Knee Cap: Sheet metal trim that fits over a panel rib after it has been cut and bent.

Knot: An imperfection or non-homogeneity in materials used in fabric construction, the presence of which causes surface irregularities.


Laitance: An accumulation of fine, powdery aggregate particles on fresh cement caused by the upward movement of water; indicates that too much water was used in the mix resulting in poor surface adhesion for a waterproofing layer.

Laminated Shingles: See "Dimensional Shingles" or "Architectural Shingles".

Lap: The area where adjacent roof components overlap eachother.

Lap Cement: Asphalt-based roof cement used to adhere overlapping plies of asphalt roll roofing.

Lap Seam: Where two material that overlap are sealed together.

Leader Head: A component used to direct water from a scupper or gutter to a downspout. Also known as a Conductor Head.

Leader Pipe: A conduit for carrying water from a gutter, scupper, drop outlet or other drainage unit from roof to ground level. Also known as Downspout.

Live Loads: Temporary items on a roof such as equipment, people, snow, etc., which the roof must be designed to support.

Loose-Laid Roof Membranes: Roofing material attached only at the perimeter and at penetrations and held in place by ballast, pavers, or other materials.


Mansard: (1) A steep-sloped roof section located at the perimeter of a building and usually used for decorative purposes. (2) The upper story formed by the lower slope of a mansard roof.

Mansard Roof: A steeper roof that terminates into a lower sloped roof at its high point.

Masonry: Refers to bricks, concrete, or concrete blocks.

Mastic: See "Asphalt Roof Cement".

Mat: A thin layer of woven, non-woven, or knitted fiber used to reinforce a material.

Material Safety Data Sheets (MSDS): Written descriptions of the chemicals in a product provided by the product’s manufacturer. MSDS also contain other information such as emergency procedures and safe handling.

Mechanical Damage: Damage to a roof by means of items puncturing or otherwise unnecessarily penetrating the roof system or any of its components. Screws or nails stuck in the roof and heel marks along base flashings are examples of mechanical damage.

Mechanical Fasteners: Devices such as screws, plates, battens, nails, or other materials that are used to secure roofing materials.

Membrane: The portion of the roofing system that serves as the waterproofing material. Can be composed of one material or several materials laminated together.

Metal Flashing: Roof components made from sheet metal that are used to terminate the roof membrane or material along roof edges and at roof penetrations.

Meter: Metric unit of length measurement equal to 39.37 inches.

Mil: A unit of measure equal to 0.001 inches (1/1000 in.). Sometimes used to indicate the thickness of a roofing membrane.

Mildew: A superficial coating or discoloration of organic materials caused by fungi, especially under damp conditions.

Millimeter: Metric unit of measure equal to one thousandth (0.001) of a meter, or 0.03937 inches.

Mineral Fiber: Inorganic fibers of glass or rock.

Mineral Granules: See "Granules".

Mineral-Surfaced Roofing: Roofing materials with a top surface consisting of mineral granules.

Miter: The joint made by two diagonally cut pieces put together.

Model Codes: A group of codes and standards accepted by more than one of the Building Code regulatory agencies such as SBCCI, BOCA, and ICBO

Modified Bitumen: A bitumen modified by one or more polymers such as Atactic Polypropylene (APP), styrene butadiene styrene (SBS).

Moisture Relief Vent: A vent installed through the roofing membrane to relieve moisture vapor pressure that has been trapped within the roofing system.

Moisture Scan: A survey of a roof specifically to detect the amount of moisture present in the roof system. Devices used in moisture surveys can be electrical capacitance meters, infrared cameras or nuclear scanners.

Monolithic: Used to describe something without seams; formed from a single material.

Monomer: A simple molecule that can combine with other to form a polymer.

Mop-and-Flop: A roofers’ term where the back side of a roofing material is “mopped” with hot bitumen. The material is then “flopped” over and set in place.

Mopping: To apply hot asphalt or coat tar using a hand mop or mechanical applicator.

Mud Cracking: Surface cracking of a material that looks similar to dried, cracked mud.

Multiple Coats: More than one layer of coating applied to a substrate.


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