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Old 08-31-2010, 06:49 PM  
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Default 4x8 joists in my woodshed

About ten years ago I built a woodshed on the side of the hill where I live. It has 9 posts, (10 inch diameter fir log sections), supporting three good sized (>12 inch) log beams parrallel to each other. The logs are 24 feet long. Since they are supported by posts in the middle, they effectively span about 12 feet.

Running across the log beams I have 4x8 full dimension douglas fir joists on 24inch centres. They span about 12 feet. The floor over them is 2x6 fir planks.

The total floor area is about 24 feet by 24 feet, and the 4x8 joists each span about 12 feet, since there's the log runnding down the middle.

I'm using one quarter of this area to store firewood. This year I have more than usual - about four cords of various types of wood. Some of the new stuff is pretty heavy - pitch filled fir and arbutus. I'm wondering how much this floor can support.

I'm guessing the wood averages 2 tons per cord, or a total of 8 tons. That would mean the floor is supporting an average of about 110 lbs per square foot in that quarter of the shed. (144 square feet X 110 lbs = 15,840 lbs).

Does anyone have any idea about how safe this is? I've only got three of the cords in there, and I'm wondering about the fourth, which is arbutus, the heaviest of the wood.

I believe the logs are safe, and the posts are well braced, but the 4x8 joists - don't forget, they're full dimension - they might be close to their limit. It still feels very solid to walk on. It really seems pretty solid, but I'd like to be reassured with the math. I couldn't find much on 4x8 on the web.

Any ideas appreciated.

Last edited by camb; 08-31-2010 at 07:00 PM.
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Old 09-01-2010, 05:50 PM  
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If the joists are df-north-#2, with a 1400fb, and a 12' span, they will support 198# per ft. = 2'o.c. = 99# per sq. ft. or 2384# in 12' of joist/ o.c. spacing. The plank flooring is giving me a little more trouble finding my reference...... looking.......
Here is an older ref. pp. 734, second table, need to ratio it up:


Last edited by GBR; 09-01-2010 at 06:02 PM.
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Old 09-01-2010, 07:07 PM  
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Thanks Gary,

That is great informtion - just what I was looking for. I tried your reference, but it won't let me go to pp 734.

Anyhow, your information looks reasonable.

There are six joists involved under where I have stacked the firewood - your calcs say they can hold about 14,000 lbs, so it's close if I want to put the final cord in.

As it is, one of my rows of firewood is right over an outside log, and another is right next to the centre log. Therefore, only about 2/3 of the firewood is fully on the joists. In any case, with your info, and my caution, I've decided to put the last cord outside under a tarp. I think it would go inside with no problem, but why risk even a slight chance of a failure?

Thanks again for your help.

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