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11 thoughts on “Drop in Floor Joist”

  1. Hi Jim! Thanks for the advice on the my floor joists! I intially was going to just drop in the joists to my longitudinal sill plate. But according to the publication on the TFG site, taking that much material off the top edge of that sill will reduce its strength by 15-40%.

    Again, Im no expert. Im just trying to build it the best I can, so what is your opinion on the figure the TFG is throwing out there? My reference for that is on the TFG website under publications. Im sure you are extremely familiar with the guide: HISTORIC AMERICAN TIMBER JOINERY: A Graphic guide. Chapter, page 24, under “Tenoned Joists” section.

    Would love to hear your opinion on that because I believe your route would save me so much time, yet i dont want to sacrifice the strength of it regardless of size. Thanks so much for taking the time to share your expertise with me!

    Francesco @ Yellow Barn

  2. So you’re going to make a cabin. And it’s 20′ long and 16′ wide.

    I would assume three bents, as you are using Jack Sobon’s shed as a basis for your design.

    If you have any kind of a foundation then the sill(s) will be continuously supported. Then there should be any problem with cutting away a portion of the sill for a drop in joist.

    Ok, so let’s say it is not a continuously supported sill, and that you’re only going to support the sill where the post are. Like six points with sono tubes. Then the distance that will not be supported will be about 9′. I would say 10′ but you’re going to use at least a 10″ diameter sono tube maybe even a 12″ one.
    So the unsupported length isn’t going to be that much, like I said about 9′.

    And it’s a cabin. First floor load on a cabin is usually about 60 lbs “live load” and about 10 to 15 lbs “dead load” (dead load is the materials that actually make up the floor itself). So you have a combined load (dead load and live load added together) of about 75 lbs per square foot.

    Your drawing on your site shows a 6×8 floor joist. But it doesn’t say the area it will support. Like 2′ or 16″ on center (oc). So let’s just say 2′ oc for figuring.

    So the area one floor joist will have to support is 2′ wide (half way to the one next to it on either side) and 16′ long, less the long sills which are 8″x10″. So 192″ less 16″ or 176″ of un-supported span.
    176″ is 14′ 8″ x 2′ for square feet = 29.34 sqft. times 75 lbs = 2200.5 lbs on one floor joist. But half of that is supported on each end. So one end has to support 1100 lbs.

    If we look at the load value for the joist will support perpendicular to the grain where the joist end will sit on the drop in joist pocket we see that these values (from the NDS book) are very high per square inch.

    I can’t find what type of wood you’re going to use, but being that you’re doing this in Southern CA, I’m going to assume you’ll be using Douglas fir.

    The value for load perpendicular to the grain on Douglas fir are range from 520 to 625 for grade two timbers in the post and timber category.

    Have I lost you yet?

    That’s 520 lbs per square inch so in order to hold up your floor joist on one end you’re going to need (1100 / 520 = 2.11 square inches of area. You’ve got a 6″ wide joist and if you cut a pocket into long sill only 2″ then that area is 6×2=12 square inches. Your drawing shows a one inch housing. That’s 6″ x 1″ or 6 square inches x 520 = 3120 lbs of support. Three times what you need.

    I know you’re concerned about weakening the sill but you only need to cut a 1″ deep by 6″ wide drop in floor joist pocket to support the load you need to support.

    Again your long sill is an 8×10. If you measure it at the joist pocket it is now a 7×10. That’s a very small amount being removed.

    However, if you wanted to be sure you had a sill that’s big enough we’d have to “run the numbers” and see what size sill you’d need in order to correctly support the floor joist to make your cabin.

    In order to actually do some beam sizing calculations we’d need to know for sure what type of wood you’re going to use to make your cabin sills and floor joist, including grade.

    I hope I haven’t confused you with this story.

    Jim Rogers

  3. I love the detailed pictures you have on this site, really nice.

    I have nothing bad to say about the drop in floor joist but I think it should be complemented with a dovetail joist now and then to keep the sill plate frame from bending outward, which could lead to the floor caving in.

  4. We use a joist with a tenon on it to “tie” the sills together.
    We call it a tying joist.

    Sometimes we use this joint as well for the second floor support system. To help hold the bents together.

    Thanks for your comments.

    Jim Rogers

  5. Thanks for posting this forum. I know it is a while since this was first posted, but just wanted to confirm that this information regarding the drop in and dove tail floor joist is accurate. I live in a 250 year old colonial that has drop in floor joists. Over many years, the sill moved enough to allow the joists to “drop out”. A dove tail joist would have been handy. Hard to call back the builders though.
    I have a reconstruction project ahead and will most likely become a regular forum viewer.
    Blessings. Any information would be appreciated.

  6. Dave,
    It would be nice to go back and time and pick the brains of the builders of the timber frames. Now a days we tend to not use the dove tail that much but it sure would have helped your frame out. I have seen on some frames where every other joist has been dove tailed. The problem with that joint is the amount of time it takes to cut.
    We also screw the joist to the beam using a good structural screw by GRK or ASSY which would hold everything together.
    Let us know if you have any joints you would like us to create for you.

  7. I’ve always thought that our ‘western style’ approach to drop in joints didn’t take into account the forces of uplifting that may occur in a tectonic event. Do we have a version of this joint that resists the uplifting force? A trenail might tie the pieces together, but it would need some kind of a wedge inlet to resist popping out. Does there exist such a solution?

  8. Mark: As all these connections will be hidden by the flooring materials/deck you can secure the joist to the drop in pocket with any proper long screw. Similar to a timberlok screw. This will prevent any uplift. No one will know except you and your engineer.
    A timberlok style screw my have a flat head to make it flush. And you may need to pre-drill a hole. Try and get at lest 2″ of screw threads into sill, so plan your screw length accordingly.
    Jim Rogers

  9. Hi there,

    What are the methods used to cut the “scoop” on these joists and for the matter, the rafters? Do timber framers typically use morton cuts and chisel? or an adze maybe?

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