Term Description
Deformation

Any change of form, shape or dimensions produced in a body by a stress or force, without a breach of the continuity of its part.
Degradation

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

Separation of the laminated layers of a component or system.
Dew point temperature

The temperature at which air becomes saturated with water vapor; the temperature at which air has a relative humidity of 100 percent.

DFT

See "dry film thickness."
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.
Differential movement

In roofing and waterproofing, dimensional changes in dissimilar interfacing materials characterized by different and incompatible rates of change of dimensions, such as membranes and flashing materials, resulting from a temperature change or change in loading. See "thermal movement."
Diffusion

(1) The movement of water vapor from regions of high concentration (high water vapor pressure) toward regions of lower concentration; (2) spreading of a constituent in a gas, liquid or solid tending to make the composition of all parts uniform; (3) the spontaneous movement of atoms or molecules to new sites within a material.
Dimensional stability

The degree to which a material maintains its original dimensions when subjected to changes in temperature and humidity.
DOE

U.S. Department of Energy.
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 wider than the exposed portion, resulting in two layers of roofing material over the deck.
Double tees

Structural precast, prestressed concrete members of characteristic cross-section used as roof and floor deck systems.
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."
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; also called a conductor or leader.
Drag load

The external force—for example, 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 or waterproofing area. See ‚"primary drainage‚" and ‚"overflow drainage.‚"
Drainage course

A separate layer of material that provides a location for moisture to move laterally through a protected-membrane or vegetative roof system. A drainage course relieves hydrostatic pressure from a material's surface and the associated weight of water.
Drainage mat

A fabric composite or a nondegradable plastic configured to allow drainage of water, typically with adhered filter fabric to prevent growth medium and fines from blocking the drainage path.
Drainage system

Prefabricated materials that facilitate the drainage of water away from the structure.
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

(1) Free or relatively free from a liquid, especially water; (2) to remove water or moisture.
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-slope roofing. In low-slope roofing, it is usually called a temporary roof.

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