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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 Slope.)

Dead loads
the weight of a structure itself, including the weight of fixtures or equipment permanently attached to it.

Dead-level asphalt
see Asphalt.

Deck
a structural component of the roof of a building. The deck must be capable of safely supporting the design dead and live loads, including the weight of the roof systems, and the additional live loads required by the governing building codes and provide the substrate to which the roofing or waterproofing system is applied the structural surface of a building to which a roof assembly is installed. Decks are either non-combustible (e.g., corrugated metal, concrete, or gypsum) or combustible (e.g., wood plank or plywood).

Deflection (bowing, sagging)
(1) the deformation of a structural member as a result of loads acting on it; (2) any displacement in a body from its static position, or from an established direction or plane, as a result of forces acting on the body.

Degradation
a deleterious change in the chemical structure, physical properties or appearance of a material from natural or artifical exposure (e.g., exposure to radiation, moisture, heat, freezing, wind, ozone, oxygen, etc.)

Degree days
a unit used in estimating the fuel consumption for a building; equal to the number of degrees that the mean temperature, for a 24-hour day, is below the "base temperature"; the base temperature is taken as 65 degrees F (18.3 C) in the U.S.A.

Delamination
separation of the laminated layers of a component or system.

Design loads
the total load on a structural system for the most severe combination of loads and forces which it is designed to sustain.

Dew-point temperature
the temperature at which air becomes saturated with water vapor. The temperature at which air has a relative humidity of 100%.

Diaphragm
a floor slab, metal wall panel, roof panel, or the like, having a sufficiently large in-plane shear stiffness and sufficient strength to transmit horizontal forces to resisting systems.

Diffusion
the movement of water vapor from regions of high concentration (high water vapor pressure) toward regions of lower concentration.

Dimensional shingle
a shingle that is textured, overlayed, or laminated and designed to produce a three-dimensional effect. (also see Laminated shingle and Architectural shingles.)

Dimensional stability
the degree to which a material maintains its original dimensions when subjected to changes in temperature and humidity.

DOE
Department of Energy.

Dome
a roof that is shaped like a half-circle, or a variation of one.

Dormer
a structure projecting from a sloping roof usually housing a window or ventilating louver.

Double coverage
application of asphalt, slate, or wood roofing such that the lapped portion is at least 2 inches (50 mm) wider than the exposed portion, resulting in two layers of roofing material over the deck.

Double Graveling
the process of applying two layers or flood coats of bitumen and aggregate to a built-up roof. Loose aggregate should be swept from the first application prior to the second coating of bitumen and aggregate. Approximately 50% of the second aggregate application will remain adhered in the bitumen flood coat unless physically removed.

Double lock standing seam
in a metal roof panel or metal cap, a standing seam that uses a double overlapping interlock between two metal panels. (see Standing seam.)

Double pour
to apply two layers or flood coats of bitumen and aggregate to a built-up roof.

Downspout
a vertical pipe or conduit used to carry runoff water from a scupper, conductor head or gutter of a building to a lower roof level or to the ground or storm water runoff system.

Drag load
the external force (e.g., from the weight of ice and snow) applied to a steep-slope roof system component forcing the component downslope.

Drain
an outlet or other device used to collect and direct the flow of runoff water from a roof area.

Drip edge
a metal flashing or other overhanging component with an outward projecting lower edge, intended to control the direction of dripping water and help protect underlying building components.

Dry bulb temperature
the temperature of air as measured by an ordinary thermometer.

Dry film thickness
the thickness, expressed in mils, of an applied and cured coating or mastic. For comparison, see Wet film thickness.

Dry-In or Dry-In Felt
usually the underlayment or the process of applying the underlayment for steep roofing.

Dry
free or relatively free from a liquid, especially water; (2) to remove water or moisture.

Drying time
the time required for the loss of volatile components so that the material will no longer be adversely affected by weather conditions such as dew, rain, or freezing.

Dual level drain
in waterproofing, an outlet or other device with provisions for drainage at both the wearing surface level and the waterproofing membrane levels used to collect and direct the flow of runoff water from a horizontal slab.

Durability
the ability to withstand physical, chemical, or environmental abuse.

Dust Free
a surface is considered dust free when a finger can be lightly run over the surface without picking up any dirt, dust, or chalk on the finger.

Dynamic load
any load which is nonstatic, such as a wind load or a moving live load.