BSP-020: Water Management Details for Residential Buildings

Flashings are used as a means to direct water back out to the exterior and away from the various construction assemblies.  They are fundamental to any water management system.  Flashing can be created using any number of materials or products such as metal, plastics, and flexible membranes.  The application is to provide a means to direct water from the drainage plane of the assembly, through the cladding and out to the exterior.  When installed properly, they provide superb protection from water penetration into building assemblies, when installed poorly; they can become water injection devices into building assemblies.  

Installation Concerns  

Flashings should all be sloped in the direction of drainage.  This may seem like a simple enough concept to incorporate into the design, however, surprisingly the most fundamental failure mechanism for a flashing installation is through installing the flashing with a reverse slope.  With reversed sloped flashings water is held by the flashing and drains to the side and off the ends of the flashing back into the assembly.  This can also hold water up against claddings and other materials, potentially reducing the service life of the products.  All flashings should be installed with a positive slope to drain to the exterior.

A second common problem with flashing installations is not properly integrating the flashing into the drainage plane of the wall assembly.  Flashings should be shingle lapped into the drainage plane of the wall assembly.  For head flashings, if the top edge is left exposed, water can potentially drain behind the flashing and into the assembly.  Depending on the location, taping or sealing the top edge of the flashing with a flexible membrane may be adequate, though it is always more reliable if the flashing is shingle lapped into the drainage plane.

Special Flashing Details  

Flashings are used in many different ways and different locations.  Most common are through wall flashings at floor separations and head flashing above windows.  There are also some specialty type flashings that should be incorporated into the design of the water management system.  

Step Flashing/Roof Wall Connection

Step flashing are a very common and long used approach to integrate the drainage from the wall assembly above a sloped roof to the shingle installation used on most sloped roof applications.  The step flashing weaves the drainage plane from the wall into the laps of the roof shingles.

Traditionally, the step flashing is installed by the roofing contractor as a method to terminate the roof installation.  This has created problems in the past, when the step flashing is not properly integrated into the drainage plane of the wall.  It is important that the wall drainage plane lap over top of the step flashing to direct water in the wall assembly out onto the roof shingles.  With siding and stucco installations, this can be easily accomplished.  

Brick/stone veneer assemblies require more specific details.  With older mass masonry wall construction, the step flashing was regletted into the exterior face of the masonry assembly.  This prevented water from roof drainage from penetrating into the wall assembly; however it did not provide a means for water, which had leaked past the exterior face of the masonry, to be directed back onto the roof.  This worked adequately in mass reservoir type construction due to the face of the masonry being the primary drainage plane and the ability of the wall construction to manage a certain amount of water leakage and absorption.  This detail however does not function well for veneer applications as masonry veneers do not follow the same rain water management principles.  

The drainage plane in a brick/stone veneer installation is behind the masonry.  With the traditional step flashing installed to the exterior face, there is no method to direct the water back out onto the roof and therefore the water leaks into the construction below.  

For brick/stone veneer installations a through wall flashing must also be incorporated into the design in addition to the step flashing.  One method is to create complicated stepped down through wall flashings above the roof to wall intersection.  This requires meticulous folding of sheet metal or other flashing material to be able to follow the step down nature of the masonry veneer.  

An alternate approach is to use a flexible membrane flashing installed from the drainage plane of the wall onto the roof deck below the masonry to water proof the joint between the wall and the roof assembly.  The masonry above can either be installed directly on top of this (for small masonry loads) or supported by a steel angle fastened back to the structure (for larger masonry loads).  The step flashing can still be installed to the exterior face of the masonry; however the step flashing installation is now more of an aesthetic termination for the masonry than a critical water management component.  

Kick-out flashing

Kick-out flashings are used at the bottom edge of a roof to wall intersection, where the roof ends and the wall continues on.  They are used to direct the water out from below the cladding at the step flashing plane and away from the wall.  This prevents water from draining off the roof and into the wall system below.  It also helps aesthetically, as it prevents unsightly stains on the wall cladding from roof drainage.

In order for the kick-out flashing to be effective, the bottom seam must be properly sealed or folded to ensure that water does not leak through the joint.  Improperly folded or sealed kick-out flashings will cause a high water concentration to be poured behind the cladding.

Kick-out flashing are also required with brick/stone veneer installations.  The flashing must extend from the drainage plane of the wall behind the veneer out through the front face of the masonry.  The geometry of the kick-out flashing will depend on the shape of the veneer cladding in order to be integrated into the masonry pattern; however the same principles will still apply.  

Other Penetrations

Often miscellaneous penetrations such as mechanical penetrations, balcony railings, and or porch framing attachment are improperly integrated into the design and construction of the water management system of the building.  Being less common than many other details they can be overlooked or improperly sequenced.

Mechanical penetrations, including exterior lighting, electrical boxes, dryer vents, gas utility penetrations etc. need to be properly flashed and integrated into the design of the water management system.  This can often be achieved through the use of self adhered membrane flashings.  Some products are also available that provide flanges that can be sealed and shingled lapped into the drainage plane permitting these penetrations to be more easily incorporated into the water management system.

Saddle flashings are used in locations where building elements penetrate through the cladding and are attached to the back up wall construction.  This occurs at such places as balcony railings, porch framing attachments, retaining walls, and some mechanical penetrations.