At what point do you need the center spine for the roof?

[fornesto]

At what point is this center spine necessary? It is the blocking that runs from front to back. I am building a 52 1/2" wide tear.

[myjeepcherokee]

Thats my tear in the picture! I put that center spline in because the tear is just under 5ft wide. Being that I could only get 1/8" ply in 4x8 sheets and that I didnt want to run the plywood aginst the grain I decided to run it long ways, hence the center spline so I have something to screw the wood to in the middle that runs the length on the trailer...

[Arne]

If the cross pieces are close enough (mine are 12"), I'm not sure a spine is needed. My tear is 60" wide inside (queen size) and I did not use a spine and just let the plywood form a natural curve.... It wasn't a problem.

[steve wolverton]

myjeepcherokee - I dig those running boards. Those are groovy.

[SteveH]

What about running the plywood the other direction? There's a lot of waist no matter what you do when the trailer is more than 4' wide.

[myjeepcherokee]

myjeepcherokee - I dig those running boards. Those are groovy.

It's amazing what you can do with 1"x6" pine and a little rustoleum spray lacquer! Thanks!

[Arne]

re waste. My tear has an inside measurement from front to back of about 16 feet... I ran the inside ply (and the outside as well) from front to back, not side to side.

I wound up with visible seams because I didn't have any backing on the seams that would have been there if I had run the pieces side to side and had cross pieces on the seams.... I could have put backing pieces on, but it seemed to be lined up well.... till the paint went on.

There is no structural problem and the outside is doubled up 1/8 and the full sheet on the bottom is on one side, and the top is on the other... but the seam is visible.

Given it would have only taken another 3 or 4 sheets, or less than $50 for the ply, I would probably run it across instead of length-wise.

Or, I would put 'backers' (thin overlapping strips) glued under the seams to get it to line up better.....

[darockrider]

My teardrop is 69"wide and the roof spars are 2by2 fir. The aluminium skin is directly over the spars with no sheeting (only foam board insulation). The roof has no problems with supporting the weight of my body (190lbs). I would think that a center spline is overkill!

[angib]

At what point is this center spine necessary? It is the blocking that runs from front to back.

I'd question how much strength it actually adds - each length only runs from rib to rib, and the joints to the ribs cannot be as strong as the spine itself. I bet it does stiffen the structure up somewhat, but I also bet it's a lot of work to fit!

Andrew (structural engineer on submarines in a former life)

[IraRat]

My teardrop is 69"wide and the roof spars are 2by2 fir. The aluminium skin is directly over the spars with no sheeting (only foam board insulation). The roof has no problems with supporting the weight of my body (190lbs). I would think that a center spline is overkill!

Rock--you just used douglas fir? My spars go up this weekend, and this is one I've been debating, what to use. Poplar is a fortune, so I was going to go for 1 X 2 fir spars doubled up, for an actual laminated spar of 1.5 by 1.5.

And ALSO just going to lay the aluminum on top of the spars and insulation, no "subfloor."

[WoodSmith]

Rock--you just used douglas fir? My spars go up this weekend, and this is one I've been debating, what to use. Poplar is a fortune, so I was going to go for 1 X 2 fir spars doubled up, for an actual laminated spar of 1.5 by 1.5.

And ALSO just going to lay the aluminum on top of the spars and insulation, no "subfloor."

There is nothing "just" about Douglas Fir. It is VERY HARD, and the older it gets the harder it is.

Did a quick Google, found some strength info here: http://www.woodbin.com/ref/wood/strength_table.htm
The headings didn't translate well from the table format there, but I hope you get the idea. It is called a "hardwood" ONLY becasue it is deciduose (That spelling ain't right - it looses its leaves in winter.) Softwoods keep thier leaves (or needles) all year long.
--------------------------------------------------------------------------------

Tree Species
Average Specific Gravity, Oven Dry Sample (0-1.0)
Static Bending Modulus of Elasticity (E) 10^6 psi
Impact Bending, Height of Drop Causing Failure inches
Compress. Parallel to Grain, Max Crushing Strength PSI
Compress. Perpen. to Grain, Fiber Stress at Prop. Limit PSI
Shear Parallel to Grain, Max Shear Strength PSI

--------------------------------------------------------------------------------


Yellow-poplar 0.42 1.58 24 5,540 500 1,190
Douglas-fir, Coast 0.48 1.95 31 7,230 800 1,130
Douglas-fir, Interior West 0.50 1.83 32 7,430 760 1,290
Douglas-fir, Interior North 0.48 1.79 26 6,900 770 1,400
Douglas-fir, Interior South 0.46 1.49 20 6,230 740 1,510

Do not confuse the SYP or SPF crap lumber you are probably getting from the big box stores with REAL Douglas Fir. They are as different as night and day.

IMHO of course.

[darockrider]

Irarat,

Not sure if it is "douglas fir" or not! Just off the wood rack at HomeDepot fir! Has a reddish color and does not seem particularly hard. It sells for about $5~6 dollars for a 6ft length of 2by2. I have attached each spar to the iside ceiling panel with glue prior to mounting the spars between the walls with 2- 2 1/2" deck screws per side of each spar. Glueing the ceiling plywood (1/8" birch) to the spars allows easy to install ceiling with no inside scres visible. I was kneeling on the roof to install the roof vent and it didn't collapse or seem to flex overly so seems plenty of strong! no need to be on the roof really anyway right?

"darockrider"

[Arne]

I haven't read all the notes, but I'm not sure what good a spine would do unless the tear was extremely wide. A spine usually is beefier than the x-bars and runs the short way, as that is how it gets its strength.

If strength is really a concern, I'd put a slight curve in the roof (a good idea anyway) and make the x-members another 1/2" higher. I'm assuming the inside of the roof is skinned. A roof without an inside skin is much weaker. Think of it as an I-beam..