Pricing a re-roof without a proper takeoff is how jobs lose money. Under-order and you're chasing a part-load delivery mid-job. Over-order and you're sitting on cut sheets you can't return. A methodical quantity survey takes less than an hour on most residential roofs and gives you a materials list accurate enough to price confidently.
This guide walks through the full process: plan area and pitch factor, sheet count by profile, flashings by the lineal metre, fixing quantities, insulation and rainwater, and waste allowances.
Step 1: Measure Plan Area and Apply a Pitch Factor
Start on the floor plan or site measure, not on the roof surface itself. Measure the building footprint covered by each roof plane in plan view, length by width, and record each rectangle or triangle separately.
Pitch increases the actual roof surface area above the plan area. The relationship is straightforward: divide the rise by the run to get the pitch ratio, then use the pitch factor from the table below to convert plan area to true roof area.
Common pitch factors:
- 5° (approximately 1:12 pitch): factor 1.004
- 10°: factor 1.015
- 15°: factor 1.035
- 20°: factor 1.064
- 22.5°: factor 1.082
- 25°: factor 1.103
- 30°: factor 1.155
- 35°: factor 1.221
For a 25° gable roof with a plan area of 120 m², the true roof area is 120 × 1.103 = 132.4 m². That extra 12.4 m² is roughly four or five sheets depending on profile, so the pitch factor is not optional arithmetic.
Step 2: Break Complex Roofs into Simple Shapes
Hip roofs, L-shapes and roofs with dormers or skillion sections need to be broken into rectangles and right-angled triangles before you can measure them accurately.
For a rectangle: length × width.
For a right triangle: (base × height) ÷ 2.
Mark each plane on a sketch and label the dimensions. Add the plan areas for all planes, then apply the pitch factor for each plane separately if the pitches differ. A skillion addition at 10° attached to a main roof at 25° needs two separate calculations.
Check your total against the building footprint as a rough sanity test. The sum of all plane plan areas should equal the footprint area of the building, minus any uncovered sections.
Step 3: Calculate Sheet Count by Profile
Every corrugated or ribbed profile has a cover width: the net width of roof covered per sheet after laps are accounted for. This is not the sheet's overall width.
Typical cover widths:
- Corrugated (Lysaght Custom Orb): 762 mm cover
- Trimdek (Lysaght): 762 mm cover
- Spandek (Lysaght): 700 mm cover
- Klip-Lok 700 (Lysaght): 700 mm cover
- Monoclad (Stramit): 300 mm cover per panel
Divide the width of each roof plane (measured along the eave, perpendicular to the sheet run) by the cover width to get the number of sheets per run. Round up to the nearest whole sheet.
Example: a hip plane 6.5 m wide using Trimdek at 762 mm cover width.
6,500 ÷ 762 = 8.53, round up to 9 sheets.
For triangular planes on a hip roof, the sheet lengths taper from the ridge to the hip rafter. The standard approach is to calculate the average sheet length for the plane and multiply by the sheet count. Average length equals (longest sheet + shortest sheet) ÷ 2. This gives you the total lineal metres of sheet for that plane, which you then order as a mix of lengths or as full-length sheets cut to a pattern.
Step 4: Set Sheet Length from Ridge to Gutter
Sheet length runs from the ridge (or apex) down to the gutter line, plus an overhang into the gutter. Standard overhang is 50 mm for most profiles into a standard gutter, though some details call for up to 75 mm. Confirm with your gutter profile.
Measure the rafter length from ridge to fascia on the roof surface (not the plan dimension), then add the overhang. That is your sheet length.
Order full-length sheets wherever possible. A sheet running the full rafter length from ridge to eave in a single piece eliminates the end lap entirely. End laps require additional sealant, extra fixings, and introduce a potential water entry point. They also add labour. Custom-cut-to-length sheets are available from ACS up to the maximum rollforming lengths for each profile, which means most residential rafter runs can be covered in one piece.
If the rafter length genuinely exceeds the maximum sheet length for the profile, an end lap becomes necessary. In that case, the minimum end lap is 150 mm for pitches above 10° and 300 mm for pitches at or below 5°, per AS 1562.1. Seal all end laps with compatible neutral-cure sealant.
Step 5: Measure Flashings by the Lineal Metre
Flashings are ordered by the lineal metre. Walk each flashing type separately and measure carefully, because getting short on valley or apron flashing mid-job is a frustrating and avoidable problem.
Flashing types to measure:
- Ridge capping: measure the full length of every ridge line, including any hip ridges. Add 150 mm per join for laps.
- Barge (rake) flashing: measure both rake edges on gable ends. Each gable has two rakes.
- Valley flashing: measure the full length of each valley from ridge junction to eave. Valleys on complex roofs are longer than they look on plan.
- Apron flashing: measure wherever a roof plane meets a vertical wall at its lower edge, such as a skillion abutting a two-storey wall.
- Step and over-flashing: measure wall abutments along the raking edge of a roof plane. Step flashing is installed in individual pieces per batten course; over-flashing (soaker) runs continuously over the top. Measure the raking length and allow one step flashing piece per sheet or batten course.
- Eave trim and fascia cover: measure the full eave perimeter if you are replacing these.
Add 10 percent to all flashing lineal metre totals for waste, cuts and joins.
Step 6: Count Fixings
Fixing quantities depend on the profile, the wind region and the batten spacing, but the following rates are a practical starting point for residential work.
Corrugated and Trimdek (screw-fixed profiles):
- Field fixings: 1 screw per pan per batten, approximately 9 to 11 screws per m² depending on batten spacing.
- Edge and eave rows: double the fixing rate for the first and last sheet in each run and for the eave and ridge rows.
- Laps: 2 screws per lap per sheet width at end laps.
Klip-Lok and concealed-fix profiles:
These use clips rather than through-fixed screws. Count one clip per rib per batten, then add screws for the flashings and trims separately.
For a 130 m² roof using corrugated sheeting with battens at 900 mm centres, a reasonable starting estimate is 10 screws per m² in the field plus 20 percent uplift for perimeter rows, giving approximately 1,560 screws. Buy in boxes of 500 or 1,000 and round up. Running short of screws on the last two sheets is a common and entirely preventable delay.
Match the corrosion class of the screw to the environment. Class 3 is the minimum for standard inland residential use. Class 4 is required within 1 km of a surf coast or in industrial environments. COLORBOND and ZINCALUME sheeting require screws with a compatible coating to avoid galvanic issues; check the BlueScope fixing specification for the product you are using.
Step 7: Add Insulation and Rainwater
Anticon blanket and sarking are ordered by the square metre of roof area. Use the true roof area figure (post pitch factor) calculated in Step 1. Allow for 150 mm overlaps at joins when ordering anticon; a 10 percent addition to the net area is usually sufficient.
For rainwater, the gutter and downpipe sizing is driven by roof catchment area and rainfall intensity rather than a simple lineal measure. That calculation is covered in detail in the ACS post on rainwater system sizing. For the takeoff, measure the eave perimeter for gutter length and count the downpipe locations. Order gutter in standard lengths and plan your joins to minimise waste.
Step 8: Apply a Waste Allowance
Waste on a re-roof comes from cuts around penetrations (skylights, pipes, vents), hip cuts on triangular planes, and any damage or miscut during installation.
- Simple gable roofs with no penetrations: 5 percent.
- Hip roofs or roofs with multiple planes: 7 to 8 percent.
- Complex roofs with dormers, valleys and multiple penetrations: 10 percent.
Apply the waste factor to sheet area only. Flashings already have their own 10 percent allowance built in from Step 5.
Putting the Takeoff Together
A completed re-roof takeoff should list:
- Sheet profile, colour and total lineal metres per length (if ordering cut-to-length) or sheet count per length
- Each flashing type with lineal metre quantity
- Fixing type, corrosion class and total count
- Anticon or sarking area in m²
- Gutter type and lineal metres, downpipe count and lengths
- Sealant tubes (allow one tube per 20 lineal metres of flashing as a rough guide)
With that list in hand, a quote request to ACS gives the trade desk everything needed to price accurately. For large or commercial re-roofs, the request-a-quote function at acsupplies.com.au handles bulk orders with custom cut lengths and scheduled delivery of long materials.
A takeoff done once properly saves the back-and-forth of two or three revised quotes and means the job starts with the right materials on site.