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Author Topic: Square Rule and Sills  (Read 1510 times)

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Offline Michael NY TF

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Square Rule and Sills
« on: November 14, 2014, 09:43:46 AM »
I am interested in several questions related to square rule as it applies to sills/plates and floor joists. Specifically:

  • Why wouldn't one cut a 'housing' in the top of a sill plate where it meets a post, using the bottom of the sill as a reference face? If sill plates vary slightly in height, for example one is 8-1/8", another is 8", or has crown, wouldn't cutting down to 7-1/2" uniform height from a bottom reference face even things out? I've never seen this in drawings.
  • The same applies to top plates - are they typically housed/cut down in height using the top face as a reference in order to accommodate for variations in timber sizes and any crown?
  • If you are using 4 x 6 drop in tenon floor joists, would you cut the ends down to 3-1/2" in width in order to have uniform mortise/pocket sizes? Again with the question of uniformity/efficiency, and not cutting all your pocket mortises to 4" to find out later that slight variations in timber size impacted you.

Thank you!

Offline woodworker9

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Re: Square Rule and Sills
« Reply #1 on: November 14, 2014, 10:06:10 AM »
Sobon's book recommends this very thing.  Basically, using a width that can be found in every single post, like 7.75" or 7.5" allows you to make sure your frame is square when you're done.  I am reading the chapter on the Sobon Shed right now, over and over, as I prepare to build a modified version of it on my property, so this is fresh in my mind.  I believe he calls it "housing" the joint.

Others with way more TF experience should be along shortly, but it certainly makes a lot of sense from a mathematical point of view.  How else will you keep your frame square?
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Offline timberwrestler

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Re: Square Rule and Sills
« Reply #2 on: November 14, 2014, 10:59:13 AM »

What you're asking about is the fundamental theory of square rule.  I would also recommend reading Sobon's books.  The timbers are reduced where they intersect.

You could do that on the sills, but you need to thing about the floor system.  Ideally the sills are all the same.  If they weren't and they were supported intermittently (like on piers), then I would reduce the bottoms.  If it was continuously supported, then I would perhaps do what you described, and hire a new sawyer.

Yes on the plates.

Yes on the joists too although I would reduce the joists even more to 3" (aim for leaving 2/3 to 3/4).  The other side of the equation is how much meat you're removing from the beam where the joists are coming in.

Offline Michael NY TF

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Re: Square Rule and Sills
« Reply #3 on: November 14, 2014, 03:22:08 PM »
Thanks for the quick answers!

I'm definitely familiar with square rule theory in general, but haven't seen examples of reducing the thickness/width (not height) of floor joists and reducing the top of the sill where it meets the posts. I'll dig in more.

Offline Jim_Rogers

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Re: Square Rule and Sills
« Reply #4 on: November 14, 2014, 03:39:04 PM »
Sills and floor joists are a bit different then posts and plates.
First of all you have to understand square rule layout and which faces are the reference faces, which are the adjacent faces and which are the opposite reference and opposite adjacent.
Normal layout rules say that the timber is reduced in size on the opposite reference and opposite adjacent faces. Not on the reference face and usually not on the adjacent face. But there are exceptions to every rule.

On sills and floor joists the side of the timber that the decking will be nailed to is a reference face. That way all the joints are flush to that face and all the decking will lay down flat.
The adjacent faces to the two long sills and the two cross sills in the Sobon shed are the outsides of the building. Those sides have to be flat for the siding to hook to.

So that leaves only the opposite reference and opposite adjacent faces to have the reductions or housing cut into. On a sill if it wasn't truly 8x8 on the end then the thin end would have to be "shimmed" up to make the sill level with the thick end. Once the long sills are shimmed or leveled by whatever means necessary then the cross sills will automatically be level if the joints are cut right.

When Jack Sobon taught us to do drop in floor joists he suggested we cut all of them first, before the long sills were cut, with the drop in floor joist pockets. Then after all the floor joist are done, we'd measure each one and find the smallest one in thickness. Let's say for example we found one on one end to be 3 7/8". All the rest were 4" or larger. What Jack would have us do is make all the drop in floor joist pockets in the long sills 3 7/8" so that they'd all be the same.
And reduce each "fat" joist at the end to 3 7/8" so that all the joist ends would be the same.
That way we wouldn't have to number or test fit each and every drop in floor joist. They would or could go anywhere in the floor system.

What I do when I cut a floor system is to cut all the drop in floor joist pockets 4" wide and then reduce any floor joist that is wider then 4" to 4" by hand planing off the opposite adjacent side.
That way all my floor joists will drop in and they don't have to be custom fit or labeled. If a floor joist is 3 7/8" then it should drop in no problem. And once the decking is nailed down to all the joists and sills nothings going to move.

I don't have time to post any pictures of this today.

But I hope my descriptions have explained it to you.

Jim Rogers
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Offline Heartwood

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Re: Square Rule and Sills
« Reply #5 on: November 17, 2014, 08:56:00 AM »
What Jim said.

Top of sills should be the same level as top of joists and the same all around; shimming the underside where necessary is not hard. Housing the post bottom is harder and unnecessary.

Reducing the width of joists to the smallest one is our standard practice; same with rafter pockets. Our sawyers provide material within 1/8" of nominal, so even cutting the pockets to nominal (say 4") is OK with a few narrow ones being a little loose (flooring locks it in) and spokeshaving the wide ones down a bit. Shaving all down 1/2" under nominal in width is too much work and unnecessary.

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