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Old 11-05-2008, 11:39 AM   #1
DigitialFusion
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Default Roof Sheathing Calculation?

Hey all,

I did a few calculations based on some formulas from both here and pathagoras. However, my calcuations are coming out way higher than what I was told I needed by Menards (a regional Home Depot/Lowes like store). They told me I needed 45-4' x 8' x 1/2" sheets of OSB for the roof. My calculations come to 52 sheets. Am I doing something wrong or did they screw up?

32x45 garage, with 2 foot overhangs and 1 foot flys.

Anyone else care to do the math and see where you are coming up?
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Old 11-05-2008, 12:20 PM   #2
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Default Re: Roof Sheathing Calculation?

well I decided to just bite the bullet and model it in SketchUp. Im pretty confused how to lay it out tho... my garage with flys is 47'. There doesnt seem to be any good way to layout the sheathing to get it to line up on the trusses.
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Old 11-05-2008, 12:31 PM   #3
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Default Re: Roof Sheathing Calculation?

what's the roof pitch? I had to get more than calculated for mine since there was just too much waste, it was only a few sheets though.

Here's menards calculation - it doesn't take into account the overhangs. [(16x45)/32] x 2 = 45 sheets

Here's my calc. [(18*47)/32] x 2 = 52.8 = 53 sheets
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Last edited by dipper; 11-05-2008 at 12:39 PM.
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Old 11-05-2008, 12:34 PM   #4
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Default Re: Roof Sheathing Calculation?

sorry, forgot to specify that... its 4/12

Last edited by DigitialFusion; 11-05-2008 at 12:38 PM.
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Old 11-05-2008, 12:44 PM   #5
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Default Re: Roof Sheathing Calculation?

is there any reason I cant do something like this with my trusses? Have two close together like that as long as the rest are 2' O.C?
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Old 11-05-2008, 12:45 PM   #6
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Default Re: Roof Sheathing Calculation?

nevermind... that wont work
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Old 11-05-2008, 12:59 PM   #7
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Default Re: Roof Sheathing Calculation?

I got it... a variation of the above... will have a count soon
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Old 11-05-2008, 01:06 PM   #8
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Default Re: Roof Sheathing Calculation?

okay, i'm no pro but here goes a try at a better calculation.
cos(18.4)=16/x; x=16.86' or 16'-10"
2'oh: cos(18.4)=2/x; x=2.1' or 2'-1"
add those together to get 18'-11" (round up to 19')
45' + overhangs = 47'.

total roof area = 47 x 19 = 893 sqft.
# 4x8 panels = 893/32 = 27.9 = 28.
that's only half the roof, gotta do the other side.
28x2 = 56 sheets.

45 is too little.
you will be somewhere between 52 - 56 sheets.
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Old 11-05-2008, 01:10 PM   #9
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Default Re: Roof Sheathing Calculation?

exact count is 60 sheets
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Old 11-05-2008, 02:54 PM   #10
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Default Re: Roof Sheathing Calculation?

I was always told to brick lay sheathing, don't know if that is relevant... and also to leave 1/8" for expansion and contraction at the seams...
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Old 11-05-2008, 02:58 PM   #11
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Default Re: Roof Sheathing Calculation?

the seams are staggered about 2'7"... is that good enough or will the inspector not go for it? I cant find it in the codes.

EDIT: the reason it looks like all the seams line up is 'cause I modeled the sheathing just slightly below the top of the trusses so I could see where they lined up.
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Old 11-05-2008, 03:14 PM   #12
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Default Re: Roof Sheathing Calculation?

If your trusses are 2 O.C. how can you get 2'7" stagger between joints?
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Old 11-05-2008, 03:50 PM   #13
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Default Re: Roof Sheathing Calculation?

47'-0" Roof length divided by 8' sheets = 6 pieces per row. 1 sheet will be cut in half in every other row to give you a 4'-0" stagger between rows. Use one half at one end and the other half at the other end of the same row. A lot of inspectors will reject sheathing that does not span at least three joists or trusses.

18'-0" horizontally from fascia to ridge at 4/12 pitch = 18'-11 11/16" or basically 19'-0" along the slope. Call it 20'-0" for easy estimating purposes .

20'-0" divided by 4' sheets = 5 rows.

6 pieces per row x 5 rows = 30 pieces per side x 2 sides = 60 pieces of sheathing.

Set your trusses at 2'-0" centers to line up with 4'-0" staggering of joints in sheathing. You have some trimming to do at each end but it won't be too bad. You'll also have to about 1' off of each piece at the ridge.

Make sure the sheathing is rated (and marked) for 24" truss spacing and that you have H-clips half-way between each truss at each joint between sheets.
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Old 11-05-2008, 05:42 PM   #14
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Default Re: Roof Sheathing Calculation?

I have it set up now with each sheet spanning at least 3 trusses with 2'0" staggers. Is that going to be a problem?
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Old 11-05-2008, 06:16 PM   #15
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Default Re: Roof Sheathing Calculation?

Quote:
Originally Posted by DigitialFusion View Post
I have it set up now with each sheet spanning at least 3 trusses with 2'0" staggers. Is that going to be a problem?
Perfect...
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Old 11-05-2008, 09:32 PM   #16
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Default Re: Roof Sheathing Calculation?

Quote:
Originally Posted by DigitialFusion View Post
I have it set up now with each sheet spanning at least 3 trusses with 2'0" staggers. Is that going to be a problem?
I'd go with 4ft staggers. This works with trusses/rafters that are 2'OC or 16"OC. This puts the seams on each row in the center of the rows above and below it.

FWIW On my garage design (25x42' w/ 1 ft overhangs) this pattern produces the same "amount" of waste but the waste is of more consistent lengths.

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Old 11-05-2008, 10:57 PM   #17
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Default Re: Roof Sheathing Calculation?

If I go with 4' staggers I have a LOT more waste and have to get a LOT more sheathing. Mine garage is an odd number length. If it was 44 or 46 foot instead of 45 I could do 4' staggers without the added waste.
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Old 11-05-2008, 11:16 PM   #18
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Default Re: Roof Sheathing Calculation?

Actually your roof is 47' long overall. You can get away with any sort of stagger that you want. It is going to take you 6 sheets to run one row. If you elect to start any row with a 3 foot piece (1 foot fly plus 2 feet to get to second truss in) the remainder of that piece will be 5 feet long, more than enough to finish the other end of that row. If you start a row with a 5 foot piece (1 foot fly plus 4 feet to get to the third truss in) the remaining 3 foot piece will finish out the other end of that row. You can run any stagger that you want, you will always have about 1 foot of waste from each row. Clear as mud huh?
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Old 11-05-2008, 11:24 PM   #19
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Default Re: Roof Sheathing Calculation?

I'm not sure why you're overthinking this so much. Calculate the area to be covered (using pythag etc), start your layout at one end and mark 2' o.c. all the way across. The last truss before the gable end won't end on the right o.c. but don't worry about it. Either run past it over the barge rafter or cut back the second last sheet so your last seam isn't over the soffit.

A few things to note:

Framing is a system that is designed to work for other building products. Sheathing at 8', insulation at 2' (22.5"), drywall etc. You're going to end up cutting sheets, don't sweat it.

When they say on center ~o.c. it means that you are going to offset your fist 24" measurement by 3/4". That is half the width of a 2x piece of lumber. This is done so that when you start your row of sheathing, they will span the distance between the next 8' and land half way on that joist. ~Sorry if you already understand this but it's a mistake commonly made by 'DIY enthusiasts'.


I could quite easily se an employee of a DIY'er supply store overlooking calculating the roof overhangs in his/her calculation. How far is the trip back to the store form your build site?
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Old 11-06-2008, 09:55 AM   #20
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Default Re: Roof Sheathing Calculation?

Boiler has it right.
Inspectors like to see the same thing as they have seen before.
The idea is not to just cover the top, but to stiffen the whole structure.
Lay them horizontal to overlap as many trusses per sheet as possible.
Half sheet over lap for the same reason.
The odd sized holes should be in the center one third of the row so the lager pieces are at the ends to stiffen the whole thing against racking.
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