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Joist Sizing #25072 01/08/11 04:02 AM
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Andre L Offline OP
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I have a question regarding joist sizing and spacing. I see there is already and old thread on this topic but it doesn't seem to work when I click on it. I'm going to build the shed out of Jack Sobon and Roger Schroeder's book 'Timber Frame Construction' and use it as a cabin. I want to increase the width of the cabin to 14'. The book recommends that you increase the size of the joists to do this but doesn't specify the size needed. Am I looking at 4"x8" joists? I'm assuming that spacing the 4"x6"'s at 16" o.c. won't solve the problem as it's joist sag that is the issue. Thanks.

Re: Joist Sizing #25077 01/08/11 01:32 PM
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Jim Rogers Online Confused
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What type of wood would you like to use for the floor joists?

As a cabin, the floor load could be considered a "first floor" combined load of 50 lbs per sqft(40 lbs live load, 10 lbs dead load).

Jim Rogers


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Re: Joist Sizing #25078 01/08/11 03:29 PM
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Paul Freeman Offline
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I haven't looked at the cabin in a long time but I seem to remember the joists running over the tops of the beams? If not then the longer spans can create problems in any notching of the underside of the ends of the joists. There are some "relatively" simple calcs that can be made to determine the allowable notch size or you can "scoop" the material out and gradually diminsh the timber size to your beam pocket's depth... within reason.

By the way, if you're using pine and it's a first floor load as Jim defined above, the 4x8's would work, but they're bouncy with a 1/2" of deflection fully loaded.

Re: Joist Sizing #25084 01/08/11 05:22 PM
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Andre L Offline OP
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The wood would be Red Pine.

The plans in the book call for 4x6 joists cut down to 4x4 where they bear on the sills to fit into 4x4x3 inch pockets. Can I fix the problem of the bouncy floor by adding another plate across the length of the cabin to cut the span of the joists down to 7'?

Re: Joist Sizing [Re: Paul Freeman] #25092 01/08/11 11:39 PM
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Jim Rogers Online Confused
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Originally Posted By: Paul Freeman
I seem to remember the joists running over the tops of the beams?


The first floor joists are drop in joists into the two long sills, with a tying joist in the middle. As mentioned they are dropped into 4x4x3" pockets.

If the flooring is something line 2" tongue and groove you will spread the floor load out over several beams and hopefully reduce the bounce factor. My calcs show a 7/16" deflection in the middle of the floor (not considering spreading the load).

Red pine would be grade #2, size 4x8 at 16" oc.

You can add a center long sill if you wish but it will need to be supported at both ends and probably the middle as well to reduce the bounce.

Good luck with your project.

Jim Rogers


Whatever you do, have fun doing it!
Re: Joist Sizing #25093 01/09/11 12:58 AM
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Paul Freeman Offline
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Hmmm, I wouldn't recommend a 4" notch in an 8" joist without checking horizontal shear. FWIW, in white pine, a 6x8, at 48" o.c. with a 40# total load doesn't make horizontal shear.

Last edited by Paul Freeman; 01/09/11 12:59 AM.
Re: Joist Sizing #25099 01/09/11 03:14 PM
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Jim Rogers Online Confused
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Here is the standard detail we use for the long sill drop in floor joist pocket:


Last edited by Jim Rogers; 01/09/11 03:15 PM. Reason: wrong label

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Re: Joist Sizing #25104 01/10/11 12:54 AM
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Paul Freeman Offline
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Jim, I'm not worried about the sill but rather the split that occurs in the joist at the underside of the notch, I refer to it as a horizontal shear failure, but my engineering school training only lasted long enough to make me dangerous.

Re: Joist Sizing #25116 01/10/11 01:56 PM
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Yes, Paul, I see your point, now. And I agree that there is reason for concern.

Of course the joist could be full length (no center sill) and 4x6 if supported underneath the middle with a hidden sleeper timber, that is supported by some foundation pier at each end and in the middle.

And the proper joist end should be cut to prevent the splitting of the joist.



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Re: Joist Sizing #25119 01/10/11 06:04 PM
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Why not cut the pocket deeper? With a 8x8 sill you would still have 2" below a 6" pocket. If I recall, Brungraber once said 2" under a pocket would work. Anyone else heard of this? What is the rule for pocket depth? 5/8 the depth of the sill is one choice so you could safely go to 5" deep pockets on 8x8's. How about 9" deep sills?

I see a ratio of 1:4, I also see a steep curve leading to the reduction. At least this is the case with my adze, I could not cut that curve with it. It would have to be extended for the adze to work properly. When ever I get the chance I put them in upside down and adze to the top of the sill then flip them over. In general I use the length of the adze head plus the width of the blade back from the end to gauge where to start hacking. This is adjusted to how deep I have to go.

I just noticed the "or more" note. Well, maybe it would be close, the curve. At one time I measured out the curve, silly me.

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