Daily standard count—The counts per minute with the device resting on a calibrated reference standard. Daily refers to historical need to calibrate older instruments each day.

Damper—A baffle used to open or close the throat of ventilators.
Dampproofing—Treatment of a surface or structure to resist the passage of water in the absence of hydrostatic pressure.

Dead level—Absolutely horizontal, or zero slope (See also Slope).

Dead level asphalt—A roofing asphalt conforming to the requirements of Specification D 312, Type 1.

Dead load—The dead load of a building is the weight of all permanent construction, such as floor, roof, framing, and covering members.

Deck—The structural surface to which the roofing or waterproofing system (including insulation) is applied.

Deflection—The displacement of a structural member or system underload.

Deflection—Crosshead movement after the loading plates con tact the specimen. It is expressed in millimeters or inches. (ASTM D-1621)

Deflection—The change in mid-span position of a test specimen during a creep test.

Degradation—A deleterious change in the chemical structure, physical properties, or appearance of a plastic, rubber or other material due to heat, ozone, oxygen, etc.

Degree days—The difference between a reference temperature (usually 65°) and the mean temperature for the day times 24 hours times the number of days in the period. Degree days is used to compare the severity of cold or heat during the heating or cooling season.

Delamination—Separation of the plies in a membrane or separation of insulation layers after lamination.

Denier—A unit used in the textile industry to indicate the fineness of continuous filaments. Fineness in deniers equals the mass in grams of 9,000 meter length of the filament.

Depth of measurement—The maximum thickness of a roof system upon which a given survey method is effective.

Design loads—The "live load" (i.e., superimposed loads) that a structure is designed to resist (with appropriate safety factor) plus "dead load" (i.e., weight of permanent loads).

Detector—A device which generates electrical signals based upon the intensity of some phenomenon such as neutrons or light waves striking it.

Dew point—The temperature at which water vapor starts to condense in cooling air at the existing atmospheric pressure and vapor content.

Dewar vessel—A thermos-type container used for storing very low temperature (cryogenic) liquids.

Diaphragm action—The resistance to racking generally offered by the covering system, fasteners, and secondary framing.

Dielectric constant—A number which defines the relative efficiency of a dielectric material for passing lines of electric force compared to that of vacuum. Number will be greater than one.

Dielectric seaming—See Heat seaming.

Differential price escalation rate—The expected percent difference between the rate of increase assumed for a given item of cost (such as energy), and the general rate of inflation.

Diffusion—The material permeation of two or more substances due to the kinetic activity of their molecules, so that a uniform mixture or solution results. Diffusion occurs with all forms of matter; it is more rapid for gases, somewhat slower for liquids and for solids in solution.

Discounting—A technique for converting cash flows that occur over time to equivalent amounts at a common time.

Discount rate—The rate of interest reflecting the investor's time value of money, used to determine discount factors for converting benefits and costs occurring at different times to a base time.

DISCUSSION—The discount rate may be expressed as nominal or real.

Discount factor—A multiplicative number (calculated from a discount formula for a given discount rate and interest period) that is used to convert costs and benefits occurring at different times to a common time, usually the present.

Double pour—Doubling of flood-coat, graveling-in operation, to provide additional waterproofing integrity for a BUR membrane.

Downspout—A conduit used to carry water from the gutter of a building to the ground or storm drain.

Drift pin—A tapered pin used during erection to align holes in steel members to be connected by bolting.

Dry (n.)—A material that contains no more water than one would find at its equilibrium moisture content.

Duck-board—A boardwalk or slatted flooring laid on a wet, muddy or cold surface.