N2Lb8 no. 10 BLDG

HOW

DRY IS

YOUR

MSEMENT F m R ?

by

C.S. Strelka

Division of

Building Research, National Research

Council

of Canada

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HUW

DRY

IS YOUR BASEMENT FLOOR? by

C.S.

Stralka

Finishing the basement in one's own house, and especially the installation

of

flooring, seems like a simple, worthwhile,

"do-it-yourself" project. Few people are aware, however, of a p o t e n t i a l hazard t h a t might frustrate t h e i r efforts: e x c e s s i v e moisture in the basement concrete floor. A floor t h a t appears d r y by casual examination may contain s u f f i c i e n t moisture t o cause trouble when it is under

f l o o r i n g , such as tile or carpet

with

foam

cushion

backing. THE PROBLEM

Most flooring materials are attached t o concrete by means of an adhesive. The f l a o ~ i n g and a properly applied, continuous adhesive layer provide an effective seal against any evaporation o f moisture from t h e concrete i n t o the air. Moisture penetrating into the concrete s l a b from below can reduce the bond between adhesive and concrete. Flooring tiles can then lift and peel off; carpets may bulge in places.

Sometimes this is accompanied by the formation of

a

white

powder deposit, usually first evident a t the j o i n t s of a t i l e floor. This can happen at any time, but usually begins a few weeks after the flooring has been installed. The white powder is an alkaline salt in the soil water which is carried to the surface of

the

concrete and appears as a deposit when the water evaporates. Salt can also accumulate on the concrete surface

in

sufficient quantity to affect t h e bond of the flooring adhesive and cause t h e deterioration of the flooring itself.

TESTING FOR DAMPNESS

There are s e v e r a l simple tests that can be carried o u t _to_check _the moisture content o f a concrete floor:

I . Place a rubber m a t or heavy polyethylene sheet, at least 1 rn by 1 m,

on the floor, preferably with the edges sealed t o t h e f l o o r with masking tape. If, after at least 24 hours, a damp spot i s found under the mat or polyethylene, there is excessive moisture in the

concrete and the installation of flooring will probably n o t be

successful. This test should be repeated at several locations including areas close to walls and in the centre of the floor.

2. Apply patches of tile cement, approximately 50 mm by 5 0 mm, i n several spots close to the walls and

in

the centre of the f l o o r . After the

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patches have been d r y for at least 24 hours, adhesion to the concrete shauld be checked by trying t o remove the

adhesive with

a putty

knife.

I f it can be scraped or simply peeled

off

without leaving substantial remnants an the concrete floor,

then

enough moisture is present to prevent proper application.

3. A more sophisticated test requires a piece

of

plate glass (approximately 100 mm by 100 nun) or a watch glass about 87 mm in diameter, ordinary putty and anhydrous calcium chloride c~ystals [available at any laborarory or building materials supply

hause),

a)

The

test areas should be close to the walls and in t h e centre of the

floor.

After these have been selected, mark

the

outline of

the

glass on the floor w i t h a pencil.

b) Spread

a

narrow bead of putty on the floor to fom a bed

for

t h e edge of

the

glass.

c ) Place 8 to 10 crystals of calcium

chloride

on a

piece of

ordinary wax paper on t h e floor in the centre of the t e s t spot and

immediately cover with t h e

glass, which must

be

pressed

firmly into rZle prepared bead of putty,

d) F i n i s h the! seal with

an

additional bead of putty and smooth it down around the edges.

e )

The

test sample should be l e f t undisturbed for at least 48 hours.

If

the

calcium chloride crystals

axe

completely dissolved after

this

period

or

sooner,

the

moisture c a d i t i o n is considered

unsatisfactory for installation of flooring. If the edges

of

the crystals become slightly rotmded but otherwise remain unchanged, some moisture is present but not in sufficient quantity to be a prob 1

em.

DISCUSSION

Even

if

any of these t e s t s indicate the presence of excessive

moisture in

the concrete floor s l a b ,

the

project need

not be

abandoned. Moisture

content in

concrete can change from time t o t i m e acca~ding to the weather and s o i l

conditions

and may only be a temporary conditicm. Extreme rainfall, high a i r humidity, due to porn ventilation

of

a

basement, can cause an increase of moisture

in

the concrete floor.

There are several ways to reduce the moisture content to a suitable level. h e of the simplest and s.ametimes the most e'ffective is to improve drainage around t h e house. This can he done by sloping the g r m d surface away from the building and installing eaves tmughing with downspouts that carry roof runoff away

from

the foundation. A basement can also be dried out by opening all basement windows and ventilating the basement during p e ~ i o d s of good drying weather, Sometimes it proves more

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satisfactory t o f i n i s h a basement

in

winter when both the moisture level in t h e ground and humidity of the air are at a low level.

If these measures prove successful and tests indicate that the concrete is free of

moisture,

flooring

can

be installed, If however, the tests show that the concrete contains excessive moisture the cause of the excessive dampness r i l l have to be determined. Professional engineering advice may be required. The following publications of the Division of Building Research may

provide

some guidance for the control

of moisture in basements:

Ball, W.H. The control o f

moisture

i n basements. Nat. Res. Council, Div. Bldg. Res., Building Res. Note No. 27, 1956.

Crocker, C.R, Moisture and thermal considerations

in

basement walls. Nat. Res. Cauncil, Div. Bfdg.

Res.,

Canadian Building Digest No. 161. 1974.

Williams, G.P. Drainage

around

buildings. Nat. Res. Council, Div. Bldg. Res., Canadian Building Digest No- 156. 1973.