Roofing Formulas

Before reading this article, please make sure you are familiar with Measurement - Parts & Labour in Groundplan.

Parts, labour and the formula used are unique to each project and company, the information below details an example of a 26 deg pitch Tin Roof being measured on plan and the formulas used to calculate additional parts and labour. The same principle applies to most roofing styles such as Hip & Valley, Gable ends, Skillion/Lean-to, etc.

The formula given is a guideline to be used to calculate your measurement parts and labour.

Adding the roof pitch to the area measurement:

Calculating the total area for a 26 deg pitch roof.

Formula: area * 1.1126

Breakdown:
area - the sqm area measured from the 2D plan.
* 1.1126 - to allow for the roof pitch

Reference for Roof Pitch and Multipliers

Here's our Roof Pitch Chart to assist you with common Roof Pitches, Angles and their corresponding multipliers.

If you know the roof pitch in degrees, find the secant of the slope using a scientific calculator. For example, if the roof pitch is 26°, then sec(26) = 1.1126019404753957. That's your roof pitch multiplier.

Adding screws and insulation

1. Roofing Screws per screw: Calculating 8 screws per sqm of roofing material.

UOM: screws or ea
Formula: area * 1.1126 * 8

Breakdown:
area - the sqm area measured from the plan;
* 1.1126 - to allow for the 26 degree roof pitch;
* 8 - to calculate 8 screws per sqm.

2. Roofing Screws per box of 250: Calculating 8 screws per m2 of roofing material when purchasing screws in boxes of 250.

UOM: box
Formula: ceil(area * 1.1126 * 8/ 250)

Breakdown:
area - the sqm area measured from the plan;
* 1.1126 - to allow for the 26 degree roof pitch ;
* 8 - to calculate 8 screws per sqm;
/ 250 - to get the number of boxes of 250 screws required;
ceil(xxxx) - (optional) to show the number of boxes required by rounding up a decimal value to the next whole number.

3. Insulation/wool/sarking: Calculating how many rolls to purchase, when each roll has an 18sqm coverage.

UOM: rolls
Formula: ceil(area * 1.1126/ 18)

Breakdown:
area - the sqm area measured from the plan;
* 1.1126 - to allow for the 26 degree roof pitch;
/ 18 - to get the number of 18 sqm rolls required;
(optional)ceil(xxxx) - to show the number of rolls required by rounding up a decimal value to the next whole number.

Guttering and brackets

1. Half round 125mm gutter in 7m lengths: Calculating the guttering to go all the way around the roof perimeter, when purchasing the guttering in 7m lengths.

UOM: lengths
Formula: ceil(len/7)

Breakdown:
len - the perimeter around the area/s measured
/ 7 - to get the number of 7m lengths required
ceil(xxxx) - (optional) to show the number of lengths required by rounding up a decimal value to the next whole number.

2. Gutter Brackets at 1500mm: Calculating the number of Gutter brackets needed, when allowing for 1 bracket every 1500mm.
UOM brackets or ea
Formula ceil(len/1.5)

Breakdown:
len - the perimeter around the area/s measured
/ 1.5 - to allow 1 bracket every 1.5m (1500mm)
** ceil(xxxx)** - to show the number of brackets required by rounding up a decimal value to the next whole number.

The final measurement

Downpipes

1. Downpipe - allow 1 downpipe per count.
   UOM: ea
   Formula: count

Breakdown:
To quantify one part per count, the formula remains simply “count” as count x 1 = count.

2. 100mm 90 degree bends - allow 3 bends per downpipe.
   UOM: brends or ea
   Formula: count*3

Breakdown:
count - the number of downpipes;
* 3 - to allow for 3 bends per count.

3. 100mm DWV 6m PVC pipe - allowing for 8m of pipe per downpipe, when the pipe is purchased in 6m lengths.
   UOM: lengths
   Formula: count * 8/6

Breakdown:
count - the number of downpipes;
* 8 - to allow for 8m of down pipe per count;
/6 - to get the number of 6m lengths required.

For additional formula assistance, please see our Measurement Parts & labour FAQs or click on help and click send us a message to contact one of our team.