Glossary of Terms – Subfloor

Term Definition
ACI American Concrete Institute – a trade organization of the concrete industry.
Refers to the use of a mixture of muriatic acid and water on concrete either to
neutralize the surface if it shows signs of alkali or to open the surface to allow
a good bond with adhesives or powder underlayments. This can actually cause
problems in bonding due to acid residue.
Adhesive A substance which is capable of holding materials together by surface attachment.
Adhesive is also called cement, glue, mastic, and paste.
Aggregate   Hard, inert material used in concrete. Fine aggregate is 1/4″ or less  in diameter and
consists of sand. Coarse aggregate is 1/4″ up to 1-1/2″ in diameter and usually consists
of crushed gravel.
Contains tiny air bubbles formed by adding soaplike resinous or fatty materials to
the cement or to the concrete when mixed.  Bubbles give the water in the concrete
enough room to expand as it freezes.
A soluble mineral salt (obtained from the ashes of plants and consisting largely
of potassium or sodium carbonate) present in some soil and natural water. A by-product
of portland cement.
Salts that are carried to the surface of concrete, normally by water coming up from
the ground below. These may cause installation failure by destroying the adhesive bond.
Salts can be deposited on the flooring by being carried up through joints. In dry concrete,
normal alkalinity on the pH scale is 9 or less.  When alkali readings on a slab are above 9,
alkali can cause problems with a flooring installation. There is no guarantee any treatment
will keep the surface free of alkali, but washing the surface with clear water or soda water
will lower the alkalinity.
Wood underlayments approved by the standards of the APA – the engineered Wood Association.
Below-grade   A below-grade floor is partially or completely below the surrounding grade level in direct
contact with ground or over a fill in direct contact with ground.
Bond The adherence of one material to another.
Butted Positioned edge to edge, such as butting tile or underlayment  boards.
Cement Usually refers to a portland or hydraulic cement.
Cementitious Having the properties of cement, being made of cement and other binders such as fly ash,
pozzalons or gypsum.
The ability of a material, such as concrete to withstand loads. Compressive strength is measured
in pounds per square inch (PSI).
If the compressive strength is 3500 psi, it means the subject material will withstand a load up to
3500 pounds per square inch without breaking down.
Concrete A mixture of Portland cement, water, fine aggregate and coarse aggregate. The concrete is bound
together by the Portland cement and water paste which surrounds the aggregated and fills all
spaces between particles.
Occur whenever concrete work is concluded for the day. They separate  areas of concrete placed
at different times. In slabs on grade, construction joints usually align with and function as control
or isolation joints.
An adhesive applied to both surface to be bonded and is allowed to dry to the touch. It bonds to
itself instantaneously on contact. Since this type of adhesive does not remain tacky, it must not be
allowed to dry.  
Control or
Joints grooved, formed or sawed into slabs so that cracking will occur in these joints rather than
in a random manner. They extend to a depth of 1/4 the concrete thickness.
Coverage   To overlay or spread with something usually measured in square feet or square yards.
Crazing Occurrence of many fine cracks in the surface of newly harden concrete.
Curing Process of keeping concrete moist for an extended period of time.  Necessary to ensure proper
hydration and for strength and quality.

In order for concrete to achieve its optimum strength and quality, the water needed for complete
hydration of the cement must be retained in the slab until hydration is complete, which is usually
28 days. This process is called curing. If the concrete surface dries by evaporation before it cures
by hydration, cracks can form and the surface will become weak, friable, and dusty.
The American Concrete Institute (ACI) recommends that if a slab is to be covered with resilient flooring
or any other floor finish, curing should be achieved by keeping the surface damp with wet straw, sand,
burlap, etc. or by sealing with plastic film or waterproof paper. However, it has become common
practice today to use curing compounds to seal in the water.  

There are three methods for curing concrete. They are: maintaining the presence of mixing water
in the concrete during the early hardening period. This includes ponding or immersion, spraying
or fogging, or wet coverings such as cotton mats, rugs, and burlap. Methods that prevent the loss
of mixing water from the concrete by sealing the surface. This includes impervious paper or plastic
sheets and a liquid membrane-forming curing compound. Methods that accelerate strength gain
by supplying heat and additional moisture to the concrete. This includes live steam, heating coils,
or electrically heated forms or pads.
Cutback Refers to asphalt adhesives which have been liquefied with solvents.  When the lighter factions
are boiled away from petroleum oil the thick residue left is asphalt. To make it fluid again, solvent
is added or the asphalt is ‘cut back’.
Deflection A variation in the position or shapes of a structure or structural element due to the effects of loads
or volume change; usually measured as a linear deviation from an established plan rather than an
angular variation.
Dry to
the Touch
Dry to the touch is when you place your fingertips lightly on the adhesive ridges, no adhesive
transfers to them.
Dusting Appearance of powdery material on the surface of newly harden concrete. Sometimes caused by
allowing the surface to dry too rapidly without curing.
Separations between adjoining parts of a concrete slab allowing separate movement of the parts.
They are usually filled with an elastomeric type of material. Expansion joints should never be filled
with a cementitious underlayment product because any movement of the separate parts may
cause the underlayment to break up and be pushed out of the joint. 
Exposure 1 Panels which have a fully waterproof bond between plies and are designed for applications
where construction delays may be expected prior to providing protection for the panels.
Exterior Panels which have a fully waterproof bond between plies and are designed for applications which
are subject to permanent exposure to moisture.
A technique which reduces a patch or underlayment material to the subfloor level.
Fill Sand, Gravel, or dirt used to bring subgrade up to desire level.
Freeze-thaw stable adhesives are not as prone to the cold weather damage as other adhesives but
they must still be treated with care. Most freeze-thaw stable adhesives can be thawed and used if
the temperature has not dropped below 10 degrees F.
Full Spread
Spreading the adhesive over the entire substrate before placing the flooring.
When used in reference to plywood underlayment, it means the boards have been sanded smooth
during manufacture so they meet the requirements of their intended use – which is a smooth
surface for the installation of flooring.
Flake Board See particleboard.
Grade The level of the subfloor in relation to the surrounding ground.
Concrete which is fairly new and has not had a chance to completely cure and/or dry.
Chiefly used in cold weather. Made with high-early-strength Portland cement and hardens
more quickly than ordinary concrete.
Hydration The chemical reaction between water and Hydraulic or Portland cement, which causes the concrete
to attain its ultimate compressive strength.
Pressure which forces water up through a below-grade slab, generally causing installation
problems due to moisture. This occurs when the water table is higher than the slab.
Hydrostatic pressure is caused by the weight of the water pressing down on itself.
IIC is the measurement of how well a product resists the direct transfer of an impact, over a
wide frequency range, from an elevated floor to the room below.
Joints   The junction of precut surfaces butted together, such as tiles or underlayment boards.
Can be made two ways; may use lightweight aggregate such as shale, clays, pumice, etc. or may add
chemicals that foam and produce air spaces in the concrete as it hardens.
Milk   Refers to latex liquid used to prime dusty substrates or mix with an underlayment powder.
Monolithic Placed in one continuous pour without construction joints.
On-grade An on-grade floor is in direct contact with ground or over a fill in direct contact with ground.  A slab on
ground level is an example.
Amount of time recommended for the adhesive to set before it is covered with the flooring. Open
time is affected by temperature, humidity, and porosity of the substrate.
Compounds used on wood or steel formwork for concrete to make it easier to remove the cured
concrete from the form. Parting agents are normally silicone or paraffin waxes and must be removed
before installation of flooring. Also known as bond-breakers.
PCA   Portland Cement Association – a trade organization of the concrete industry.
Panel products designed and engineered to meet performance criteria for specific end-use applications.
Plywood A term used to determine the number of thicknesses of wood veneer for plywood underlayment when
laminated together. A five-ply thickness of plywood has five separate layers of wood, glued or laminated
together, which each layer lying at right angles to the graining of the wood layer beneath or above it.
This cross lamination allows for added dimensional stability of the panels.
Porosity   A matter which is porous or contains pores which are able to absorb liquid. Subfloors, which are porous,
are normally concrete and wood.  If there is any doubt as to the porosity of a subfloor, put a few drops
of water on the surface. If the water is quickly absorbed, the surface is porous. If the water remains beaded
on the surface, the surface is nonporous.
The amount of time a product, which must be mixed, remains workable in the original mixing container.
Generally applicable to two-part epoxy adhesives and powder underlayments.
A finely pulverized material used in the making of concrete. When mixed with water, it causes hydration
to occur. Named for its color – like the Isle of Port off the English coast.
Usually made by casting concrete around steel cables stretched by hydraulic jacks; after concrete hardens,
jacks are released and cables compress concrete. Concrete is strongest when compressed while steel is
strong when stretched or in tension.
A subfloor which also serves as a means to heat an area. Generally, heating coils, pipes or ducts are built
into the subfloor.
Made by casting concrete around steel rods or bars.
Panels where the faces are sanded smooth or touch sanded during manufacture in order to meet
the requirements of their intended use.
Scaling Breaking away of hardened concrete surface about 1/16″ to 3/16″ deep; usually happens at an early age
of concrete.
Scarify A mechanical means of roughing a surface to obtain a better bond.
The period of time which a manufacturer guarantees the unopened adhesive will be usable.
The date of manufacture is normally stamped somewhere on the adhesive container.
Slab A layer of monothlic material which serves as a substrate for finished flooring to be installed.
Consists of wood subfloor installed over or on an existing concrete subfloor on or below-grade without
18″ of well ventilated air space.
Slump A measure of the consistency of concrete in inches. The distance the concrete slumps from its original
12″ molded form.
A conical mold is filled in three layers with the concrete. After each layer, the concrete is puddled with
25 strokes of a rod. Concrete is evened off at the top of the mold and the mold removed. The slump is
the space between a rod laid across the top of the mold and the molded concrete. ASTM C 143.
Usually concrete which has a pitted, marred, often dusty, uneven face, which must be leveled with a
topping layer of concrete or with a mastic underlayment or other type of underlayment prior to the
installation of floors.
The amount of coverage which can be expected from a given amount of adhesive when spread using
the recommended trowel.
STC   Sound Transmission Class (STC) is the rating of airborne sound transmission. The STC of floor/ceiling
(or wall) structure is a measure of the decibel difference between the airborne sound energy striking
one side of the structure and the sound energy radiated into a receiving room on the other side.
Typical floor/ceiling structure STC values range from 25 to 35 for lightweight single family residential
construction to upwards of 50 to 60 for commercial construction.
STURD-I-FLOOR An APA performance rated panel specially designed as combination subfloor/underlayment.
Subfloor A subfloor is selected for structural purposes and is the substrate (supporting layer) for the underlayment.
A surface that must meet structural requirements and have a smooth surface suitable for the floor covering.
Substrate The smooth surface prepared to accept the floor covering, such as concrete, underlayment or
existing resilient floor covering.
Suspended A suspended floor is one with a minimum of 18″ of well-ventilated air space below. Also referred to as
above grade.
Underlayment The smooth surface used as the substrate for the floor covering.
Trowel A hand tool with notches used for spreading adhesives onto the substrate. Trowels are recommended with
notches that leave adhesive ridges of a size which will ensure complete contact with the flooring being installed.
TSP Tri Sodium Phosphate commonly used to remove surface contaminates from substrates.
Any material used to stop the migration of vapor through walls, floors or ceilings.
Veneer Thin sheets of wood which plywood is made. Also refereed to as “plies” in the glued panel.
Ratio by weight between the water and the cement. Only a small amount of water is needed for hydration
and the rest is used to make the concrete more workable. The water/cement ratio controls the characteristics
of the paste and ultimately the concrete. Allowance must be made for water in the aggregate when adding
water to the concrete batch.
Updated on June 16, 2022