How to Measure and Order Metal Roof Panels and Trim

Step by Step

1. 1Step 1: Break the roof into planes and label them

   Give every plane a letter: A for the main south slope, B for the main north slope, C for the garage and so on. For each, record the eave length, the slope length from eave to ridge (the rafter length), and the pitch. A roof report from an aerial measurement service gives plan dimensions and pitch, and you then calculate the real slope length.

   Real slope length equals plan run times a pitch multiplier. For example, a 4/12 roof multiplies by about 1.054, 6/12 by 1.118, 8/12 by 1.202 and 12/12 by 1.414. Rafter length is the number panels are cut to, not the flat distance from the eaves to the ridge on a drawing.

   Tip: Photograph each plane and mark its label on the photo. When a panel arrives cut to 'Plane C, 18 foot 4 inch', a photo saves a lot of second-guessing.

2. 2Step 2: Get the coverage width and length limits from the profile data sheet

   Coverage width is how much roof one panel covers after the lap or seam, and it is always less than the overall width. A 16 inch snap-lock panel covers about 16 inches, but an R-panel with a 36 inch coverage is about 38 inches overall. Use coverage width for every count.

   Also note the maximum length the forming machine and delivery truck can handle. Panels of 30 to 40 feet are common, and longer runs mean transport and handling problems and much bigger thermal movement. If the rafter length exceeds what you can handle, plan a mid-roof splice with the lap required by the manufacturer, or a transverse joint designed for the panel. See thermal movement in metal roofs.

3. 3Step 3: Calculate panel count per plane

   Take the eave length and divide by coverage width. For a 30 foot (360 inch) eave and a 16 inch coverage, that is 22.5 panels, so 23. Round up, but decide before ordering how the last panel will sit. A narrow sliver panel (under roughly 6 inches of coverage) is hard to seam or lock and looks wrong, so it is often better to trim the first and last panels so both are a similar partial width.

   Work rake to rake, not wall to wall. On a gable roof with a rake trim, the panels stop short of the wall by the amount the rake trim requires. On a hip roof, each plane is a trapezoid, so panel lengths change from one side to the other, and every panel needs its own cut length. Hips and valleys mean a cut list, not a single number.

4. 4Step 4: Calculate each panel length

   Start with the slope length. Add the eave overhang past the fascia, typically 1 to 2 inches, so the drip edge or eave trim throws water clear. Subtract the ridge gap required by the ridge detail, often 1 to 2 inches for a vented ridge, or add a hem allowance if the panel is folded at the ridge as an end dam. See how to hem and bend panel ends.

   For a hip roof or a roof with a valley, panels are cut on an angle at the hip or valley, so order the longest length in that run and plan to trim on site, or order angled cuts from the supplier if they offer them. Write the list so every panel that is identical is grouped, for example '23 pieces at 18 feet 4 inches'. That shortens the order form and lets the roll former run them in one batch.

   Watch out: Order panels one to two inches longer than calculated only if the manufacturer's eave detail calls for a hem or drip. Extra length you did not plan for has to be cut off, and cutting pre-finished panels risks finish damage.

5. 5Step 5: Build the trim list, one piece at a time

   Go around the roof and write each edge and junction on the diagram: eave trim, rake or gable trim, ridge cap, hip cap, valley flashing, sidewall and endwall flashing, transition trim where a low roof meets a wall, and trim at each dormer or skylight. Convert each to a length and divide by the standard trim length, usually 10 feet, to get a count. Add about 4 to 6 inches of lap at every joint, or follow the manufacturer's lap dimension.

   Closures are separate from trim. Foam or rubber closures that seal the panel profile at the eave, ridge and sidewalls are ordered to match the panel. See the guides for eave trim and closures, rake trim, ridge and hip caps and valleys for what each needs.

6. 6Step 6: List fasteners, clips, sealants and accessories

   Screws are ordered by the box, often sized from the roof area and the screw pattern. The count depends on the profile: a clip system needs clips at the spacing the panel specifies, and an exposed-fastener roof needs screws at each purlin or strap line across each panel. See metal roof screws for types and lengths.

   Add butyl or sealant tape for laps and penetrations, matching touch-up paint for cut edges and scratches, pipe flashings sized to the actual pipes (flashing pipes on metal), snow guards if the roof needs them (snow guards), and any vents or caps. Underlayment and strapping belong to the same order.

7. 7Step 7: Add a sensible allowance and recheck the numbers

   Panels themselves usually need very little allowance on a simple roof, because each is cut to length. On a complex roof with many hips and valleys, angled cuts waste material, and adding one or two spare panels for a mistake or transit damage is cheap insurance. For trim, an allowance of about 10 percent covers laps and cutting mistakes.

   Recheck every measurement from a second source, such as the roof report against a tape. A quick test is to add up the panel coverage widths along each eave and confirm they match the eave length within an inch or two. Also verify colour, gauge and finish on the order form against the quote.

8. 8Step 8: Confirm lead time, delivery and staging

   Lead times change with season and colour. Stocked colours may ship in days, custom colours and long cut lengths may take weeks, and the busy season before fall in BC is the slowest. Confirm the date in writing and order the roof before tearing the old one off, never after.

   Panels arrive banded in bundles on wood dunnage. Plan where they will rest: flat, off the ground, slightly sloped so any water drains, and covered without trapping moisture. Wet bundles stored flat and closed can develop white rust between panels, known as wet stack staining, which can mar the finish.

Working From a Roof Report

Colour and Lot Matching

Common Mistakes

• Using overall width instead of coverage width. The panel count comes up short and the last bay has no panel.
• Calculating from plan dimensions and forgetting the pitch multiplier, so every panel is ordered too short.
• Forgetting the eave overhang and ridge allowance, leaving panels that stop short of the drip or cover the ridge closure.
• Ordering trim by the total roof perimeter and missing the laps, which leaves the last trim piece a few inches short.
• Ordering panels and trim in separate orders from different paint lots, which shows as a colour change along the ridge.
• Storing panels flat and wet, causing white rust staining before they are installed.
• Ordering custom lengths before verifying the deck and fascia are square, then finding the eave line is not straight.