Roof Spar Joint - Alternatives to Butt Joint

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Roof Spar Joint - Alternatives to Butt Joint

Postby Andrew Herrick » Thu Nov 17, 2016 10:47 am

Hello all!

I apologize if this topic has been addressed else where. My quick n' dirty search hasn't turned up any in-depth results.

The Project
Most builders seem to use a butt joint to connect their roof spars to their sidewalls. The spar butts up against the sidewall and is screwed into the end-grain from the outside using a 3.5-inch deck screw. Some people apply glue to the joint as well.

The Problem
I'm interested in alternatives. The butt joint has the following weaknesses:

- It can be difficult and time-consuming to position the spars to predrill the screw holes.
- Using glue is (arguably) useless and messy.
- If you don't predrill correctly, the spars are prone to splitting from shock impact.
- Intrinsically, the joint only works well in tension. In shear (standing on a roof, for instance), the entire load is borne by the cross-section of the deck screws. And the joint provides almost no racking resistance whatsoever.

- Using a 3-inch deck screw makes you feel good inside, but I'm not convinced it's any better than a shorter screw. According to the following chart ...

http://www.zillarac.com/Portals/0/Docum ... ll-out.pdf

... and when downgraded 25% for the end grain effect, you're looking at 80 - 150 pounds of pull-out strength per inch of thread. The thing is, the countersunk head of the screw will rip through the sidewall long before the screw pulls out.

The Solution
Now, the butt joint has a long history and works fairly well. And it's certainly easier than cutting dovetails! But I'm interested in something that's FASTER to assemble and STRONGER over time, particularly for 4WD use.

Looking for ideas.

- Replace the deck screws with lag bolts?
- Some sort of structural connector?
- Mating wood joint?
- Extra bracing?

Thanks for the help!
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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby tony.latham » Thu Nov 17, 2016 11:07 am

A butt joint for something so critical makes no sense to me. I fall into the sandwiched-wall crowd and cut a mortise into the wall. It's not going anywhere.

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Here's my wife and fellow teardrop builder working on a wall. You can see the mortise cut into the 3/4" skeleton. The oak spar is fitted and then glued in place.

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Brute strong. :thumbsup:

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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby yrock87 » Thu Nov 17, 2016 11:40 am

I also used sandwich construction and while I did not gas as far as Tony, creating a mortise for the roof spars, I did have my roof spars resting on top of the inside skin and frame. With only the outside skin joined to the end grain of the spar. That way I could screw through from the top, plus use wood glue with an cross grain to cross rain connection. Oh, and I glued the end grain to the outside ply for good measure. Oh again, the inner roof skin was glued to the wall frame all along the one inch top of both walls. Lots of bonded area, lots of strength. Dispersed loads and stressed skins make for strong and light build.

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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby lfhoward » Thu Nov 17, 2016 12:04 pm

I did the same as yrock87 and Tony, creating a mortise and screwing the spars down into the sidewall from above (and gluing also).

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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby QueticoBill » Thu Nov 17, 2016 2:08 pm

I'm new but planning and I agree with yrock on this - set spar on top of frame and bore, countersink, and screw into frame. In the end, I believe the diaphragm of the inner and outer skins along with good glue joints of the skins to the spar will provide the strength. If xps is laminated in between spars, it will serve to strengthen the skins.

Since I'm going for a woody look, I'm planing on both skins and fame all flush, and spars on top of it all, and let a trim piece cover the spar ends. I think. :oops: Something like a 10 x 3.
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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby bobwhite215 » Thu Nov 17, 2016 2:56 pm

I think it is all a question of how strong is strong enough. I'm as guilty as the next guy, but I think most of us DIYers tend to go way overboard.

Is a screwed and glued butt joint the strongest possible solution, definitely not. Is it strong enough for the application, probably so.

I do disagree that glue is messy and useless. It may be messy, but definitely not useless. In most cases, the screws are only there as clamps to support the spars while the glue dries. A properly glued joint should be very strong.

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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby Andrew Herrick » Thu Nov 17, 2016 5:46 pm

We got us a good conversation going here! Excited to hear what people have to say!

I've used mortises when building sandwich walls, too, but they're difficult when using solid 3/4-inch plywood walls. I supposw you'd have to recess a square mortise into the wall using a router, I guess? Not a bad idea, but a little slow ...

Bobwhite, I'm a not an expert woodworker, but my understanding is that wood-glued joints work best in a) slow, static loads) and b) best on cross-grain interfaces. I've had my glued end-grain butt joints fail from a few blows of a hammer in the middle of the spar (and I'm pretty sure they were good joints :p ). But, willing to be educated. I know the butt joint works for simple travel on maintained roads. Don't like to trust it on 4WD roads ...

So, while we've got some good information, the original question remains outstanding: We need an alternative join a 2x2 or 1x2 roof spar to a 3/4-inch solid plywood wall.
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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby bobwhite215 » Thu Nov 17, 2016 6:32 pm

If your looking for an alternative, what about running a threaded rod instead of a wooden spar.

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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby Wolfgang92025 » Thu Nov 17, 2016 6:48 pm

Mine is a mortise pocket in the wall.

145631

Glued with construction adhesive in the pocked and two screws per side that are hidden under trim.
Because it is buried under trim, I had to make sure it is rock solid.
Been down many a dirt trails without a concern that it might come apart.
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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby Tomterrific » Thu Nov 17, 2016 8:06 pm

I used butt blocks on either side of the spar. The blocks went along the curves but I used straight sections between spars on the flat sections. I guess the butt blocks could be called cleats. I don't know a correct term.

I first installed the spars like everyone. Then I put in the ceiling. The butt blocks were then glued to both the spar and the installed ceiling and the wall. This spreads the glue joint and makes for a very strong edge.

My walls and roof are only 3/16" underlayment so the thick edge from the front to the back gives much strength while being lightweight.

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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby Nobody » Thu Nov 17, 2016 10:09 pm

When I built my TD in 2006 I used sandwich wall construction. I cut the profile from 1/4" oak plywood for the inner wall, then used 1x4 spruce ripped to appropriate widths to make the borders & dividers. Everything was pre-fitted & joined with pocket screws before gluing it all to the quarter inch inner plywood wall. After the walls were done I 'notched' each wall at the appropriate location for the spars. I used 'half lap' construction to attach the spars; spars were ripped from spruce 2x4s & each end was rabbetted 1/2" by 1 1/4" & the wall notches were 1" deep by 1 1/4" to accommodate the rabbetted spars. After all spars were pre-fitted I used Titebond II & a screw through the top of each spar to hold everything together 'til the glue cured. The wall door facings & the final spar where the hatch would be hinged were made of solid oak, rough cut to dimension & trued/finished on my little 4" jointer. After all the walls/spars were finished I installed the 1/8" Luan ceiling panels & added 1" blue/pink styrofoam insulation (3/4" on sides & 1 1/2" in the roof) to fill all the 'voids' created by the spars & the wall support pieces. Outside walls & roof were then covered with 1/8" Luan before sealing in prep for the aluminum siding. I built the floor on the trailer chassis first & after pre-fitting the walls/spars I fastened the walls to the floor using 3 1/2" & 4" 'deck' screws vertically into the floor frame after spreading PL1200 construction adhesive. The 1/8" Luan outer skin covered the entire cabin including the 1x6" floor trim, which probably aided in wall stiffness. The 4' x 114" x 1 1/8" thick walls were light enough for one old man to handle easily, tho kinda bulky. The aluminum outer skin was 'floated' except for a little 'temporary' PL1200 adhesive used to secure it in place 'til the door & edge/roof trim was fastened in place. After more'n 10yrs & being towed nearly 30K miles it still feels as solid as the day I finished.

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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby Andrew Herrick » Thu Nov 17, 2016 11:28 pm

I'm liking this half-lap idea. Fixes the shear issue, the racking issue, but still gives you plenty of surface for gluing (if you want it) and you can still screw through the sidewall if you want. However, making a mortise pocket in the wall for a standard spar does have the advantage of only needing to modify one part instead of two.

Thanks for the inspiration! Keep 'em coming!
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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby QueticoBill » Fri Nov 18, 2016 7:40 am

Slept on this and it occurred to me that if you can achieve a good bond between the plywood skins and the foam, you can skip the spars entirely. Many stressed skin structures have nothing but a foam or honeycomb core and skins. Fasten lower skin to wall tops, lay up foam, lay up top skin.

Whether or not I or other home builders can achieve the bond is worthy of consideration.
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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby noseoil » Fri Nov 18, 2016 8:04 am

Sandwich wall construction, butt blocks, screws, glue & skins. Not going anywhere, ever...

Rout rabbet joint into wall panels after interior skins are in place & finished
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Ceiling skinned (pre-glued canvas headliner) & spars fastened in place. Everything is glued.
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Blocked edges & insulated with 2 layers of 3/4" foam for the headliner, prior to skinning exterior
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Re: Roof Spar Joint - Alternatives to Butt Joint

Postby aggie79 » Fri Nov 18, 2016 9:41 am

I would say that most of us, even when trying to build light, overbuild _______ (fill in the blank for any portion of teardrop construction). I am guilty of using more cross-sectional area of fasteners (screws and staples) in attaching my roof structure to the sidewall than the single 5/8" pin that holds my draw bar to my hitch receiver.

I built my roof structure to be independent of the sidewalls. I framed it in place using 1" x 2" spars and 3/4" ply outer "ribs". Spars are connected to the ribs using 2 pocket screws per end. Then I removed the roof structure, applied 3mm plywood and hull liner for the ceiling and re-installed the roof. The ribs are connected to the sidewall using 1 screw in between every two spars. I removed one of the pocket screws for each end of the spar and replaced it with a longer screw that spanned the rib into the sidewall. I also added one deck screw from the outside of the sidewall into each spar. An outer layer of 3mm plywood was glued to the spars/rib/sidewall using construction adhesive. Then aluminum sheeting was perimeter glued to the plywood and staples were driven through the aluminum and outer 3mm plywood into the framing. Finally, the insert trim was applied with screws, through the aluminum and plywood into the sidewalls.
:?

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