Author Topic: Jetty/overhang joinery detail (Read 861 times)

Jacobezzell · « **on:** April 04, 2019, 03:29:51 PM »

This is what I've been working up for my big playhouse, based on the drawings I see in Benson's "Building the Timberframe House" book for an overhang. In my other thread there were a couple of questions about the strength of the joints and whether the mortises would weaken things too much. I think the concern was the mortises in B for Post A and Girt C? 1.5" tenons.

In another design I had lowered C so it was shouldered into A, But then it wasn't clear to me how I would strongly support the 2nd level flooring, and still have a solid surface to skin.

All timbers except D are 6x6, D is 4x6.

I havn't gotten as far as adding all the braces to my design yet. but was planning on braces from A to B and A to C. There was also a suggestion to add knee braces from E to B and E to D?

D L Bahler · « **Reply #1 on:** April 04, 2019, 04:03:53 PM »

I'd raise b+d so b sits on top of c, which extend over top of a. But that's the Central European in me

Jim_Rogers · « **Reply #2 on:** April 04, 2019, 04:10:30 PM »

The tenons are too short. Tenon on B should be a through tenon.
Tenon on C should/could be a through tenon.
You have to have enough wood beyond the peg hole to prevent pull out.
Do you really want the upper post to hang down below B?
It would be stronger to set E on top of B.
but that's just my ideas.
Jim Rogers

Jacobezzell · « **Reply #3 on:** April 04, 2019, 08:13:37 PM »

Quote from: D L Bahler on April 04, 2019, 04:03:53 PM

I'd raise b+d so b sits on top of c, which extend over top of a. But that's the Central European in me

If you do that, how do you frame the floor joists on the 2nd floor? do you run a summer beam or something from D across the top of C? So that beam sits parallel and at the same height as B?

Jacobezzell · « **Reply #4 on:** April 04, 2019, 08:19:18 PM »

Quote from: Jim_Rogers on April 04, 2019, 04:10:30 PM

The tenons are too short. Tenon on B should be a through tenon.
Tenon on C should/could be a through tenon.
You have to have enough wood beyond the peg hole to prevent pull out.
Do you really want the upper post to hang down below B?
It would be stronger to set E on top of B.
but that's just my ideas.
Jim Rogers

This was what I was going off of, from the Benson book. I did like the look of the drop on the overhang posts... They don't show a through tenon but this could just be outdated.
If I added knee braces would that help? I know it wouldn't affect the shear of pulling out the wood beyond the peg, but it would reduce the racking of that joint, more than just having a standard brace at the top?

ljohnsaw · « **Reply #5 on:** April 04, 2019, 08:35:27 PM »

Quote from: Jacobezzell on April 04, 2019, 08:19:18 PM

This was what I was going off of, from the Benson book.

That does look really nice, but I'm guessing that the post is something like a 8x8 or bigger. Then you have something to work with for the joints. You can always scale up but not down in most cases.

Jacobezzell · « **Reply #6 on:** April 04, 2019, 11:47:41 PM »

Quote from: ljohnsaw on April 04, 2019, 08:35:27 PM

That does look really nice, but I'm guessing that the post is something like a 8x8 or bigger. Then you have something to work with for the joints. You can always scale up but not down in most cases.

makes sense, tho I thought I could get away with a 6x6.

I redid it as a through tenon, and got my braces added. Does that seem more reasonable?

Don P · « **Reply #7 on:** April 05, 2019, 07:46:44 AM »

My concern is at the post mortises at ABC. I like your idea of dropping C or D L Bahler's idea of running C out on the post and setting B on it, I like that idea better, you're not damaging B with a mortise over the post then. Your smaller joists could run over C to D from the mid joist. The smaller joists could be notched for 1x blocking or 6x blocking to drop into shallow dados for draft sealing.

Do check B as a beam overhanging a post with a point load on the end.
http://forestryforum.com/members/donp/ptloadohang.html
This is all guesswork until you quantify things. Although if the smaller joists run across C to D you've just picked up another possible redundant support path. I would not count on it unless B failed, load goes to stiffness and B-E would be the stiffest load path initially compared to small joist over C to D-E.

ljohnsaw · « **Reply #8 on:** April 05, 2019, 09:29:30 AM »

No one is going to mention the horizontal tenon on C? I always though that was bad form. Leads to C splitting open along the tenon if the joint is not perfect.

Don P · « **Reply #9 on:** April 05, 2019, 09:36:24 AM »

Hopefully it goes away but point taken, it appears in the mid joist. Running C and D above B might be worth exploring.

Mad Professor · « **Reply #10 on:** April 05, 2019, 10:01:45 AM »

The shouldered joint on the bottom of the top posts is better but you can put a 2nd tenon there. Make those posts 6" X 8" so the shoulder has more to bear on.

Put the exterior ties below that joint with a full tenon.

For lower posts don't put you're ties in same place, then you can use full tenons.

I'm sure many will have better solutions

Jacobezzell · « **Reply #11 on:** April 05, 2019, 10:28:32 AM »

Quote from: ljohnsaw on April 05, 2019, 09:29:30 AM

No one is going to mention the horizontal tenon on C? I always though that was bad form. Leads to C splitting open along the tenon if the joint is not perfect.

Quote from: Don P on April 05, 2019, 09:36:24 AM

Hopefully it goes away but point taken, it appears in the mid joist. Running C and D above B might be worth exploring.

Ljohnsaw the tenon at the end of C is fully housed, doesn't that greatly reduce any chance of a split forming from the tenon? Don P do you mean the failure would appear mid joist on C?

I'll look at dropping C 6" on the post so I don't have all the mortises on B. That will also offset the braces on the post 'a'.

Mad Professor are you saying to make a pair of through tenons on the end of B?

D L Bahler · « **Reply #12 on:** April 05, 2019, 10:41:52 AM »

If working with small timbers, my thought would be to use the example of a tradition that works with small timbers. The Germans do that a lot, and they are key to set things atop each other with light cog joints (they call these "combs") rather than mortise and tenon things together, whenever practical. I think this is because as ljohnsaw points out, you can't scale joinery down very well, you have to redesign it. (Technically, the Germans lucked out here. They already used joinery that just so happened to work well with smaller wood when they made the switch, but I digress)

With this sort of system,
you primary lower posts would join into the plate above with stub tenons, probably 2 inches

I'd actually put b onto the posts. Raise C and D to just above, maybe with 1 to 1.5" cog into B for C and other joists. That is if you want to have timber joists. Maybe chunk up E a little but if you really want the pendant, for the sake of joinery. You could use the extra size to add some nice ornamentation further up as well.

D L Bahler · « **Reply #13 on:** April 05, 2019, 11:00:03 AM »

Here's a quick mockup
Nothing is refined, just wanted to clarify what I was talking about with example

Mad Professor · « **Reply #14 on:** April 05, 2019, 04:19:18 PM »

It has many weak sisters. The joinery is not strong.

I was learned from Dick Babcock, but he was a happy drunk I learned a lot from.

Me too, framing and drinking!!!

So didn't lots that didn't, like Ted Benson, a poser.

Let ANYBODY here try to say Dick was wrong, Sobon (he taught), TFG et.al. I was a friend of Dick and tossed more than a glass or six with him.

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