Flashing Design for Built-up Roofs
The flashing element is the most vulnerable part of any roof system because it is the point at which the horizontal roof deck and vertical surface join. It is also an intersection of two different materials, such as parapet walls. In addition, flashings are vulnerable because they are applied around all roof penetrations such as skylights, HVAC units, vents, expansion joints and other areas where the membrane is interrupted or terminated.
Selecting Proper Flashing MaterialProblems with the flashing element cannot be corrected at the application phase; thus a roofing contractor should never have to design a flashing detail at the job site. To achieve successful flashing, the designer must thoroughly analyze the roof system. The material selected should have an in-service life expectancy that meets or exceeds that of the membrane, because the flashing generally has to perform in more severe conditions than the membrane. The selection of inferior materials to save money up front generally results in future economic losses caused by premature failures.
If the flashing material specified does not conform to the roof system and surface to which it is attached, then even the most detailed flashing plans will fail. It is essential that this material is compatible with all adjoining materials and that it has the durability to last the lifetime of the system.
The material must also have the ability to withstand all thermal and load-induced movements. An allowance must be made for differential movement between the membrane and all other parts of the application. Base flashing should not be anchored to parapet walls, unless the parapet and the substrate are continuously connected and cannot expand or contract independently. Counter flashings should not be connected to base flashings unless the possibility of relative movement between them can be positively prevented. If the flashing material cannot sustain the strains of the roofing system movement, cracks and tears will develop and deteriorate the flashing.
Flashing DesignOn remedial roofing projects, the roof designer can determine the extent of differential movement to the roof system by focusing attention on exposed flashing details on the current system. The roof designer should pay particular attention to the layout of the seams, the joints and all connections to the roofing membrane, such as at walls, curbs and vents.
The roof designer must be certain that the flashing detail for the walls, parapets and other vertical surfaces provides for differential movement of the roofing system in all areas. In structurally independent building elements, such as walls that do not support the structural deck, base flashing should not be directly connected to the walls.
A wood nailer should be horizontally secured to the structural deck with appropriate fasteners every 24 inches on center. A vertical wood nailer should then be installed behind the cant strip to form a blocking backer for the base flashing. The vertical backer must also be installed to the structural deck and not the wall. A compressible insulation board should be inserted between the back of the vertical wood nailer and the wall to prevent heat from escaping from the building in the winter months.
A flexible vapor retarder should be installed over the base flashing, the vertical wood nailer and the insulation to serve as an insulation retainer. Appropriate fasteners should be secured at the front facing of the vapor retarder at approximately every 8 inches on center. The vapor retarder is utilized as an obstruction to the vapor flowing up through the insulation. Counter flashing is attached over the vapor retarder and secured to the wall through a cutout reglet.
On buildings where the structural decking is supported by the wall, the need for a vertical wood nailer is eliminated. The horizontal wood nailer should still be secured over the structural deck; however, the base flashing can be applied directly to the wall. Masonry walls must be adequately coated with primer before the flashing application. The appropriate fasteners should be used to secure the top of the flashing to the wall at approximately every 8 inches on center. Plastic cement and membrane fabric are then applied over the top of the flashing and at all seams where the counter flashing is installed into a cutout reglet.
Flashings and Drainage SystemsStanding water is more detrimental to flashings than it is to the membrane because of the increased possibility of leakage at the seams. There is also greater possibility of relative movement. Flashings must be designed to divert the flow of water away from them. To accomplish this, a tapered cant strip must always be installed at all flashing areas between the roof and the vertical surface. In addition to allowing for the positive drainage of water away from the flashings, tapered cant strips also provide structural strength and serve as a gradual angular transition from the horizontal surface to the vertical surface. Flashings should be positioned at a maximum 45-degree bend to reduce the risk of cracking that would occur if they were positioned at a right angle.
To further ensure water-tightness at the flashing areas and reduce the risk of leakage at the joints, flashings should be located at the high roof areas. When preparing details on a remedial roofing project, the layout of the existing roof plan may present a challenge. In instances where roof sumps are located at the building's perimeter edge in areas where no vertical surfaces exist or directly next to equipment penetrations, the most feasible way is to elevate the flashings above the water level resulting from build up around the sumps. To do this, install a cricket around the roof sump and elevate the perimeter edge or equipment base with wood nailers. It is important that the seams are adequately coated with roof mastic and membrane fabric in these locations.
The base flashings at the vertical surfaces should be a nominal height of no less than 8 inches and no higher than 12 inches above the finished roof surface. The flashing should extend above the surface 8 inches in order to be protected from the effects of weathering, such as rain and snow. A vertical dimension of 8 inches is also necessary to provide sufficient working room for the application procedure. The maximum height limit of 12 inches provides a safeguard against flashing sagging. When the vertical height exceeds 12 inches, the asphalt has a tendency to flow downwards when a high range of roof temperatures are experienced. On vertical surfaces that extend beyond 12 inches in height, such as walls, proper measures must be taken to adequately waterproof the remaining vertical surface area.
Perimeter FlashingThe design of flashing details at the perimeter edges that do not have walls, parapets or other vertical surfaces requires diligence. A study conducted by Factory Mutual indicates that of 145 roof failures, 85 occurred due to inadequate perimeter flashings. The majority of roof failures caused by Hurricane Hugo began at the perimeter edge.
When designing flashing at the perimeter edges, follow these steps: a wood nailer must be secured to the structural deck at the perimeter edge of the building where the deck adjoins the outside wall. The attachment of the wood nailer to a masonry wall should comply with Factory Mutual Data Sheet 1-49, which specifies that bolts be installed in the top edge of the wall to anchor the wood nailer. If installation of a scupper or ventilation is required, the wood nailer can be slotted.
Wood nailers are secured parallel to the perimeter edge and the roof surface. They are also installed as a baseboard on which the counter flashing is installed. On structural metal decks, the wood nailer serves as a shield against any wind that may penetrate into the flutes under the insulation casing uplift.
The installation of the tapered cant strip is necessary to raise the roof elevation at the flashing location. It is essential to elevate the flashing above the roof surface. The tapered cant strip must be secured to the wood nailer and should be positioned above the roof surface. If the tapered cant strip were positioned below the roof surface, the flashing would be vulnerable at the joints due to the possibility of ponding water.
The most important aspect of the perimeter edge flashing is the use of metal flashing. At perimeter edge locations, the metal flashing is the distinctive attribute that determines the success or failure of the flashing, because it shields the base flashing at the joints. A gravel stop is a proficient type of metal flashing used at the perimeter edge, performing these basic functions:
It serves as a barrier that prevents loose aggregate from rolling off the roof.
It is used as an edge termination for the membrane.
It serves as a rain shield.
It serves as top surface for the anchorage of the fascia strip.
The metal used for the gravel stop in these areas must be rigid, watertight at all end laps and connections, and flexible enough to withstand thermal and differential movement. If the roof designer intends to restrain the gravel stop from expansion and contraction, a thin-gauged metal should be fastened closely at approximately every 3 inches on center. In these cases, the recommendation is to use 16-ounce copper, 24-gauge galvanized metal or 0.040-inch aluminum.
When the intention is to accommodate linear expansion and contraction of the gravel stop, a thicker 10-inch extruder section is required and it should be fastened sparingly at approximately every 12 inches on center. In these applications, the fasteners should be staggered. The distinctive attribute in this type of situation is that the ends of the metal are left free to expand and contract. For this type of detail it is recommended that the applied metals be fabricated from 22-gauge metal, 0.050-inch aluminum or 24-gauge stainless steel.
The gravel stop should always extend over the base flashing when it is installed at the perimeter edge to act as a shield over the flashing joints. The perimeter edge flashings that are not sloped or adequately shielded with proper metal tend to create the most problems.
It is also essential that a layer of plastic cement be applied over the membrane to isolate the gravel stop from the membrane. Sheet metal should never be directly applied over the roof membrane. The plastic cement will not only serve as an added waterproofing sealant, it will guard against the splits that often occur when the gravel stops are stripped in with the membrane. This detail is of extreme importance in colder climates like the North and Midwest where this type of split is more prevalent.
Flashing problems account for the majority of roof leaks on low-slope built-up roof systems. Proper flashing design is the responsibility of the roof designer. Adequate research of flashing conditions and the use of proper flashing details are vital to the long-term success of the roof system. Although design is not the responsibility of installers, awareness of proper details for all types of situations is required to ensure that the correct installation procedures are followed. This will eliminate flashing problems and costly callbacks, which will satisfy the designer and owner and add to the bottom line.