Spar height

[madjack]

...had some 5/4x7/4 red oak left over from a cabinet job so that is what we used. We set them at 9" centers 'cause the math worked out and we used bendy ply for ceiling and roof and wanted plenty of support for it. They also allowed for a full 1.5" of insulation
madjack

[Hardin Valley Magic]

I used 1 1/2 by1 1/2. i put them at 12" centers. Got 2 pieces of 3/4 blue board insulation sandwiched between the outer and inner skins works like a champ..

[randy chesnutt]

i used 1 1/2" x 1 1/2" . got some 2" x 4" and ripped them down. i just drawn a blank on the spacing. i allso run 1 1/2" x 1 1/2" long way in between the spars. if you look at my album page 4 & 5 you will see what i'm talking about

[Arne]

I used ripped 2x4s also, spacing was 'that looks just about right'.... closer on the front bend, a bit further apart on the roof....

[SteveH]

I also used ripped 2 X 4's spaced 12" except where the vent went and that is of course 14". Also insulated with 2 layers of the 3/4" foam.

Like others on here, I'm not saying my way is the way to do it... just saying that is the way I did it and the roof has not fallen yet.

[cracker39]

It looks like a lot of people have used spruce or fir ripped from 2x4s. I plan on ripping 3/4" thick x 1 1/2" to 3" wide from quality 1x8s (clear spruce) for the side walls, and ripping 1 1'2" deep spars from 2x4s (either spruce or fir). They'll be from 1" wide for the top/end spars, and up to 2" wide for use use in corners where the ends meet the top, or for framing openings for fan, A/C, etc. I plan on 12" OC for the top spars.

I see no need to use expensive hardwoods for framing or spars...JMHO. I've used this type of wood for framing a boat covered in Luan ply and it worked great. If 2x4s are used, the better grade should be purchased as they have fewer knots. They can be bought in different lengths to save wastage. My spars need to be 62" long, so I'll get 12' 2x4s and have less than 20" left over from each pair of spars that I can use in framing the A/C and fan.

BTW, I found out long ago, when making crafts for shows, that the longer the piece, the better the quality and there will be fewer knots. I usually bought 1x8s and 1x10s in 16 foot lengths for that reason, even though I had to have them cut in half to take them home. They must have thought I was nuts, buying 16' and then cutting them down to 8'.

[JLaman]

I used ripped 2x4s at about 12" oc. I suspect most use 2x4s because the 2x2s at Lowes and elsewhere are like spaghetti. You can purchase premium 2x4s for less then $4 and end up with better product for less money.

If you calculate the section modulus (strength) and moment of inertia (stiffness) for a 3/4 by 1.5" deep member with 1/8 ceiling liner and 1/4" roof spanning a 5ft wide tear, the construction will support more than 40 pounds per square foot. This is considerably more snow load or wind load than could be expected in most cases. This requires a "composite" section consisting of the ceiling, inner spar, and outer plywood all carefully glued. This evaluation ignores the substancial benefit of the arching action that can be expected from the curved shape of most of the tears being constructed.

Hope this helps.

Jeff L.

[bledsoe3]

If you're building 5' wide, buy the 2" X 4" X 10'. Less waste.

[cracker39]

I used ripped 2x4s at about 12" oc. I suspect most use 2x4s because the 2x2s at Lowes and elsewhere are like spaghetti. You can purchase premium 2x4s for less then $4 and end up with better product for less money.

My point exactly. the better quality 2x4s have few knots and are strong, as well as cheap, compared to hardwood.

If you calculate the section modulus (strength) and moment of inertia (stiffness) for a 3/4 by 1.5" deep member with 1/8 ceiling liner and 1/4" roof spanning a 5ft wide tear, the construction will support more than 40 pounds per square foot. This is considerably more snow load or wind load than could be expected in most cases. This requires a "composite" section consisting of the ceiling, inner spar, and outer plywood all carefully glued. This evaluation ignores the substancial benefit of the arching action that can be expected from the curved shape of most of the tears being constructed.

That's why i am only going to 1" wide and 1 1/2" deep. That's plenty strong. I may even rip 1x8s to just 3/4" wide x 1 1/2" deep to save a little more weight. Using clear pine (e.g. spruce) 1x8s will cost a little more than the 2x4s, but the wood is better quality. I'm not at all worried about the strength of the roof. Besides, I won't be in snow or ice to have a heavy roof loading.

[jay]

even though it is more expensive per foot, buying the longest lengths of wood available sometimes yields the least waste and best lumber [read: fewest knots].

[Greg M]

Why not just rip your spars out of ply? They're more dimensionaly staple ans wouldnt the bending modulus be higher?

-Greg

[Hardin Valley Magic]

I didn't because I wanted something substantual to attach the outside and the interior lining to.I guess you could use ply. if you were running your spars top to bottom in the front but i prefer running them left to right....Just my 2 Lincoln's worth though..

[doug hodder]

Greg M....I would think that a fastener into the edge of a ply spar wouldn't hold as well...maybe not a real problem on something like a Baltic Birch...but just about any other ply will want to separate....Doug

[cracker39]

I agree with Doug. I've used screws into the edges of 3/4" plywood and under any stress, they usually pull out or work loose. Personally, I'd not use plywood for a spar.

[JLaman]

I have found that narrow strips of plywood will split open if screws are applied parallel to the grain.

Making box spars sounds like a lot of work for little gain. The spars are practically a spacer to hold the stiffening ceiling ply and outer ply. The plywood is where the majority of the transverse strength comes from, not the spars. If the aluminum is attached directly to the spars, then the spars are much more structurally significant and need more careful design.

Jeff L.