Plastic Wood Calculator

How to calculate plastic wood materials for your decking subframe

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To determine how much Manticore Plastic Lumber you will require to build a suitable subframe for your deck use the method below:

Calculating plastic wood for deck substructure

Step 1

Start off by measuring your proposed decking area(s) width, length and height off the ground

Step 2

Based on the square meterage of the area(s), multiply this by 4 and add 10% for wastage to determine the total linear meters of deck bearers required.

Step 3

Divide the total linear meters of bearers, by the individual length of bearer you require (3.1 or 3.4m) to get the total quantity of bearers
Example 20m² Decking area:
( 20m² x 4 ) x 1.1 = 88 linear meters of bearer
88LM / 3.4m length bearer = 26 bearers (rounded up)

Step 4

You now need to decide on the type of bearer profile you require: 50x50, 50x100 or 50x150mm, as this will determine the required structure support. The allowable deck height and the ground conditions will also determine what support system would best suit your build, either:
  • 100x100mm posts to be cement into soft ground, or;
  • Adjustable support pedestals placed on hard flat ground
Bearer support span
One factor to take into account will be how often the bearer profiles require supporting:
Bearer ProfileMax. Support SpanPedestals per m²

Step 5

If the deck frame is planned to sit on top of a flat roof, it will also be important to have the correct amount of protective rubber mats for each support in order not to damage the roof membrane