A reader's Asbestos question: Roof quantities?

John G.   from   Timmins, Canada   had this Roof quantities question.

I am building a roof. trusses are 6/12 pitch. Wall to wall is 34 feet x 9 feet high. Trusses are 37 feet wide and the roof is 67 feet long. Overhang is included in measurements. Can you tell me how many sheets of plywood i would need. Can you tell me if I was rightby guesstimating about 82 sheets. Your help would be gteatly appreciated

Bill's reply

John if you sign up for my newsletter,(you can cancel it straight away) you will get access to my free E-book "Using a calculator etc". Above is a part of it.
It goes into what I do below in more depth.
The basic point to get out of this is that you can easily get the ratio of the rater length to the plan length, and from that one ratio apply it to the whole roof area. You do not have to calculate each section separately. This is if all the roof surfaces are at the same angle.

I make it 87 sheets.
I did it with a calculator, but you can do it easier with a spreadsheet.

Your pitch is formed by a right triangle 12 on the base and 6 high.
Get the sloping side by Pythagoras.
The sloping side is 6x6=36 plus 12x12=144 = 180
punch in square root of 180 = 13.416


So for every 12' run on the plan the rafter is 13.41'
the ratio is 12:13.41
divide 13.41 by 12 = 1.118


From what you say your roof on the plan is 37' x 67' 37 x 67 = 2479 sq ft on plan.

you know that for every ft on plan the roof has 1.118 ft.
2479x1.118=2771.5 sq ft.
1 sheet = 8x4=32 sq ft
2771.5 div 32 = 86.61 sheets.
If I have misread your sizes or the size of your sheets just alter and recalculate.

In John's example above I used the way of getting the rafter length by using Pythagoras, because I was given the run to rise ratio.

If it was a UK or Aussie job I would most likely be given the roof pitch angle instead. In that case I would use a calculator and the cosine rule as shown in the above grab from the e-book.

Cheers Bill.