8.1 General Information

8.1.1 Flashings are used to make watertight any roof area where the roof membrane terminates, is interrupted, or intersects an area or projection having a marked change in slope or direction. This condition can occur at gravel stops, curbs, parapets, walls, built-in gutters, expansion joints, skylights, vents, drains, pipes, and other penetrations.

8.1.2 Flashings are generally divided into two categories: base flashings and counterflashings. Base flashings are, in a sense, a continuation of the membrane, turned up onto a surface that is in a different plane from the field of the roof, and installed as a separate operation. They are usually fabricated using a non-metallic material, such as an asphalt-coated felt. Counterflashings, or cap flashings, can be of metal, coated felt, or other materials. They shield and seal the exposed edges of the base flashing.

8.1.3 The combined use of non-metallic materials for base flashings and metal for counterflashings brings out the best in each material. Bituminous base flashing materials have the same coefficient of expansion and contraction as the roof membrane, and they work together as a unit. They are the only type of material acceptable for use in constructing base flashings.

8.1.4 Because of the rigidity of metal and its extreme movement with temperature changes, its use is not acceptable for base flashings. Cap flashings or counterflashings made of metal, removed from any area of possible standing water, are acceptable, provided they are properly installed in accordance with industry-accepted sheet metal details, in conjunction with an approved JM base flashing specification. Bituminous base flashings must extend at least 8" (203 mm) above the surface of the roof.

8.1.5 In some instances, it is necessary to provide wall coverings on the area of the wall above the base flashing. This section includes specifications for two general constructions, however, JM does not typically include these wall coverings in the guarantees offered for built-up and modified bitumen systems.

8.1.6 All health, safety, and environmental procedures involving the storage, use, and disposal of roofing materials should be followed. These precautions are outlined in the "Introduction" section of the current Johns Manville Commercial/Industrial Roofing Systems Manual.

8.2 Principles of Flashing Design

8.2.1 The following considerations should be given to all bituminous flashing systems:

A. Allow for differential movement in the flashing system, particularly when the deck and wall are not directly tied to each other (non-load-bearing construction). Differential movement between the roof deck and the wall is usually evidenced by diagonal wrinkles in the base flashing. Continued movement can cause tears in the flashing, particularly at the most restricted areas, usually in corners. If the movement cannot be handled by standard base flashing systems, roof-to-wall expansion joint details should be used to accommodate this movement. See Specifications FE-1 (NLB) and DFE-1 (NLB).

B. Locate flashed metal joints above the highest water level on the roof. Good roofing practice dictates that roof edges should be raised above the roof level, if possible. If problems do occur, the edges will be out of the area of standing water. The top edge of all base flashings must be carried a minimum of 8" (203 mm) above the roof.

C. Avoid sharp bends in built-up and modified bitumen base flashings. Right angle bends in bituminous flashings create high stress areas and can result in premature aging of the flashing material. To alleviate this condition and to provide solid backing, the use of cant strips is necessary. Cant strips are made either of perlite board, wood fiber board, wood, or other materials, and their 45° (0.79 rad.) angle face modifies the sharp angle between horizontal and vertical. They also reduce the danger of damage from foot traffic at these vulnerable areas.

D. Connect metal solidly. Sheet metal sections should be well attached. The solid securement of all flashing accessories to wood nailers is mandatory. The wood nailers must extend horizontally beyond the metal flange. Movement due to poor securement will result in abnormal stress on the membrane and flashing, which can cause leaks.

E. Secure all base flashings at the top edge with appropriate mechanical fasteners, spaced 6" (152 mm) o.c. maximum, or with some other positive means to prevent sliding or sagging.

8.3 Inspection and Preparation of the Flashing Substrate

8.3.1 All surfaces to be flashed should be inspected before any flashing work is started, since the success of the flashing system greatly depends on a properly constructed base.

8.3.2 Masonry Construction: Walls should be built with hard burned brick, sound reinforced concrete, or waterproof concrete block construction. Common faults encountered are:

1. Soft or scaling brick or concrete.

2. Poor mortar or faulty pointing of joints.

3. Broken copings and inadequate pointing of joints between copings.

Walls of ordinary hollow tile, or other materials which in themselves are not waterproof, should not be accepted as suitable to receive flashings unless they are properly waterproofed, to prevent moisture infiltration into the flashing system. All masonry surfaces which are to receive hot bitumen or bituminous-based cement shall be primed with JM Concrete Primer. The primer must be allowed to dry thoroughly prior to application of flashing.

8.3.3 Frame Construction: Frame walls are not acceptable to receive flashing unless suitable solid backing for the flashing is provided. Walls of gypsum wall board are not acceptable as a substrate for bituminous flashings. Suitable stops should be provided at the top of the flashing in stucco construction, to ensure a watertight seal for the base flashing.