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Forum topic by phlepper posted 1133 days ago 843 views 0 times favorited 6 replies Add to Favorites Watch
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phlepper

21 posts in 1848 days


1133 days ago

Topic tags/keywords: question cedar joint mortise tenon

Hello!

I’ve been asked to build a mission style swing for a family member using plans from a 1912 Popular Mechanics article (http://howtobuildplans.com/2010/09/11/mission-style-porch-swing-plans/). The project will be made out of left over cedar from a backyard pergola (so the swing will match).

The plans call for a lap joint between the bottom rails and the front and back posts. Then they call for a single 4” lag screw through the joint and into the end of the end rail.

I would prefer to make this without the exposed hardware so I’m wondering if I could instead use a mortise and tenon to attach the end rail to the front and back posts (through the center of the lapped joint).

Would this create a sturdy joint or would the mortise in the half-lap joint cause too much structural stress and actually weaken the joint? If so, any other ideas on creating a strong joint without the exposed bolt?

For particulars, the post and rails are all 2×4 cedar planks.

Also, any advice on the size of the tenon? All things being equal, I would plan on cutting the tenon to 1” x 3” (so 1/4” all around) and 1” long (not a through tenon). Although, if it would be better structurally, I have no problem with a through tenon for a little extra detail showing through the joint. Also, I’m concerned that at 3” tall, the mortise would cut through most of the lapped rail, so maybe the tenon should be shorter (1” x 2” perhaps)?

Below is a picture from the article (the joints are at the bottom on each corner).

Any and all opinions appreciated!

Thanks for your help!
phlepper

-- "A hammer in search of a nail..." (thelepperts.wordpress.com)


6 replies so far

View BobTheFish's profile

BobTheFish

361 posts in 1147 days


#1 posted 1133 days ago

can you do the screwing in from the back inside? If not, have you considered dowels, and hide them within where the lap joint holds and the side adjoins? Barring that, go with the screws and inset them, hiding the screws with plugs?

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crank49

3333 posts in 1566 days


#2 posted 1133 days ago

I would be very wary of doing anything to reduce the strength of the rails for three reasons.
1. The entire weight of the swing is carried on the cantilevered ends of the rails.
2. The strength of the rail is already cut in half by the lap joint itself.
3. You are wanting to use cedar to produce a design that was intended for quarter sawn oak. That is a significantly weaker material to start with.
Why don’t you counter bore the face of the post slightly and use 4” long, flat head deck screws? Then cover the head with a wood plug. The plug could be round or square. I think square would look more appropiate for the style you are after.

Sorry Bob, you posted while I was writing.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View tenontim's profile

tenontim

2131 posts in 2339 days


#3 posted 1133 days ago

The reason they used a lap joint instead of mortise and tenon, is because of the way the chains that it hangs from are attached. If you use a m&t joint, the through tenon would be considerably thinner than the way that it’s designed.
I will also second the remark by crank49, about the cedar not having the strength of the white oak. You might want to consider making the bottom frame out of oak and the rest out of your cedar. Then stain it to get the colors close.

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phlepper

21 posts in 1848 days


#4 posted 1133 days ago

crank49, I’m not sure where you’re seeing oak. The plans call for “southern pine”. I tend to rely on the “sagulator” to calculate various wood “strengths” and there is very little difference between pine and cedar (although the pine is slightly stronger).

Given the concerns about the cedar, I’m thinking I probably shouldn’t make any deviations and just use the screws. I could counter-bore them and use the plugs as suggested by you and Bob, but I might not even do that as I’m concerned it might weaken the lap joint (or at least not hold the outside post as well).

However, now I’m thinking I might be better off gluing up two 2×4s to create a 4×4. I could then replace the front and back rails with these 4×4’s versus 2×4. That would essentially double the strength of the cross rails. The half lap wouldn’t even touch the second 2×4 of the two and I could put the screw for the chain in the “inside” half of that 4×4 (the uncut part).

Would this be overkill? I’d really like to stick with all cedar since it matches the existing pergola.

Thanks again,
phlepper

-- "A hammer in search of a nail..." (thelepperts.wordpress.com)

View BobTheFish's profile

BobTheFish

361 posts in 1147 days


#5 posted 1133 days ago

Can I suggest hard maple (preferably “country” maple) instead of oak tenontim? The grain is probably a bit closer match. :S

View crank49's profile

crank49

3333 posts in 1566 days


#6 posted 1133 days ago

Sorry, I didn’t read the link. The picture just looked like oak, and oak is often used in Mission style pieces so I just went with that. If its intended for pine you would probably be ok with cedar, IMHO.

A screw through a lap joint is not a bad thing; and as others have also said, it can be hidden.
A screw into endgrain, “into the end of the end rails” is not so good, but that may not matter with a 4” screw. I do a similar joint on my workbenchs and some machine stands for my shop. I put a maple dowell through the rail, perpendicular to the screw and let the screw bit into that. That is extremely strong.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

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