BASIC ROOFING CONCEPTS

From: The Roofing Industry Educational Institute, Synopsis: Introduction to Roofing

From the time mankind left cave, the first successful roofs were those that shed water.  Sloped roofs could be fabricated out of an readily available raw material including: thatch, bark, sod, animal skins, wood shingles and shakes, slate and soft metals such as lead and copper.  Later on, asphalt, aluminum and concrete shingles or tilles were also used, as well as steel and aluminum roof panels.

Membrane roofing is a relatively recent development.  It requires the use of materials that can be sealed together to resist water pressure.  Bituminous materials which were inherently resistant to water and easy to stick together were introduced in the 1800's and have now been joined by plastics, elastomerics, polymer modified bitumens, low slope metal roofing, and sprayed-in-place polyurethane foam systems.

In a steep roof system, rain-water cascades down the slope, with gravity pulling water away from the head lap.

Even though the shingles are not sealed together and have holes punched through by the roofing nails, the water- shedding feature prevents leakage.

On lower-slopes, this feature is inadequate and waterproof underlayments are necessary.

When slope is inadequate, a sealed membrane must be used.  Must be continuous "skin" with watertight "flashing"

Some examples include Built-Up Roofing, Single Ply, Modified Bitumen, Sprayed Foam, Structural Metal Panels with suitable sealant, Liquid Applied Waterproofing.

Besides keeping the water out of a building, a roof system must:

1. Protect the structure and occupants from fire hazard

2. Be practical to install

3. Be affordable and cost effective

4. May need to be aesthetically pleasing

5. Provide human comfort

6. Resist the flow of air or water vapor

7. Keep water out

8. Resist not only weather elements but rooftop traffic as well

The roofing business is a big business!

The roofing market has attracted major industries, competing for their market share.  It's not unusual to see total different materials considered for a roofing project.

Built-Up Roofing - assembled in place from alternating layers of bitumen (asphalt or coal-tar) and reinforcements

Polymer Modified - mixtures of polymers and bitumen, with coated fabrics produced in the factory and field-joined together.

Polymeric Sheets - which are factory premanufactured, requiring relatively narrow seams to be field joined.

Spray-in-place Foam - chemicals are field-mixed, expanding and polymerizing in place.  Followed with surfacings, usually of the liquid polymeric type.

Metal Panels - may sometimes be used on slopes as low as 1/4" per ft (1:48)

A roof system is subjected to many elements.  Combinations of membranes and surfacing materials are frequently synergistic with the surfacing screening out ultraviolet energy and lowering surface temperature.

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ROOFING SYSTEMS GUIDE

This section contains photographs of different roofs in order to give you a better understanding of the many different types of roof systems that exist.  If you have any questions about these different types, click the Quick Facts link beside each sub-heading.  You may also view a larger version of the pictures below by clicking on the picture you wish to see.

Built-Up Roofing Systems  -    Quick Facts

Modified Bitumen Roofing Systems    -    Quick Facts

Single Ply Roofing Systems  -   Quick Facts

Shingle Roofing Systems  -    Quick Facts

Slate Roofing Systems    -    Quick Facts

Metal Roofing Systems    -    Quick Facts

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ROOFING TERMINOLOGY

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Aggregate - (1) Crushed stone, crushed slag or water worn gravel used for surfacing a built-up roof; (2) Any granular mineral material.

Alligatoring - the cracking of the surfacing bitumen on a built-up roof, producing a pattern of cracks similar to an alligator's hide; the cracks may or may not extend through the surfacing bitumen.

Application Rate - The quantity (mass, volume, or thickness) of material applied per unit area.

Area Divider - a raised, double wood member attached to a properly flashed wood base plate that is anchored to the roof deck.  It is used to relieve thermal stresses in a roof system where no expansion joints have been provided.

Asbestos - a group of natural, fibrous, impure silicate materials.

Asphalt - a dark brown to black cementitious material in which the predominating constituents are bitumens, which occur in nature or are obtained in petroleum processing.

Asphalt Felt - an asphalt-saturated felt or an asphalt coated felt.

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B

Backnailing - the practice of blind-nailing roofing felts to a substrate in addition to hot-mopping to prevent slippage

Blind Nailing - The practice of nailing the back portion of a roofing ply in a manner that the fasteners are not exposed to the weather in the finished product.

Blister - An enclosed pocket of air mixed with water or solvent vapor, trapped between impermeable layers of felt, or between the felt and substrate.

Blocking - Wood built into a roofing system above the deck and below the membrane and flashing to stiffen the deck around an opening, act as a stop for insulation, or to serve as a nailer for attachment of the membrane or flashing.

Built-Up Roof Membrane - a continuous, semi-flexible roof membrane assembly, consisting of plies of saturated felts, coated felts, fabrics or mats between which alternate layers of bitumen are applied, generally surfaced with mineral aggregate, bituminous materials, or a granule surfaced roofing sheet.  (Abbreviation: BUR)

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C

Cant Strip - a beveled strip using under flashing to modify the angle at the point where the roofing or waterproofing membrane meets any vertical element.

Cap Flashing - (See Flashing)

Cap Sheet - a granule surfaced coated sheet used as the top ply of a built-up roof membrane or flashing.

Caulking - a composition of vehicle and pigment, used at ambient temperatures for filling joints, that remains plastic for an extended time after application

Coal Tar - a dark brown to black, semi-solid hydrocarbon obtained as residue from the partial evaporation or distillation of coal tar.

Coal Tar Pitch - a coal tar used as the waterproofing agent in dead-level or low slope built-up roof membrane, conforming to ASTM Specification D450, Type I.

Coal-Tar Felts - A felt that has been saturated with refined coal tar.

Cold Process Roofing - a continuous, semi-flexible roof membrane, consisting of plies of felts, mats, or fabrics that are laminated on a roof with alternate layers of cold-applied roof cement and surfaced with a color-applied coating.

Condensation - the conversion of water vapor or other gas to liquid as the temperature drops or the atmospheric pressure rises.

Coping - The covering piece on top of a wall exposed to the weather, usually sloped to shed water.

Counterflashing - formed metal or elastomeric sheeting secured on or into a wall, curb, pipe, rooftop unit or other surface, to cover and protect the upper edge of a base flashing and its associated fasteners.

Course - (1) the term used for each application of material that forms the waterproofing system or the flashing; (2) one layer of a series of materials applied to a surface (i.e. a five-course wall flashing is composed of three applications of mastic with one ply of felt sandwiched between each layer of mastic.

Coverage - the surface area continuously covered by a specific quantity of a particular roofing material.

Crack - a separation or fracture occurring in a roof membrane or roof deck, generally caused by thermal induced stress or substrate movement.

Cricket - a relatively small, elevated area of a roof constructed to divert water around a chimney, curb, or other projection.

Cutback - solvent-thinned bitumen used in cold process roofing adhesives, flashing cements, and roof coatings.

Cutoff - a detail designed to prevent lateral water movement into the insulation where the membrane terminates at the end of a day's work, or used to isolate sections of the roofing system.  It is usually removed before the continuation of work.

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D

Dead Level - absolutely horizontal, or zero slope. (See Slope)

Dead Level Asphalt - (See Asphalt)

Dead Loads - Non-moving rooftop loads, such as mechanical equipment, air conditioning units, and the roof deck itself.

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

Delamination - separation of the plies in a roof membrane system or separation of laminated layers of insulation.

Dew Point - the temperature at which water vapor starts to condense in cooling air at the existing atmospheric pressure and vapor content.

Double Pour - The process of applying two layers of aggregate and bitumen to a built-up roof.

Drain - a device that allows for the flow of water from a roof area.

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E

Edge Stripping - application of felt strips cut to narrower widths than the normal felt roll width to cover a joint between flashing and built-up roofing.

Edge Venting - the practice of providing regularly spaced protected opening along a roof perimeter to relieve moisture vapor pressure.

Elastomeric - a rubber like synthetic polymer that will stretch when pulled and will return quickly to its original shape when released.

Embedment - (1) The Process of pressing a felt, aggregate, fabric, mat, or panel uniformly and completely into hot bitumen or adhesive; (2) the process of pressing granules into coating in the manufacture of factory prepared roofing.

Emulsion - The intimate dispersion of an organic material and water achieved by using a chemical or clay emulsifying agent.

Envelope - a continuous membrane edge seal formed at the perimeter and at penetrations by folding the base sheet or ply over the plies above and securing it to the top of the membrane.  The envelope prevents bitumen seepage from the edge of the membrane.

Equiviscous Temperature (EVT) - the temperature at which the viscosity is 75 centipose for asphalt and 25 centipose for coal tar products; the recommended temperature plus or minus 25 F at the time of application.

Expansion Joint - a structural separation between two building elements that allows free movement between the elements without damage to the roofing or waterproofing system.

Exposure - (1) The traverse dimension of a roofing element not overlapped by an adjacent element in the roof system.  The exposure of any ply in a membrane may be computed by dividing the felt width minus 2 inches by the number of shingled plies; thus, the exposure of 36 inch-wide felt in a shingled, four-ply membrane should be 8 1/2 inches; (2) the time during which a portion of a roofing element is exposed to the weather.

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F

Fabric - a woven cloth of organic or inorganic filaments, threads or yarns.

Factory Mutual (FM) - an organization that classifies roof assemblies for their fire characteristics and wind up-lift resistance for insurance companies in the United States.

Felt - a flexible sheet manufactured by the interlocking of fibers through a combination of mechanical work, moisture and heat.  Felts are manufactured principally from vegetables fibers (organic felts), asbestos fibers (asbestos felts) or glass fibers (glass fiber felts); other fibers may be present in each type.

Fine Mineral Surfacing - Water-insoluble, inorganic material, more than 50 percent of which passes the No. 35 sieve, used on the surface of roofing.

Fishmouth - (1) a half-cylindrical or half conical opening formed by an edge wrinkle; (2) in shingles, a half-conical opening formed at a cut edge.

Flashing - the system used to seal membrane edges at walls, expansion joints, drains, gravel stops, and other places where the membrane is interrupted or terminated.  Base flashing covers the edge of the membrane.  Cap slashing or counter flashing shields the upper edges of the base flashing.

Flashing Cement - a trowelable mixture of cutback bitumen and mineral stabilizers, including asbestos or other organic fibers.

Flood Coat - the top layer of bitumen into which the aggregate is embedded on an aggregate-surfaced built-up roof.

Fluid Applied - an elastomeric material, fluid at ambient temperatures, that dries or cures after application to form a continuous membrane.  Such systems normally do not incorporate reinforcement.

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G

Glass Felt - Glass fibers bonded into a sheet with resin and suitable for impregnation in the manufacture of bituminous waterproofing materials, roof membranes, and shingles.

Glass Mat - a thin mat composed of glass fibers with or without a binder.

Glaze Coat - (1) the top layer of asphalt in a smooth surfaced built-up roof assembly; (2) a thin protective coating of bitumen applied to the lower plies or top ply of a built-up roof membrane when application of additional felts or the flood coat and aggregate surfacing are delayed.

Gravel - Course, granular aggregate, with pieces larger than sand grains, resulting from the natural erosion of rock.

Gravel Stop - a flanged device, frequently metallic, designed to provide a continuous finished edge for roofing material and to prevent loose aggregate from washing off of the roof.

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H

Headlap - The minimum distance, measured at 90 degrees to the eaves along the face of a shingle or felt. from the upper edge of the shingle or felt to the nearest exposed surface.

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I

Ice Dam - a mass of ice formed at the transition from a warm to a cold roof surface, frequently formed by refreezing meltwater at the overhang of a steep roof, causing ice and water to back up under roofing materials.

Inorganic - Being or composed of matter other than hydrocarbons and their derivative, or matter that is not of plant or animal origin.

Insulation - (See Thermal Insulation)

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L

Live Loads - Moving roof installation equipment, wind, snow, ice, or rain.

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M

Membrane - A flexible or semi-flexible roof covering or waterproofing layer, whose primary function is the exclusion of water.

Metal Flashing - (See Flashing) Metal flashing is frequently used as a through-wall flashing, cap flashing, counterflashing or gravel stops.

Mineral Granules - Opaque, natural, or synthetically colored aggregate commonly used to surface cap sheets, granule surfaced sheets, and roofing shingles.

Mineral Surfaced Roofing - Built-up roofing materials whose top ply consists of a granule-surfaced sheet.

Mineral Surfaced Sheet - A felt that is coated on one or both sides with asphalt and surfaced with mineral granules.

Modified Bitumen - Are composite sheets consisting of a copolymer modified bitumen often reinforced and sometimes surfaced with various types of films, foils, and mats.

Mopping - The application of hot bitumen with a mop or mechanical applicator to the substrate or to the felts of a built-up roof membrane.

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N

Neoprene - A synthetic rubber (polychloroprene) used in liquid-applied and sheet-applied elastomeric roof membranes or flashings.

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O

Organic - Being or composed of hydrocarbons or their derivatives, or matter of plant or animal origin.

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P

Parapet Wall - That part of any wall entirely above the roof.

Perlite - An aggregate used in lightweight insulating concrete and in preformed perlitic insulation boards, formed by heating and expanding siliceous volcanic glass.

Perm - A unit of water vapor transmission defined as 1 grain of water vapor per square foot per hour per inch of mercury pressure difference (1 inch of mercury - 0.49 psi).  The formula for perm is: P = GRAINS OF WATER VAPOR / SQUARE FOOT * HOUR * INCH MERCURY

Permeance - An index of a material's resistance to water vapor transmission. (See Perm)

Phased Application - the installation of a roof system or water proofing system during two or more separate time intervals.

Picture Framing - A rectangular pattern of ridges in a roof membrane over insulation or deck joints.

Pitch - (See Coal Tar and Incline)

Pitch Pocket - A flange, open bottomed, metal container placed around columns or other roof penetrations that is filled with hot bitumen or flashing cement to seal the joint.  The use of pitch pockets is not recommended by NRCA

Plastic Cement - (See Flashing Cement)

Ply - A layer of felt in a built-up roof membrane system.  A four-ply membrane system has four plies of felt.

Pond - A roof surface that is completely drained.

Positive Drainage - The drainage condition in which consideration has been made for all loading deflections of the deck, and additional roof slope has been provided to ensure drainage of the roof area within 48 hours of rainfall.

Primer - A thin, liquid bitumen applied to a surface to improve the adhesion of subsequent applications of bitumen.

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R

Re-covering - The process of covering an existing roofing system  with a new roofing system.

Reglet - A groove in a wall or other surface adjoining a roof surface for use in the attachment of counterflashing

Reinforced Membrane - A roofing or waterproofing membrane reinforced with felts, mats, fabrics, or chopped fibers.

Replacement - the practice of removing an existing roof system and replacing it with a new roofing system.

Re-roofing -   The process of re-covering or replacing an existing roofing system.  (See Re-covering and Replacement)

Ridging - An upward, tenting displacement of a roof membrane frequently occurring over insulation joints, deck joints and base sheet edges.

Roof Assembly - An assembly of interacting roof components (including the roof deck) designed to weather-proof and, normally, to insulate a building's top surface.

Roof Cement - (See Flashing Cement)

Roofer - The trade name for the workman who applied roofing material.

Roof Hatch - (See Scuttle)

Roof System - A system of interacting roof components (not including the roof deck) designed to weather proof and, normally, to insulate a building's top surface.

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S

Saddle - A small structure that helps channel surface water to drains, frequently located in a valley, and often constructed like a small hip roof or like a pyramid with a diamond shape base (See Cricket).

Saturated Felt - A felt that has been partially saturated with low softening point bitumen.

Scuttle - A hatch that provides access to the roof from the interior of the building.

Seal - (1) A narrow closure strip made of bituminous materials; (2) To secure a roof from the entry of moisture.

Sealant - A mixture of polymers, fillers, and pigments used to fill and seal joints where moderate movement is expected; it cures to a resilient solid.

Selvage - An edge or edging that differs from the main part of (1) a fabric, or (2) granule-surfaced roll roofing material.

Selvege Joint - A lapped joint designed for mineral-surfaced cap sheets.  The mineral surfacing is omitted over a small portion of the longitudinal edge of the sheet below in order to obtain better adhesion of the lapped cap sheet surface with the bituminous adhesive.

Shingle - (1) A small unit of prepared roofing material designed for installation with similar units in overlapping rows on inclines normally exceeding 25%; (2) To cover with shingles; (3) To apply and sheet material in overlapping rows like shingles.

Shingling - (1) The procedure of laying parallel felts so that one longitudinal edge of each felt overlaps and the other longitudinal edge underlaps, and adjacent felt.  (See Ply.)  Normally, felts are shingled on a slope so that the water flows over rather than against each lap; (2) the application of shingles to a sloped roof.

Slippage - Relative lateral movement of adjacent components of a built up membrane.  It occurs mainly in roofing membranes on a slope, sometimes exposing the lower plies or even the base sheet to the weather.

Smooth-Surfaced Roof - A built-up roof membrane surfaced with a layer of hot mopped asphalt, cold applied asphalt clay emulsion, cold-applied, asphalt cutback, or sometimes with an unmopped inorganic felt.

Softening Point - The temperature at which bitumen becomes soft enough to flow, as determined by an arbitrary, closely defined method.

Solid Mopping - (See Mopping)

Split - A membrane tear resulting from tensile strength

Spudding - The process of removing the roofing aggregate and most of the bituminous top coating by scraping and chipping.

Square - The term used to describe 100 square feet of roof area.

Stack Vent - A vertical outlet in a built-up roof system designed to relieve the pressure exerted by moisture vapor between the roof membrane and the vapor retarder or deck.

Stripping or Strip Flashing - (1) The technique of sealing a joint between metal and the built-up roof membrane with one or two plies of felt or fabric and hot-applied or cold-applied bitumen; (2) The technique of taping joints between insulation boards or deck panels.

Substrate - The surface upon which the roofing or waterproofing membrane  is applied (i.e., the structure deck or insulation).

Sump - An intentional depression around the drain.

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T

Tapered Edge Strip - A tapered insulation strip used to (1) elevate the roof at the perimeter and at curbs that extend through a roof; (2) provide a gradual transition from one layer of insulation to another.

Tar - A brown or black bituminous material, liquid or semi-solid inconsistency, in which the predominating constituents are bitumens obtained as condensates in the processing of coal, petroleum, oil-shale, wood, or other organic materials.

Tarred Felt - (See Coal-Tar Felt.)

Test Cut - A sample of the roof membrane that is cut from a roof membrane to: (a) determine the weight of the average interply bitumen moppings; (b) diagnose the condition of the exiting membrane (e.g., to detect leaks or blisters).

Thermal Resistance (R) - An index of a material's resistance to heat flow; it is the reciprocal of thermal conductivity (k) or thermal conductance (C).  The formula is R = (1/C) or R = (1/k) or R - (Thickness in Inches / k)

Thermal Shock - The stress-producing phenomenon resulting form sudden temperature changes in a roof membrane when, for example, a rain shower follows brilliant sunshine.

Through-Wall Flashing - A water resistant membrane or material assembly extending through a wall and its cavities, positioned to direct water entering the top of the wall to the exterior.

Tuck Pointing - (1) troweling mortar  into a joint after masonry units are laid; (2) Final Treatment of joints in cut stonework.  Mortar or a putty-like filler is forced into the joint after the stone is set.

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U

Underwriters Laboratories (UL) - An organization that classifies roof assemblies for their fire characteristics and wind up-lift resistance.

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V

Vapor Retarder - A material designed to restrict the passage of water through a roof or wall.

Vent - An opening designed to convey water vapor or other gas form inside a building or a building component to the atmosphere, thereby relieving vapor pressure.

Vermiculite - An aggregate used in lightweight insulating concrete, formed by the heating and consequent expansion of a micaceous mineral.

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W

Water Cutoff - (See Cutoff)

Waterproofing - Treatment of a surface or structure to prevent the passage of water under hydrostatic pressure.

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