A trim profile that runs right on the brake comes down to four things: leg lengths, bend direction, which face carries the paint, and where the hems go. Get those four onto the drawing and the shop can build the part without a phone call.
Leg lengths: measure outside, write every leg
Every leg on the profile gets a number, including the short ones. A 1/2 in. kick or a 3/4 in. return is a real leg that takes real material.
Dimension to the outside of the metal, corner to corner. That is how the operator sets the back gauge and checks the finished part with a tape. On 24 and 26 gauge the metal is thin enough that most shops simply add the outside leg lengths to get the flat.
Measure off the building, not off a catalog; fascia depths and wall plumb vary.
Stretch-out and pieces per sheet
Stretch-out, or girth, is the flat width of the blank before any bends. Add up every leg and every hem. A drip edge with a 3 in. deck flange, a 2 in. fascia drop, a 1/2 in. kick and a 1/2 in. hem is 6 in. of girth.
Girth decides pieces per sheet, and that decides what the trim costs. On a 48 in. sheet a 6 in. girth yields 8 pieces with nothing left over. Stretch the deck flange to 4 in. and the girth is 7 in., which yields 6 pieces and a 6 in. strip of scrap. That is why the shop asks whether a 3-1/2 in. leg could be 3 in.: it is an extra piece out of every sheet.
Bend direction and angles
If a bend is 90 degrees, you do not have to say anything. Anything else, write the degrees and which way it goes. "45 degrees" can mean 45 off flat or a 45 degree included angle, and those are very different parts. Draw it, or write it as degrees from flat: "bend down 45 degrees."
Ridge and hip caps, valley trim and the fascia drop on eave trim follow the roof pitch, so the bend angle comes straight from the slope. Rake trim does not; its section is cut across the slope, so the deck-to-fascia bend is a plain 90.
| Roof pitch | Angle from flat |
|---|---|
| 3:12 | 14.0 degrees |
| 4:12 | 18.4 degrees |
| 6:12 | 26.6 degrees |
| 8:12 | 33.7 degrees |
| 12:12 | 45.0 degrees |
A ridge cap on a 6:12 roof is bent 26.6 degrees down on each side of center. An eave drip on the same roof needs its fascia drop bent 90 plus 26.6 degrees off the deck flange, or the drop leg kicks out from the plumb fascia.
Paint side: color out
Coated coil is painted on one face. The back carries a thin wash coat, usually grey or off-white, that protects the steel and is not meant to be seen. Bend the profile the wrong way and the wash coat ends up on the weather side, and there is no fixing that.
Mark the color side on every sketch: hatch marks along the painted face, or the words "color out" with an arrow.
Asymmetric profiles such as a sidewall flashing or a drip edge with a kick have a left hand and a right hand. Say how many lefts and how many rights.
Hems: open or closed, and which way they fold
A hem is a 180 degree fold on the edge of the trim. It stiffens the edge and hides the raw cut.
- Closed hem: folded flat against itself. Use it where the edge just needs to be stiff and finished.
- Open hem: folded back with a gap, roughly 1/8 to 3/16 in. Use it where the edge has to hook a cleat or receive a panel hem.
A 1/2 in. hem is the common size on light-gauge trim; go to 3/4 in. when a cleat has to engage it. Write the size, open or closed, and which way it folds. Hems usually fold toward the back so the color face stays clean, but the shop bends what you draw.
Takeoff and cut list
Pull run lengths off the roof: eave, rake, ridge, hip, valley, and every wall the roof meets. Trim comes in 10 ft lengths as standard because it is cut from 4 ft by 10 ft flat sheet. Divide each run by 10, round up, add for laps at every joint and for corners, and add a piece per run for the one that gets cut wrong.
The cut list the shop needs, per profile:
- Profile name or sketch reference
- Quantity and length per piece
- Gauge, color and paint system
- Color side, and left or right hand where it applies
- Hem size, open or closed, at each end of the profile
Sketch vs drawn profile
A pencil sketch works fine if every leg is dimensioned, every non-90 bend is labeled, the color side is marked and the hems are called out. A photo of the old piece with no numbers, or "same as last time," does not.
Frequently asked
Should trim leg lengths be measured to the inside or outside of the metal?
Dimension to the outside of the metal, corner to corner. That is how the brake operator sets the back gauge and checks the finished part, and on 24 and 26 gauge most shops add the outside legs to get the flat.
What is the difference between an open hem and a closed hem on metal trim?
A closed hem is folded flat for a stiff, finished edge. An open hem is left with a small gap, about 1/8 to 3/16 in., so it can hook a cleat or receive a panel hem.
How do I figure out how many pieces of trim come out of a flat sheet?
Add up every leg and hem to get the girth, then divide the sheet width by the girth and round down. A 6 in. girth gets 8 pieces from a 48 in. sheet; a 7 in. girth gets only 6.