Let’s see if I can explain my question - quite frequently, I see building elements that protrude 6-12 inches from the facade (could be a window feature, or the lower floor is larger than floor above it) and on the top of the protusion, the architect will draw “pre-finished metal flashing” or “metal flashing” that is supposed to act as the “roof” for the part that protrudes. More and more, I am seeing the “metal flashing” spanning greater distances. Today, I have a corridor that is added to the exterior of an existing building, about 6 feet wide and the “roof” is called out as “metal flashing”. I am pretty sure this is stretching the line, but I am curious where the line is between where you can use sheet metal for flashing over a horizontal surface (with some slope) versus needing to upgrade to a engineered metal roofing system. Let me know if that isn’t clear.
To me, flashing extends (overlaps the substrate) no more than about 4 inches, and it is not of itself watertight: It sheds water but is not seamed to the substrate over which it laps , although it could be “clipped” to secure the “loose” edge. It does not function as roofing but as an accessory.
Robin,
I think the size of your “metal flashing” roof dictates that it now be categorized as Sheet Metal Roofing, and that is what I would tell the design team to call it - with an accompanying specification section - if it were my project. I use Section 07 6100 for this which includes all the right language and requirements; substrate board, underlayment, minimum gage thickness, fasteners, performance requirements, etc. I start with a MasterSpec section.
One limitation on the flashing is its ability to resist wind uplift forces. By the time you get to 6 feet I would suggest the wind uplift forces would need to be considered.
I think whomever wrote this notation does not have the foggiest idea what a flashing is other than it may be a metal. As Forest Gump says “Stupid is as stupid does.” Sent it back the creator of the notation for a clear and concise explanation of his/her basis-of-design.
We specifiers see a lot of off the wall notations. Uusally the author of the note does not know any better. They are designers first, technical architects second, third, or 4th.
But they know AutoCAD or Microstation or Revit. Must know what they are talking about.
Just out of curiousity, have you checked SMACNA manual to see if flashing is defined?
DAFFYNITION
Flashings are components of the exterior envelope used to intercept and direct the flow of water to designed drainage paths. They prevent water from penetrating the building. Flashings are also used to direct water away from building elements susceptible to deteriorationby staining, erosion, frost damage and corrosionwhen exposed to excessive wetting.
FLASHING PERFORMANCE REQUIREMENTS
Water Barrier:
The flashing assembly including materials and joints must shed water without allowing leakage.
The ability of the flashing to seal and be detailed to avoid leakage is fundamental. Creating an effective and durable joint seal is often difficult. It is good practice to provide a secondary, continuous flexible membrane
flashing under jointed materials such as brick, stone or sheet metal.
Source:
CMHC Best Practice Guide, Building Technology - Flahings.
Perhaps the notation was intended to say SHEET METAL?
Probably the closet thing to it in SMACNA is a “ledge flashing”. The illustrations do not dimension it, but from the illustrations, one could surmise a ledge flashing to be in the 4" to 8 or 10" range. Defined as follows:
ARCHITECTURAL SHEET METAL MANUAL FIFTH EDITION
(not the latest edition)
Figure 420 illustrates various ledge flashing applications.
Figure 420A shows the elevation of a wood ledge
on a masonry building. The ledge is covered with
metal flashing (Section AA).
This flashing is attached to the face of the ledge with
a continuous cleat and is turned up the masonry wall
a minimum of 4 in.(102 mm) Step counter flashing
is applied as shown on drawing (see Figure 47A).
Figure 420B shows the elevation of a wood ledge on
a frame building. The ledge is covered with metal
flashing (Section BB). This flashing is attached to
face of the ledge with a continuous cleat and is turned
up 4 in.(102 mm) on the sheathing. Siding or shingles
serve as counter flashing.
Figure 420C shows metal flashing applied to a stone
ledge sloping away from the building. The flashing
is held in place by a continuous cleat attached with
masonry anchors to the face of the ledge. Flashing
is turned up the wall a minimum of 6 in. (152 mm)
and is held in place by counter flashing as shown (see
Figure 45A).
Figure 420D shows metal flashing applied to a stone
ledge sloping toward a building. A scupper or outlet
tube is provided to drain ledge onto the roof (see Figure
126A).
The flashing is held in place by a continuous cleat attached
with masonry anchors to the face of the ledge.
Flashing is turned up wall a minimum of 4 in. (102
mm) and is held in place by counter flashing as shown
(see Figure 43A).
Recommended minimum gage for flashings shown on
this figure is 16 oz. (.55 mm) copper, or 26 ga (.477
mm) stainless steel over rosin paper and 30 lb. (14 kg)
of felt.