How is the weight of wood calculated in psf?

Question

for example i found a chart online showing me that roof rafters that are 2'x6' @ 24 o.c. are 1.1 psf, but i dont get how its calculated in psf. Can someone explain it?

Answer 1

Basically what the chart is saying is that if you were to frame the roof with 2X6's, 24" on center and you add up the total weight of all the 2X6's, then divide by the total area of the roof, you get the value of 1.1 pounds per square foot, the 'dead load' of the beams.  

This is sometimes important to know because a roof or floor is designed to carry a dead load (beams, sheathing, shingles, drywall, etc) plus a larger 'live load' (wind, snow, etc.).  You don't want your dead load to be too high and reduce the live load to an unacceptable degree.

Answer 2

The value given in psf is the equivalent normal pressure loading due to the weight of the rafters. If the rafters were to be horizontal, The normal pressure due to the weight of the rafter spaced 24 inches apart would be:

P = 2 x 6 x 48 /(12 x 12 x 24/12) =  2 psf

(The density of white pine is about 48 lbs per cubic foot. If heavier wood is used then use the density of that wood).

However, since the rafter is inclined at an angle A with the horizontal then the normal pressure Pn is:

Pn = P x cos A

Apparently the inclination of your roofing is about 56.63 degrees  so the normal pressure equivalent of your rafters is;

Pn = 2 cos 56.63 = 1.1 psf

Answer 3

Perhaps you could supply the link to the table.  That would help understand your question.  The weight of wood is in pounds whereas psf, to me, means pounds per square foot and more likely designates what the roof would support.  But the 1.1 psf seems way too low, usually a roof structure might be more like 100 or 200 psf.  

Think of 2 feet of wet snow on a roof.  On one square foot of the roof area that could be 50 pounds or so.  So the 1.1 psf makes no sense to me.

EDIT John may be on the right track

Answer 4

When calculating the roof load this is what the 2"X6" 24" centers would amount to. The rest of the materials in the roof would be spelled out the same so as when it summed it would give you the weight of the roof.