roof flashing
Flashing is utilized on buildings or structures to help prevent water from getting within the building. Flashing occurs wherever a break in the building material occurs or when a vertical and horizontal joint exists. It is typically made up of strips of metal, asphalt strip product, or composite fabric which is bent or shaped to it’s final required form. Some of the more commonly used materials are asphalt, plastic, copper, rubber, composite material or a combination of these materials. The main premise behind flashing is that water/joint penetration can only occur when water moves either upward against the force of gravity or by way of a physically challenging path during which dissipation would eventually occur. Capillary action of moisture is also a challenge posed for flashings regarding concrete and masonry structures, which are combatted with the use of ‘breaks’ or drips.
​
Flashing can be concealed or exposed. If exposed the material makeup is most commonly aluminum, copper, painted galvanized steel, stainless steel, zinc alloy, terne metal, or copper-clad lead.There are commonly expansion joints provided to minimize deformation stress on long stretches. The material type utilized should not stain or react chemically with adjacent materials (aluminum and lead may react with cement mortar).
The following are the most common areas where flashing is utilized: Roof Valleys, ridges, hips, pitch changes, chimneys, roof drains, vent pipes, skylights, window and door openings, roof eaves, roof rakes, building/ground joint, wall expansion joints. In commercial contracting, the general rule of thumb is that any flashings within the body of a certain trades work will be under his scope of work. At the perimeter however is where the scope responsibility can become grey. Also matching colors may be a guiding factor in terms of who should carry a certain flashing scope. For example a glass/glazing contractor will typically pick up all exposed flashing systems around their work, but may not be responsible for concealed flashings. Also an EIFS contractor may or may not be responsible for any perimeter flashing aside from any perimter reglets required for containment of their work. A Masonry contractor will typically provide flashings within his system (like through wall flashings) and even some at the perimeter but may often exclude these. Masonry subcontractors however may or may not be skilled at flashing work, especially when it’s to be exposed permanently. A roofing subcontractor will typically perform all sheet flashings but metal flashings may or may not be provided. Mechanical, Electrical, and Plumbing trades will typically be responsible for flashing all of their work. Sheet metal flashings are much easier painted on the ground than once in place. This work may or may not be allocated to the painter. Any sealants required with a flashing assembly should be done by the trade installing them unless otherwise noted.
​
The following are some standard roof penetration or joint flashing assemblies and their descriptions:
Eave Flashing – to resist wind driven snow, rain, and ice buildup. Commonly premoulded and prefinished metal which goes between the eave/rake shingle and the wood sheathing.
Chimney Flashing – Firstly a cricket is commonly framed/sheathed behind the chimney to shed water away from accumulating up behind the chimney.
It is typically flashed with sheet metal and has locked and soldered joints
The front base of the chimney will receive a minimum of 4″ apron horizontal and vertical. The sides of the chimney will receive stepped apron flashing.
The vertical ‘counter-flashing’ or ‘cap flashing’ is the next piece installed lapping over the apron with a minimum of 4″ vertically and side laps of 3″+. This cap flashing commonly terminates into the mortar joints horizontally at least an inch.
Vertical Wall Flashing – Where a vertical wall meets a flat or sloped roof, base flashing will be required at least 2″ horizontally and 4″ vertically. The exterior wall finish materials in this instance will typically serve as the ‘cap-flashing’ or counter flashing.
Stack/Vent Flashing – Flashing a plumbing stack will typically be required and is done with a rubber boot and/or a flange type flashing. This is interweaving if the shingles are the utilized roof material. The boot base and/or flange flashing should not be nailed or screwed, only adhered.
Ridge Flashing (concealed) – Depending on the roof type, ridge flashing may be copper, aluminum/steel, roofing felt, or other material. It is commonly installed at the ridge flashing and concealing the roof sheathing termination peak. This will work to seal out moisture and help bridge this gap before final roofing material is installed above.
Ridge Flashing (exposed) – An exposed ridge flashing is more common with metal roofs and utilizes a preformed, mechanically attached flashing trim/system often utilized as the finished look as well.
Flat To SLoping Roof – When utilized on a flat-membrane-roof to a sloped shingle roof, a single cleat is typically fastened to the sloped roof side with an interlocking two-piece cap flashing system covering the joint. The flat roof membrane is then typically lapped at least 4″ over the top cap flashing.
Exposed Valley – Exposed valley flashing is often done with sheet metal, aluminum or galvanized-painted steel.
For wood shingles, the overall flashing piece is typically at least 16″ wide for roof slopes 6:12 or greater and 22″ wide for slopes less than 6:12. For asphalt shingles, the total width of the flashing piece is often 24″ with a minimum of 5″ exposed. There is typically a 1″ tall crimp bent into the metal which diverts splashing water from trying to make it’s way underneath the roofing materials.
​
There are also edge crimps at the end of either width which are a further deterrent to the movement of water below the roofing material.
Flat-Roof to Parapet Wall Flashing – This flashing assembly will typically start with a cant strip of wood, foam, or plastic along the perimeter of the masonry wall. Roofing membrane from the roof installation will then typically be brought up 8-12″ past the highest anticipated water line above the cant strip. Then another layer of membrane flashing will be cut and installed from 4-8″ past the cant strip horizontally to 8-12″ past the highest anticipated water line above the cant strip to line up with the previously placed membrane.
For flashing of a stud wall, sometimes a 3rd layer of flashing is lapped past the second horizontal strip and in line with it vertically. A termination bar is typically utilized on a masonry parapet, with masonry anchors adhering the bar tightly against the wall blocking out moisture.
For a masonry parapet wall, sometimes a through-wall cap flashing is utilized in lieu of or in conjunction with the termination bar to provide added flashing and moisture-penetration resistance.
