Installing a Loft in the Sobon Shed

[BCsaw]

Hey everybody. I have been going through the materials list for the Sobon shed. I want to put in a loft and want to install joists. In the book, Jack mentions framing in 4x4's for more strength. I would like to do this 3' on centre and cover with 1½" flooring. Now, my question is "Can I go with the same size joist pockets in the cross beams as are used on the sill (4x4x3)?"

Your assistance would be appreciated. Thanks.

[Brian_Weekley]

I chose to use tusk tenons with a diminished haunch for the loft joists in my Sobon shed.  I thought they would remove a little less meat from the tie beams.

[BCsaw]

That was my concern. The cross beams are substantial but is there enough "meat" for the joist pockets.

My other thought was to use a mortise and tenon.

Thanks for the pics. Its nice to see the use of the joints up front and centre!! ;D

[Jim_Rogers]

You really should ^not^ automatically assume that the loft floor joist, from the book, is the right size and spacing for your load.
You should figure it out and make them the right size and spacing and then plan the joinery based on that.

Jim Rogers

PS thanks for the edit.

[PC-Urban-Sawyer]

You really should ^not^ automatically assume that the loft floor joist, from the book, is the right size and spacing for your load.
You should figure it out and make them the right size and spacing and then plan the joinery based on that.

Jim Rogers

There, fixed it for ya...

Herb

[venice]

@Jim_Rogers

This is something i always wanted to ask the experts like Jim. Because i dont understand this part of traditional american carpentry.

Designing a joist like this means giving up stuctural strength voluntarily. There are other details in roof design that got me scratching my head as well.

What is the reason behind this design? Aesthetics?

[Jim_Rogers]

@Jim_Rogers

This is something i always wanted to ask the experts like Jim. Because i dont understand this part of traditional american carpentry.

Designing a joist like this means giving up stuctural strength voluntarily. There are other details in roof design that got me scratching my head as well.

What is the reason behind this design? Aesthetics?

I'm not sure if I understand what you don't understand?

You can't automatically assume that all loads in the garden shed are going to be the same.
Someone may store lumber up there, others may only store a few tools or a canoe for the winter.
These two items may have total different weight loads on the joists. The 4x4 joist maybe able to easily carry the canoe and a few tools, but may not be strong enough for the lumber. Depends on the lumber and what type and whether or not if it's green or dry or drying.

Every frame has to be designed for it's intended load. And when the load is unknown we design for a standard "assumed" load based on what the codes tell us is the expected load for such a space.

Keep asking questions.

Jim Rogers

[venice]

@Jim_Rogers

I´ll keep asking. Thanks.  :D

>I'm not sure if I understand what you don't understand?<

I don´t understand why a perfect, say 4x6 joist, is reduced to a 4x4(statically), cutting out the decorative? arch. Looking at it from an economical and structural point of view, the same number of straight 4x4 would serve the same purpose. No? Just a waste of time and material?

I understand that building like this today is to keep up the traditon. I also know that a building has to be designed for it´s intended use and the builder is the one responsible(as long as you´re not living in California  ;D). We are on the same page here.

What i ask myself when looking at a historic/ traditional design, is, what is the reason they went this way. How did this this particular design develop.

With all your background in traditional framing, what is your personal opinion, is it a trade off - sacrifice economics for aesthetics?

Thanks. venice

[Brian_Weekley]

Venice,

If you are asking about the purpose of the "curve" on the bottom of the joist, it does serve an important purpose and is not for aesthetics.  The curve distributes the stress along a greater number of the wood fibers in the joist.  If you were to just reduce it from 4x6 to a 4x4 directly, the joist would be much less strong and would tend to split at that location.

Brian

[venice]

Brian,

yes, that is what led me to this question. To my experience, wood under stress is failing where the long fibre is cut. That´s why i thought it might be for aesthtics. The other part is that a 4x6 is can carry more load than a 4x4.

[BCsaw]

Okay, I hear where you are coming from. I will be more specific. This "shed" will be a storage shed/cabin near my mill. The loft will see very little weight from storage items. The loads most often would be people. Is the 4x4 enough or should I go with a 4x6 joist? My thought was that the 4x4 would serve fine for the human load as compared to thousands of pounds of storage items.
The loft floor will be 1½ or 2" planks.

Also, is the joist pocket as in the sill (4x4x3), okay for use in the cross beams? Is there enough meat? Or do I need a mortise tenon?

P.S. - Thanks for the heads up Jim. I was thinking loads for the joints etc. but was not specific enough with my question!

[BCsaw]

Brian, are those 4x6's in your loft?

[Dave Shepard]

The 4x6 can be reduced at the pocket because the main stress there is shear, which is easily handled by the 4x4. Reducing the joist takes less wood out of the tie beam which is important. The main stress in the middle of the beam is bending, so having the extra thickness there is beneficial. The curved reduction is structural, and any aesthetic quality is purely coincidental.

[beenthere]

The main stress in the middle of the beam is bending, so having the extra thickness there is beneficial.

And add the benefit of stiffness to what Dave said.

And the curve lessens the shear concentration and distributes them, as I understand it.

[Jim_Rogers]

The 4x6 joist maybe needed for strength depending on the specific load and intended use of the frame.
Removing 6" of wood to create a drop in floor joist pocket in the tie beam would be excessive and leaves only 2" of wood under the joist to hold up the load. This may or may not be enough.
The Timber Framers Engineering Council, (TFEC) has been testing joist and joist pockets at the last two conferences to see what is the weakest point of the joint.
Results have not be published yet. But from my own personal observations every joint last year failed by the floor joist breaking before the bottom of the tie beam blew out.

The was a "rule of thumb" drawing give to me when I took a timber frame joints and design course at Heartwood school.
This is it:



 

There are some basic rules to insure that the joint doesn't excessively weaken either timber.
Here we see the third rule. " 'h' has to be less than or equal to 5/6 of 'D' " This sets the limit to how big the floor joist can be. When we're using a 8x8 timber it can't be any bigger then 6 and 11/16".
So a 4x6 falls in nicely.
Next we see that " 'd' has to be more than or equal to 3/4 of 'h' ". this is so that the end of the joist is large enough to support the load.
Then if this is true then 'd' is 5/8 of 'D' and therefore tells us the size of the pocket.

Now if we just cut the joist with a 90° corner instead of a curved scoop you'd have a situation where the timber can easily split:



 

So, you cut a scoop or curve cut into the bottom of the joist, for a reason. The reason is to "eliminate the point of fracture".
The timber has no spot to split and therefore is stronger.
Historical evidence has shown us that joist cut with a 90° corner split. joist cut with a scoop do not.

The ratio of how much to make the cut is shown in the drawing. For every inch you cut up into the bottom of the joist, you go over 4". So for a two inch reduction in depth you cut over 8" to create the scoop or curved cut.

I hope this has helped you to understand why the scoop or curved cut is done to the bottom of a floor joist.

If not keep asking questions.

Jim Rogers

[BCsaw]

Thanks for putting up the rule of thumb drawing. That helps.

[venice]

Thank you Gentlemen! Jim, all clear. Thanks for the details.

BCsaw, sorry for hijacking your topic.

venice

[BCsaw]

I went back to the book and found where it was suggested that the soffit tenon with diminished haunch was good the joists as it removed less meat from the beam. Duh. Did not remember that part. Same idea as what Brian used.