Custom flashings arrive wrong for one reason more than any other: the sketch was ambiguous. The folder did what the drawing said, and what the drawing said was not what the site needed. Getting this right costs nothing extra. Getting it wrong costs a return freight, a delay and, on a tight programme, a failed inspection.
This post covers the geometry of a flashing order from first principles: what girth means, how to count bends, what a hem does to the bend count, how to nominate the dimension that cannot move, and how to mark a sketch so the roll-former has no room to guess.
Girth: The Developed Flat Width
Girth is the total flat width of the sheet before any folding takes place. Imagine unrolling a finished flashing back into a flat strip. The length of that strip, measured perpendicular to the fold lines, is the girth.
This matters because the folder prices by girth, not by the sum of the finished face dimensions. A flashing with a 150 mm wall leg, a 25 mm top cap and a 75 mm apron has a girth of 250 mm before you account for bends. Bends consume a small amount of material in the radius, but for standard brake-press work in 0.42 mm or 0.48 mm steel, the bend allowance is small enough that most fabricators treat it as negligible at quoting stage. If you are working in heavier gauge aluminium or copper, ask the fabricator whether they apply a bend deduction.
At CSA, custom flashings are priced by girth, bend count and material, so an accurate girth figure on your order directly determines the price you are quoted.
Counting Bends
A bend is any change in plane. Count each fold once. A simple Z-flashing has two bends. A cap flashing with a drip edge on each side has four. The number matters because each bend pass through the brake takes time, and fabricators with automated roll-formers may have minimum bend counts or charge per additional fold above a threshold.
Draw the profile in cross-section and mark each angle change with a circle. Count the circles. That is your bend count. Do not include the hem in this count yet.
What a Hem Is and Why It Counts as Two Bends
A hem is a folded-back edge, usually 10 to 12 mm, turned 180 degrees back onto the face of the sheet. It stiffens the edge, removes the raw cut edge from view and eliminates the sharp burr that causes hand injuries on site.
A hem requires two passes through the brake: one to fold the edge to approximately 90 degrees, and a second to close it flat. Some folders use a dedicated hem die that does both in one pass, but the geometry is still two bends, and the material consumed is twice the hem depth. A 12 mm closed hem adds 24 mm to the girth and two to the bend count.
If you specify a hem on one edge only, add one hem to your bend count and 24 mm (for a 12 mm hem) to your girth. Both edges hemmed: add two hems and 48 mm to girth, four to bend count.
Leave hems off internal flashings that will be concealed. Specify them on any exposed edge that will be visible or handled regularly.
Minimum Fold Leg
The minimum leg length the brake can physically form depends on the tooling and the material thickness. As a working rule for standard COLORBOND and Zincalume in 0.42 mm BMT, allow a minimum leg of 15 mm. Thicker material, typically 0.55 mm or above, may require 20 mm minimum depending on the folder's tooling.
If you draw a 10 mm drip edge on a 0.48 mm sheet, the fabricator will either refuse the job or open the leg to the minimum they can hold. Either way, the finished piece will not match your sketch. Specify legs of 15 mm or more unless you have confirmed the fabricator's tooling can go tighter.
This also applies to the return leg on a hem. A 10 mm hem on 0.42 mm material is achievable. An 8 mm hem is not, in most shops.
Maximum Practical Piece Length
Roll-formed flashings can be produced in long lengths, but there are practical limits. For standard profiles in COLORBOND steel, 6.0 metres is the common workshop maximum for a piece that can be handled, transported and installed without permanent distortion. Some fabricators will go to 7.2 metres for simple profiles.
Beyond those lengths, the piece will bow, twist or kink in transit. The solution is a lap joint. Standard lap allowance is 150 mm minimum, with sealant between the lapping faces. On exposed cap flashings in high-wind zones, 200 mm is more appropriate.
If your run is 8.4 metres, order two pieces: one at the full run length minus 150 mm, and one at 300 mm (to give the 150 mm lap plus a working tail). Mark the lap direction on your sketch so the upper piece sheds water over the lower, not into the joint.
For transport, pieces over 4.0 metres need to be carried flat or supported at close intervals. A 6.0 metre flashing carried by one end will take a permanent set. Confirm with your delivery provider how long pieces will be handled before you order.
The Held Dimension
When a flashing has to fit a specific gap, slot or overlap, one dimension governs everything else. That is the held dimension: the dimension that cannot move.
On a cap flashing that has to fit over a 90 mm parapet wall, the 90 mm straddle is the held dimension. The folder must produce that face width exactly. The two side legs can absorb any minor variation in total girth, but the top face cannot.
On your sketch, mark the held dimension with a double underline or write HOLD THIS DIMENSION next to the arrow. Do not leave the folder to guess which leg matters. If you mark every dimension as though they are all equally fixed, the folder will prioritise the easiest one to hit, which may not be the one that matters on site.
You can only nominate one held dimension per profile. If you try to hold two dimensions simultaneously while also specifying a total girth, the geometry is overconstrained and the fabricator cannot satisfy all three. Decide which dimension the installation cannot tolerate being wrong, mark that one, and allow the remaining legs to float within the girth.
Marking Colour Side
COLORBOND and Zincalume are single-sided products. The colour coat or the bright zinc finish is on one face; the reverse is a grey primer or a plain zinc back-coat. When the flashing is folded, the colour side needs to face outward on every exposed face.
On your cross-section sketch, mark the colour side with a C or shade it with a pencil line. For a simple Z-flashing, this is obvious. For a complex profile with returns and hems, it is not. If the folder sets the sheet up the wrong way, the colour is on the concealed face and the primer is exposed to weather.
Also nominate the finish: COLORBOND colour name (for example, Surfmist, Monument, Woodland Grey) or Zincalume. If the flashing is going onto a COLORBOND roof, match the colour from the BlueScope colour range. Mismatched finishes on a visible cap flashing are a defect that requires replacement.
How to Dimension a Sketch That Leaves No Room for Guessing
A dimensioned cross-section sketch should contain the following:
- Overall girth noted in a callout box, calculated as the sum of all legs plus hem allowances
- Each individual leg dimension, measured from fold line to fold line (or fold line to edge for the outermost legs)
- Bend angles for any fold that is not 90 degrees; assume 90 degrees only if you write it
- Hem specification on any hemmed edge: open or closed, depth in millimetres
- The held dimension marked clearly, with a note that it cannot move
- Colour side marked on the cross-section
- Finish and colour name
- Material thickness in BMT (base metal thickness), not TCT (total coated thickness)
- Piece length and quantity
- Lap allowance if the run requires multiple pieces, with lap direction noted
If any of these items is missing, the fabricator will make an assumption. Sometimes the assumption is correct. Often it is not, and the piece comes back wrong.
For complex profiles, a three-dimensional sketch showing how the flashing sits in context, for example over a parapet or around a penetration, gives the fabricator the intent of the piece. The cross-section provides the geometry; the context sketch confirms it makes sense.
Ordering Through CSA
At CSA, custom flashings are quoted from a dimensioned sketch. The more complete the sketch, the faster the quote and the lower the risk of a remake. Standard turnaround for fabricated flashings varies by profile complexity and volume; contact the trade desk to confirm lead times for your project.
For straightforward profiles, the trade desk can work from a hand sketch with dimensions. For profiles with multiple bends, compound angles or tight tolerances, a drawn cross-section with all dimensions marked is strongly preferred.
Visit constructionsupplies.group/au to browse stocked flashing profiles, or contact the trade desk directly to discuss a custom quote.