A Watertight Job: The Art of Detailing and Specifying










By Karl A. Schaack
President of Price Consulting, Inc.



There are several resources available to the design professional providing general guidance specific to the preparation of details and specifications for roof flashing systems. Typical resources available include manufacturer and industry entities. Manufacturers typically provide generic guideline details and
specifications for installation of the specific materials produced by that manufacturer. Common related items, such as sheet metal, sealants, fasteners, etc., are often not depicted or are simply pictorially represented by these details, since they are usually not considered the responsibility of the manufacturer. The National Roofing Contractors Association (NRCA) publishes standard construction details in the “Roofing and Waterproofing Manual”(1) for conditions that would typically occur on a roof. The Sheet Metal and Air Conditioning Contractors National Association, Inc. (SMACNA) provides recommended details in the “Architectural Sheet Metal Manual”(2) for a wide variety of conditions with
regard to sheet metal components. Since sheet metal is the purpose and responsibility of this entity, the roofing membrane and flashing material related issues are not presented by SMACNA details.

As valuable as these resources are, critical aspects such as terminations or transitions of details are typically not represented in available standard details. The information typically available from these sources consists of either crosssectional or isometric views of standard conditions. However, quite often, the atypical condition (i.e. termination of base flashing or end of parapet wall) can create obstacles/difficulties for the installer, resulting in a sub-standard detail and possible water infiltration. In addition, the flashing conditions presented in a published detail may be a minimum standard that is suitable for the materials promoted; but may not be suitable for the existing field conditions nor meet the
requirements of the building owner or the intended use or serviceability of the roof. Therefore, it is responsibility of the designer to detail these conditions in order to achieve the desired finished product. If these specific elements are omitted, the constructability is left to the interpretation of the field mechanic, which could jeopardize the integrity of the installation through the lack of proper attention at a particular location.

The intent of this text is to provide general insight to the technical content that can be contained in a standard detail, the potential oversight of common conditions, and helpful hints to enhance the specific detail.
CONSTRUCTION DETAIL

The subject detail is a typical flashing condition at a load-bearing wall. The wall construction could be either a parapet wall or a building rise wall that is constructed of either concrete or masonry and extends vertically a minimum of 4 feet above the deck. The roof system selected is a multi-ply built-up roof with a
granule surfaced modified bitumen cap sheet and similar base flashing. The type of deck and presence of insulation are not relevant to this discussion. A two-ply modified bitumen roof system has similar characteristics as a built-up roof that is discussed herein.

SUBSTRATE

Masonry walls can be a challenging substrate on which to install flashing materials. Masonry located on the inboard faces of parapet walls is often not constructed as true and clean as exposed face brick on the exterior of the wall. Irregular surfaces, fractured partial bricks, bug holes, and untooled mortar joints
are common conditions on the inside of masonry parapet walls. Plywood or gypsum sheathing can be anchored to the wall to provide a smooth, sound, and monolithic substrate.

If plywood is installed, it is recommended that the top edge be chamfered, sloped upwardly, and back to the wall on a 30-degree to 45-degree angle. This honed edge will provide a smooth transition to extend the flashing material, or three coursing, applied along the leading edge of the base flashing up the interior of the masonry wall. A mechanically attached base sheet is typically anchored to the plywood prior to the application of other bituminous materials.

With the improvement of gypsum sheathing products, this type of product provides a sound suitable substrate to receive the various types of flashing materials now available. Direct application by the various installation methods (i.e. torch, adhesives, etc.) is possible while providing a substrate that is
inorganic and not susceptible to the affects of moisture, fire, and insects. A uniform coating of plastic roof cement can be applied to the masonry wall or the sheathing prior to the installation against the wall to serve as an additional layer of waterproofing.

CANT STRIPS

The initial step of constructing the flashing system involves installation of the cant strip at the deck and wall juncture. The cant strip could be a fibrous type, treated wood, or a quick-set cementitious based material. Fibrous cants are typically the most economical material based on initial costs. However, depending on the substrate (e.g. concrete deck) a cementitious cant material can be installed and
remain in place for future roof replacement activities. 

Whether the substrate is rigid insulation board or base sheet, a fibrous cant should be set in a uniform troweling of plastic roof cement, a cold-applied adhesive, or mopping of hot asphalt. If a wood nailer or wooden substrate is present, a wood cant may be used and nailed to the substrate. Cants should be
installed in a manner similar to those general requirements for rigid board insulation. Adjacent pieces of fibrous or wood cants should be installed with










Photograph 1: Misaligned cut

sections of fibrous cant along
curved substrate.











Photograph 2: Wood cant at
corner.











Photograph 3: Cementitious cant
at curved substrate.

joints butted snugly to provide a continuous smooth substrate (reference Photograph 1). Cants installed at inside and outside corners of walls should be constructed with mitered corners at the ends of the mating cant pieces. Forming smooth mitered corners is difficult to achieve with wood due to the rough, sharp edges formed by the saw-cutting process and the rigidity and trueness of both the wood and the underlying substrate (reference Photograph 2). Fibrous cants can be “shaved” and cementitious cants can be manually shaped to form smooth transitions (reference Photograph 3).

MEMBRANE & BASE FLASHINGS

After installation of the cant, the felt plies or the smooth surfaced modified bitumen base ply sheet should be installed continuously from the field of the roof to the top edge of the cant and terminated. In the BUR application, if the wall is parallel to the long dimension of the field plies, starter plies should be installed along the cant in order to achieve the proper ply count over the cant. If the masonry wall in the area to receive roofing is deemed to be a suitable substrate, primer should be applied and allowed to dry prior to application of the flashing plies.

Common details published by material manufacturers depict the ply sheets extending up the wall a minimum of 2-inches above the top of the cant. With fiberglass felt plies, particularly Type VI, the vertical extension is difficult to properly install. Due to the relative rigidness and “memory characteristics” of
glass felts, conforming to the two transitions (horizontal-to-cant; cant-to-vertical)









Photograph 4: Disbonded felt
plies.

is difficult to achieve in the relatively short dimension. Bridging of the felts or unadhered felts at one or both of these transitions will most likely occur. The author has observed roofing contractors, during the installation of built-up roofing felts, terminating the field plies above the cant and leaving the felts in that condition to serve as a “night seal”. This condition rarely performs satisfactorily, even if coated with plastic roof cement, because the felts often become disbonded from the vertical substrate creating an opening for water entry (reference Photograph 4). Therefore, it is the opinion of the author that felt plies of the field membrane should be terminated at the top of the cant and properly pressed into place to conform to the substrate.

Some modified bitumen manufacturers typically recommend terminating the base ply of the field membrane at the top of the cant. After installation of the field membrane plies, the author recommends installing two plies of felts (Type IV preferred as they tend to be more flexible) or backer sheets. The base flashing felts should be cut into 6-foot lengths and 12-inches to 18-inches in width or the dimension required for proper flashing height and site conditions. One-ply of smooth surfaced modified bitumen sheet material could also be utilized in-lieu of the felt plies. The flashing plies should be installed over the substrate, extending horizontally 4-inches beyond the toe of the cant and vertically 4-inches
(minimum) above the top of the cant. If the application of the cap sheet and the modified bitumen base flashings are delayed (which is more common than not), the backer ply sheets can be temporarily sealed and made watertight with a troweling of plastic roof cement applied over the top leading edge. Upon completion of the base flashing backer plies, the cap sheet is then installed up to the top edge of the cant and terminated. 







Photograph 5: Degranulating
cap sheet to receive base
flashing.








Photograph 6: Application of
three-coursing to vertical lap
seams of base flashing.

The granule surfaced modified bitumen base flashing is then extended horizontally 4-inches beyond the toe of the cant and vertically above the top of the cant a minimum of 4-inches. Granule surfacing of the cap sheet to receive the base flashing should either be primed or degranulated to provide a bituminous substrate (asphalt-to-asphalt bond) to adhere the material (reference Photograph 5). Modified bitumen finish plies should be cut into lengths of 6 feet (maximum). Flashing sheets in lengths greater than 6 feet are difficult for roof mechanics to handle and properly install, particularly using hot asphalt or torching techniques.

Since modified bitumen products that are to be applied with asphalt require that the asphalt to be of sufficient temperature in order to melt the coating on the back of the sheet, use of shorter lengths will help to achieve this. Although more vertical lap seams are created when using shorter lengths of flashing sheets, the author feels that a better installation will result. The flashing sheet can be
created by cutting sections from the full-width material roll resulting in sheet lengths of 3-feet
with a selvage edge. If a selvage edge is not present on the flashing sheet, the granules should
be primed or degranulated in the overlapped (vertical sidelap) portion. Care should be taken to
install the modified bitumen flashing sheet so that it conforms to the contour of the roof-to-cant-towall
profile. With low profile flashing heights (i.e. less than 24-inches), the flashing material should be installed on the roof surface initially and proceed upward. For taller flashing heights, the flashing sheet (commonly torch or coldadhesive application) is typically installed from the top edge and proceeds
downward to the roof surface. The modified bitumen sheet should be installed








Photograph 7: Application of
aluminized coating to threecoursing.









Photograph 8: Base flashing
nailed along top edge.

without any pulling or stretching (tension). The sheet should be allowed to fall into place and then be hand pressed into position. After adjoining sheets are installed, the author recommends that the vertical lap seams of the newly installed base flashing be treated with three coursing (plastic cement, reinforcing fabric, and plastic cement) (reference Photograph 6). A technique that can be
used to provide a neat appearance of 3-coursing is installing strips of an adhesive tape (i.e. masking or duct tape) parallel and on each side of the lap seam (approximately 6-inches apart). After the application of the 3-coursing, the tape is removed to provide straight vertical lines for the edges of the treatment. After this treatment, the three coursing can be finished by embedding granules in the wet cement or applying aluminized coating after curing of the plastic cement (reference Photograph 7). The three-coursing is not intended to substitute or conceal a poorly constructed lap, but rather to provide an additional layer of protection to the lap (exposed edge of the modified bitumen sheet). Several manufacturers do not specifically suggest the application of plastic cement over modified bitumen sheets particularly APP modified products. However, due to the advent of lower solvent bearing products
and because the plastic cement is topically applied, this concern is not as critical.

After the flashing has been adhered to the substrate and lap seams treated, the top edge of the base flashing should then be mechanically fastened. Individually placed nails, spaced 6-inches to 8-inches on-center, are often depicted in common industry published details (reference Photograph 8). Although this approach serves well to secure the material to the substrate, the long term seal of the material located between the attachment points can be jeopardized if the material becomes disbonded.








Photograph 9: Installation of
termination bar.









Photograph 10: Application of
three-coursing over termination
bar.

Therefore, a continuous metal termination or compression bar is recommended for securing the flashing (reference Photograph 9). The bar should be a minimum 1-inch wide and 1/4-inch thick, with a flat profile and should have slotted pre-punched holes at the desired spacing (6-inches recommended by author). The slotted or oval holes provide room for differential movement of the bar during changes in temperature. Termination bars can "bow" outward between the points of attachment when secured at greater than 6-inches on-center. The fasteners used to secure the termination bar into a masonry or concrete substrate should be suitable for the substrate. The author recommends a type of fastener that can be removed via screwdrill for ease in future renovation activities. Although drive pins function well, removal of these fasteners requires shearing of the head and either drilling out the fastener or auguring another hole. When installing the termination bar over a masonry substrate, the author recommends positioning the anchors into a mortar joint as individual bricks may fracture during the drilling or anchoring process.

After installation of the termination bar, a treatment of three coursing is recommended over the termination bar and top leading edge of the base flashing (reference Photograph 10). Although base flashings may be fully adhered to a wall and periodically mechanically attached (even with a termination bar), as accepted by many in the industry, over time the base flashing material can become disbonded from the substrate between the fasteners providing avenues for possible moisture infiltration. The continuous securement and three coursing will prevent this from occurring and reduces the criticality of the longterm integrity of the counterflashing element. The design of this type of base flashing construction creates a watertight detail even if the counterflashing were to be purposely or accidentally removed, dislodged, or was missing. The NRCA detail depicts 3-coursing as an optional item and material manufacturers vary in the requirement for 3-coursing treatment of the base flashing leading edge.

COUNTERFLASHINGS

After the base flashing is completed, the sheet metal counterflashing should be installed. A two piece counterflashing, consisting of a receiver and counterflashing, is recommended. This type of assembly allows for removal of the counterflashing segment without affecting the anchorage to the wall.
Removal of the counterflashing segment will allow personnel to perform specific roof related work and re-use components in the future. A receiver can be installed into a saw-cut reglet, secured in place, sealed, and then the counterflashing is secured to the receiver. A saw-cut reglet is preferred over a
surface-mounted type (often depicted in published details) for several reasons. A surface-mounted counterflashing has a caulk trough on the top edge to receive sealant at the wall interface. This detail relies heavily on the performance of the sealant to maintain the waterproof integrity of the counterflashing. The trough of a surface-mounted counterflashing is constantly exposed to water running down the face of the wall. Proper sealant application is extremely difficult to achieve on
a masonry substrate due to the irregular surface (i.e. brick and mortar joints). Proper bonding of the sealant to the masonry and concrete substrate is also difficult due to other soiled or contaminated surfaces (i.e. bituminous deposits)(3).
 
A caulk trough results in a fillet bead profile, which is a less than optimum sealant configuration. With a saw-cut reglet, the sealant is installed in proper configuration (between parallel planes), and the majority of it is recessed behind the face of the wall. The tooled face of the sealant is exposed providing little
resistance to water running down the face of the wall. A saw-cut reglet also requires proper techniques to achieve the desired performance. The reglet should be of proper depth, minimum 1-inch, into the wall and proper width, minimum 1/2-inch wide. The noted depth allows for adequate embedment of the
metal flange of the receiver, and allows for the installation of lead wedges, backer rod, and sealant. The noted width allows for the creation of a proper sealant joint width.

The reglet should also be properly cleaned to remove all debris from the saw cutting process. The cleaning process should use forced-air such as that created by a commercially available “leaf blower”. Compressed air is not recommended as moisture can be created by this process and introduced into the joint. The flange of the receiver should be fabricated to provide a downward sloping facing
at the point of protruding from the wall. This profile provides the least resistance for water cascading down the wall. The actual receiver should also be of sufficient length, 2-inches minimum, to allow proper attachment of the counterflashing. The counterflashing is then inserted and attached to the
receiver. This can be accomplished with either pop rivets or self-tapping screws. Self-tapping screws are preferred, as the screw can be removed and reinstalled to disengage the counterflashing in order to perform maintenance and repairs to the baseflashing or roof in the future. Adjacent sections of counterflashing should be lapped a minimum of 4-inches with a bead of sealant sandwiched
between the mating surfaces. The hemmed portion (backside) of the drip edge on the underlying section of the counterflashing is typically notched or removed on the lapped area so that the two adjoining pieces can nest. The lapped section can be secured, preferably with a grommeted fastener installed through an enlarged or oval hole to allow for movement.

If a surface-mounted counterflashing is the preferred or the only viable option, then the following detailing is recommended: apply a preformed sealant tape between the backside of the vertical flange of the counterflashing and concrete wall; utilize a continuous heavy-gauge bar secured 6-inches on-center to provide uniform attachment; utilize appropriate fasteners with steel washers backed with rubber seals.

If the counterflashing is designed, fabricated, and installed appropriately, the metal should hug the base flashing and eliminate the need for wind clips. The receiver should have a length of 2-inches and extend out of the reglet and be bent downward on a 45 degree angle with the vertical wall. The return should be
a minimum of 1-inch in length to engage the counterflashing. The counterflashing should have a receiver leg approximately 2-inches in length (1-inch engaged in receiver, 1-inch extending out of receiver). The angle between the receiver leg and fascia should be approximately 90 degrees. The length of fascia of the counterflashing should be approximately 3-inches. At these dimensions, the counterflashing should provide approximately 4-1/2-inches of coverage over the top edge of the base flashing and the hemmed drip edge should almost be touching the finished surface of the base flashing. The drip edge should be a hemmed return, approximately 1/4-inch in length, and bent on a 45-degree angle. If wind clips are desired, the author recommends utilizing 1-inch wide strips of the
same type of metal as the counterflashing. It is recommended that the wind clip be secured to the termination bar (after 3-coursing is applied) or height that the top edge of base flashing is secured. Typical SMACNA details depict wind clips that are installed into the base flashing material just above the bottom of the counterflashing (reference SMACNA FIGURE 4-3). However, the previous installation scenario has exhibited similar capabilities of restraining the counterflashing while the point of securement is located above the line of possible windblown water and snow and does not penetrate the base flashing material.

TERMINATIONS

When the wall terminates, particularly at the edge of the roof, a unique flashing detail occurs to transition the wall to the metal edge. Sealed terminations should be installed on each flashing component (i.e. counterflashing, base flashing, metal edge). When the base flashing is terminated at the edge of the wall, a termination bar should also be positioned vertically along the edge of the material
extending from the top of the cant to the horizontally positioned bar across the top edge of the base flashing. Three coursing can then be applied over the corner, extending 3-inches to 4-inches each side of the corner for the full height of the flashing material overlapping the three coursing applied at the top leading edge. For the metal edge, a specialty piece should be fabricated that extends up the cant and over the flashing and provides an end cap for the fascia. The flange of this fabrication should be anchored and then stripped into the flashing. An outwardly deformed hemmed kick-out should be formed on the outer edge of the metal cap to provide a trough to receive sealant. This fabrication can be made to extend up and under the counterflashing or a secondary piece of metal flashing could be installed on top of (in shingle fashion) the top edge of this unit, and extend under the counterflashing. The counterflashing should be wrapped around the end of the wall extending the same distance as the base flashing and metal edge. The counterflashing segment to be installed on the receiver should extend approximately ½-inch to ¾-inch beyond the end of the receiver. This extended portion can then be cut along the break lines and hand tonged downward to form an end closure for the counterflashing.

SUMMARY

There are many different field conditions, materials, design intents, installation nuances, and specific experiences that influence why details are constructed as they are. The purpose of this text is to show ideas and reasons for designing and constructing long term serviceable details and to present multiple areas than can be affected if not appropriately identified. It is the opinion of the author that the information and resources available to the industry are guidelines for minimum standards. It is the duty of the specifying community to present the proper information so that the product can be properly installed. Proper detailing will result in competitive and defined bidding, minimize multiple submittal processes, eliminate ambiguity between project personnel, and provide the Owner with a roof system suitable for the conditions and meeting the intended purpose of the design.

Ref: 
(1) NRCA “Roofing & Waterproofing Manual” Fifth Edition

(2) SMACNA “Architectural Sheet Metal Manual” Fifth Edition

(3) “The Use of Sealants in Roofing” Schaack Interface May 2001