Roofing and Flashings

Home | Fire Safety | Skyscrapers

Home Emergencies | Glossary

The general geometry—or shape—of the roof is an earlier decision related to the architectural character or design of the building as a whole.

Color, texture and pattern of the materials are the three major appearance variables. Wood shakes or shingles, for example, will provide a very rustic and dramatic texture, but with limited color range; whereas, asphalt or fiberglass shingles can produce a wide range of textures depending on shingle weight and profile, because they come in varying patterns or cuts, as well as a wide range of surface colors. Tiles of clay, cement and slate all have a unique character, pattern and texture which may dictate or preclude their selection. Metal roofing can be quite plain, such as the often-seen corrugated metal, or be very unique due to a particular pattern of seaming or jointwork between adjacent panels. Metals can also be quite variable in color—either as a natural material such as copper or aluminum, or via coatings, paints, and other processed finishes.



Roofing materials may or may not be required by local code to possess some degree of resistance to the effects of fire, relating to their combustibility as well as their resistance to spread fire via flying brands (the release of burning splinters or pieces). Wood shakes and shingles are extremely susceptible to burning. These materials can become the initial source of fire for a building when flying brands from an adjacent field fire, forest fire, or building fire land on the shingles, igniting them. This is an all-too-common problem in many areas of southern California where forest and brush fires spread to residences via their exposed wood shake roofs. We have all seen the news stories showing homeowners and firefighters desperately hosing water onto these kinds of roofs hoping to prevent disaster. Some building codes will require special asbestos underlayment felts and /or special roof deck Construction under wood shakes and shingles, to partially Compensate for the high fire risk aspects of this roofing.

Asphalt and coal-tar pitch are by themselves also quite Combustible materials; however, when incorporated into the various composite shingle, roll-roofing and built-up roofing assemblies with mineral or stone top surfaces, these assemblies can achieve a greater degree of fire resistivity than wood. They are therefore more readily acceptable by codes for most residential construction applications. Asphalt shingles are usually classified by Underwriters Laboratories (U.L.) as Class “C”. This classification indicates they are effective against light fire exposure, that is they are not readily flammable and don't readily carry or communicate fire; afford at least a slight degree of heat insulation to the roof deck; don't slip from position; possess no flying brand hazard; and , may require occasional repairs or renewals in order to maintain their fire-resisting properties.

The newer—and more common—fiberglass roof shingles usually carry a U.L. Class “A” classification, which signifies substantial improvement in the similar aspects of flame spread, flame communication, etc. described above for Class “C”, due to the use of fiberglass fiber mats in the base stock of the shingle.


There is an extremely wide variance in the anticipated useful life of various roofs, dependent on the following variables:

1) The degree of exposure to sun, rain, cold and temperature fluctuations.

2) Make-up of the system as to the number of plies or layers, thickness, and weight.

3) Whether the materials are organic, such as wood, asphalt, etc. or in-organic such as metal, clay, slate, etc.

The following table is included with the intent of providing only a very general indication of anticipated useful lives of various roofings:



The base surface, or deck, over which the roofing is applied is critical to the success of the roofing. Nearly all residential roofing systems require a deck which is smooth and nailable; that is, that has the ability to be nailed into, with substantial resistance to nail withdrawal. The deck must be strong enough to support the roofing materials plus any other applied loads such as snow, ice, occasional foot traffic, etc. It must not deflect sufficiently under load to cause strain, displacement nor rupture of the roofing materials. Decks usually installed in residential work are plywood, wafer-board, 3/4” tongue-and-grooved wood sheathing boards, or 2”, 3”, OR 4” thick tongue-and-grooved wood planking. Board and plank decks must be dry to prevent warp and shrinkage, which can distort and tear roofing.

For the installation of wood shingle or shake roofing, the deck may consist of 1” x 4” wood boards installed with approx. 4” spaces between each board, in order to allow air exposure to the undersides of the shingles for moisture control and for shingle dry-out. See (ill. 52).

Plywood for decking must be exterior type, or be made with water-resistant glue. The usual grade used is CDX. Required thickness is a function of Code requirements and of the span distance between rafters or supporters, but should not be less than 1/2” for adequate nail holding power.

Any holes, loose knots or other major defects in the decking should be covered with patches, 26 gauge minimum. Iron or steel should be galvanized.

(CAUTION) All decks must be clean and free of debris, loose or protruding nails, oil, grease or other contaminants, and be dry when the roofing is installed. Moisture which is trapped in materials during construction can be very troublesome and detrimental to the success of any roofing system.


A. ROLL ROOFING consists of 36” wide rolls of asphalt impregnated felt which is coated with weather-resistant asphalt, then top surfaced with colored mineral granules. When installed, it forms either a single layer (ply) weighing 80 to 90 lbs. per 100 square feet (a square), or 2 plies weighing 110 to 120 lbs. Per square. The minimum pitch for roll roofing is 1” on 12”. The normal range of pitch is from 1” to 4” on 12”. Laps are nailed and cemented with special quick-setting cement. Roofing sheets should not be installed when the air temperature is below 45 deg. F; and , the roofing should be first rolled out, allowed to flatten and be job acclimatized before being installed. All eaves and rakes should be fully cemented to 9” wide continuous strips of the same roofing which have been previously nailed along the edges of the roof deck. Upper edges of roll roofing must be nailed, with the next succeeding course lapping over the nailed portion and being fully cemented to it. All ends of sheets should be lapped at least 6”, nailed and cemented. See (ill. 53)

Roll roofing is generally considered to be more of an economical utilitarian roofing system. It does not result in a particularly aesthetic roofing pattern nor texture.


ill. 53: ROLL ROOFING: plywood decking all laps and edges are fully cemented.

B. ASPHALT SHINGLES are of similar materials to roll roofing; FIBERGLASS SHINGLES differ in that they have a fiber glass mat core. Both are made into separate shingles, usually 36” long x 12” wide. The exposed edge of each shingle has slits to divide it into sections called tabs, which are made in varying sizes and profiles to create different patterns once installed. See (ill. 54) for examples. Shingles come in varying weights and thicknesses, from approx. 235 lbs. to 300+ lbs. per square. Shingles which are self-sealing should be used. These have small patches of mastic placed by the manufacturer on the underside of the tabs of each shingle, which when exposed to the warming action of the sun, adheres the tabs to the shingles below, thereby preventing lifting and possible tearing of tabs due to wind. Heavier weight shingles are generally more durable and resistant to wind damage, dry-out, Cupping, etc., than the light weights.


(CAUTION) Shingles should be installed at roof pitches greater than 4” per foot. However, there are some shingles made for low slopes between 2” and 4” per foot, provided they are installed in accordance with special low-slope specifications.

An underlayment layer of No.15 asphalt saturated felt must be nailed to the entire roof deck, before application of shingles.

C. BUILT-UP ROOFING consists of layers of asphalt saturated fiberglass or asbestos felts alternated between field applied coatings of heated asphalt. See (ill. 55).

ill. 55: 4-PLY, GRAVEL SURFACE, BUILT-UP ROOFING ON NAILABLE DECK: nailed base sheet; plies of felt in mopped layers of hot asphalt; gravel topping in coating of asphalt

From 3 to 5 plies of felt are installed depending on the desired life of the roof, and its particular application. Roof pitch must be an absolute minimum of 1/4” per foot, and can go up to 6” per foot, depending on the surface treatment and the type of asphalt used. Pitches of ¼” up to 2” per foot are more commonly found. The steeper pitches require use of steep pitch asphalt, which is of stiffer consistency and higher softening temperatures so as to resist the tendency to soften and flow as it gets heated up by the sun.

Built-up roofing receives either gravel top surfacing, a mineral granule surfaced top sheet, or special asphalt top coatings left smooth surfaced. Smooth surfaced roofings should have aluminum or white paint-type reflective coatings applied to them. All of these surfaces are intended to prevent deterioration of the roofing by the sun and weather elements, and to reduce surface temperature build-up.

(CAUTION) It can be very difficult to locate and repair leaks in a gravel surfaced roof.

Built-up roofing is definitely intended, and more suitable, for very low-pitched roofs.

(CAUTION) it's very important that built-up roofing be applied in strict accordance with the specifications of the manufacturer of the roofing products. Any variance from this can void potential warranties and absolve the manufacturer of any responsibility for roofing difficulties or failures.

It is possible to secure written warranties, or a surety bond from the manufacturer of the roofing materials, but these will result in increased costs to the owner. it's also possible to require a warranty from the roofing installer for a specific period of time (I recommend 10 years mm.); however, the validity of this warranty is subject to the good faith and continuance in business of the roofer. It does, however, tend to increase the roofers’ care and attention to installation procedures and workmanship.

On wood board or plank decks, a separate layer of building paper should always be nailed down first, before the built-up roofing. This is not usually necessary on plywood decks. The first ply (or layer) of built-up roofing on plywood, should be nailed to the plywood, or thru the building paper to wood board decks. This is to allow differential movement due to the unavoidable swelling and shrinkage of the wood to take place as it absorbs and releases moisture, without the stresses and other effects being transmitted to the roofing membrane because of a too-rigid connection of the two materials. See (ill. 55).

D. CLAY TILE, CEMENT TILES, and SLATE: These are all much heavier, durable and more expensive roofing materials. The roof structure may need additional strength to support these materials. All should have at least a 30 lb. roofing felt underlayment, doubled at ridges, hips, valleys and other break points in the roof. All should have nail holes already provided in the tiles by the manufacturer — not be punched in at the jobsite.

All nails and fasteners for these materials should be non ferrous copper, aluminum or be of heavily zinc coated steel.

Some highly-profiled clay tile roofs, like Mission and Roman tiles, require horizontal or up-and-down strips of wood applied to the wood deck, in order to properly support and maintain the design profile of the tiles. See (ill. 56).


(CAUTION) Tile and slate roofings are brittle and therefore very susceptible to damage from physical abuse, especially foot traffic on the roof. If there is need for regular access to the roof to perform routine maintenance on equipment, or similar reasons, other kinds of materials should be used in those traffic areas, or other precautions taken to preclude damage to the tile roof.


Flashing are the materials and methods used to make watertight: (1) all penetrations thru the roofing, (2) any intersections of the roofing with other materials, and (3) any intersecting planes of the same roofing.

It is beyond the scope of this book to discuss the nitty-gritty details of various flashing conditions. However, we will illustrate some typical flashing situations and discuss certain principles.

Flashing materials include thin gauge metals (copper, aluminum, lead, tin, stainless steel, and galvanized iron or steel), roofing felts, rubber, fiberglass and various types of asphalt and roofing mastic.

All metal flashings expand and contract due to temperature changes at significantly higher rates than most other roofing materials they are used with. Therefore, provisions must be made to allow metal flashing parts to move. If they are confined or constrained by rigid fastenings or joints on all edges—especially in long lengths — the internal stresses will cause metals to fold, crease, ridge and eventually crack open.

The basic principle of flashings is that parts at higher elevations must lap over the top of parts at lower elevations — not the other way around. This is so as to shed water away from the joint, not into and under it. Joints must be so made that water could work through them only against the force of gravity.

Typical locations where flashings must be provided are the following:

— Penetration of a chimney thru the roofing (ill. 57),

— Penetration of plumbing vents and other pipes thru roofing (ill. 57),

— Valleys in roofs (ill. 58)

— Roof ridges and hips (ill. 59)

— The intersection of roof with vertical wall or parapets (ill. 47)





In cold climates, buildings with roof overhangs can experience troublesome leaks due to a phenomenon known as ice dams. In these instances, snow and ice which collects on the roof will gradually melt from the heat which escapes from the building interior. The melted water runs down towards the lower edges of the eaves. If the outside air is still quite cold, it's likely that the water will turn to ice again on roof overhangs which extend beyond the outside walls, because there is no interior heat source at these locations. This ice builds up into an ice dam, causing water to back up the roof, into and under the roofing materials, then into the unprotected building structure. Significant damage has been done to many cold-country buildings because of this phenomenon. There are two generally used alternative methods of eliminating this problem. One is to use watertight metal roofing on the overhangs, extending the metal back up the roof to at least 12” inside the interior wall line, where it's flashed to— and covered by—the regular roofing material, such as shingles, tile slate, etc. The other alternative is to install a 2-ply underlayment of No.15 asphalt saturated felts and asphalt cement from the drip edge of the eave back up to at least 12” inside the interior wall line for roof slopes over 4” on 12”, and at least 24” inside the interior wall line for roof slopes less than 4” on 12”. The finished roofing material is then installed in the regular manner over the entire roof, including the special 2 ply underlayment. See (ill. 60) for examples of both alternatives.

ill. 60: Ice Dams


Gutters, sometimes called eavetroughs, are installed at the eave drip edges of roofs, to catch run-off water and carry it to leaders, or downspouts, which deliver it to the ground at desired locations. Gutters thus prevent random, uncontrolled dripping of water from roofs, which causes soil erosion as it splashes on the ground, as well as discoloration and deterioration of the lower portions of the building.

Gutters are made of metal or wood, and may be attached to the fascia at the outside edge of the eave, or be integrally built into the eave construction, otherwise known as a “concealed gutter”. See (ill. 61).

ill. 61: ATTACHED GUTTER ; gutter, or eavetrough; CONCEALED, OR BUILT-IN GUTTER.

(CAUTION) Built-in gutters must be very carefully lined, flashed and sealed with permanent non-ferrous materials, or else they will eventually develop leaks and cause serious deterioration to the overhang structure, exterior walls and finishes.

Gutters must be:

1) Large enough to handle the quantity of water being discharged.

2) Sufficiently pitched to carry the water off quickly, and not retain pockets of water.

3) Leak-proof.

4) Installed so as to avoid possibility of snow and ice backing up under the main roofing material.

Leaders are vertical pipes which carry the water from the gutter to the ground, or to a ground level drain. They are usually made of metal, in various shapes; but, they must be sized to carry the water away as fast as it's received, so as not to cause gutter over-flow.


Wind striking a building is deflected upward, passes over and around the building. In so doing, wind produces areas of reduced pressure over the surfaces of flat or slightly inclined roofs. This pressure can be considerably lower than the pressure inside the building This differential between inside and outside pressures tends to lift the roof Uplift force from suction is highest along roof eaves and corners which face the wind, for flat or slightly inclined roofs. it's also high on the leeward slope of steeply inclined roofs See (ill. 62)

(CAUTION) As a result of these forces, roof structures, decks and roofing materials which are not securely fastened are subject to lifting and possible blow off Special care must be taken along eaves and edges to see that a sufficient number of fasteners have been used to secure the edging to the building, since this a most vulnerable area. Similar precautions apply at the corners of roofs.

(CAUTION) The fastening recommendations of roofing manufacturers must be followed, and should be considered as minimum requirements for the attachment of roofing materials. Their specifications and recommendations are based on many years of field experience as well as testing. Most manufacturers print these instructions on the wrappings used to package their materials. They are also readily available from any major roofing supply house. We have personally witnessed major blow off of built-up and shingle type roofing installations, which when analyzed were determined to be a direct result of a lack of adherence to the fastening requirements of the manufacturer. it's much better to error on the side of fastening over-kill on this issue.


Prev: The Roof Structure
Next: Interior Construction and Finishes

top of page    Home