 09 May 2022
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Roofing Formulas
 Updated on 09 May 2022
 3 Minutes to read

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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/Leanto, etc.
The formula given is a guideline to be used to calculate your measurement parts and labour.
 If there is more than one pitch on the project, each pitch will need to be a separate measurement and the parts & labour built into each measurement.
 The formula bar follows the PMDAS Rule: Parentheses first, then Multiplication, Division, Addition & Subtraction, respectively.
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
 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
.
 100mm 90 degree bends  allow
3
bends per downpipe.
UOM:brends
orea
Formula:count*3
Breakdown:
count
 the number of downpipes;
* 3
 to allow for 3
bends per count.
 100mm DWV 6m PVC pipe  allowing for
8m
of pipe per downpipe, when the pipe is purchased in6m
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.