roof

[Boanerges4]

If you use 3/4 ply. for the sides and 1/2 ply for roof is there any need for roof spars other than for insulation and a place to hide wiring. Im planning on a total electric standy with only the basics, refrig., a/c, microwave, lights. I think the wiring could be hidden with nice trim. Would the 1/2 inch ply. be sufficient for the roof with no insulation? Decisions, Decisions, Decisions

[48Rob]

Hi,

Good thoughts, but you probably need at least a few roof supports.

What span will you have?

Will the plywood roof be laid flat, or bowed into an arch shape?

Will there be any interior partitions or closets or floor to ceiling cabinetry?

If you are considering no spars as a weight saving measure, using thinner ply along with a few roof supports will be lighter.

Rob

[bobhenry]

Chubby will very soon be 4 years old and I am beginning to notice a slight concave plane to the roof as it has relaxed with age. There are 1 1/2 x 1 1/2 southern yellow pine spars located 12" apart and I felt this would be far enough support. However, if I were to do it over I would have cut them to a slight bow to allow for the natural relaxing of the wood over time.



I would be very very leary of relying solely on a layer of 1/2 plywood for a roof. In 2 years you will find you have a soup bowl. If the trailer profile is almost round this may not create a leaking problem but a stretched benroy or such with an almost flat section in the center this would almost surely be inviting disaster.

[Forrest747]

To He** with the weight. Just kidding THe roof issue has bothered me some as well since I am now close to wanting to put it on. I have spars every 12 inches and it will be a flat roof no curve to it. My son even toyed with the idea of setting the small tent up on top of it. I have a 58 3/4 inch span. I have also elected not to put the roof vent in at this time. It is framed out for it and I ran electrical to the space just incase i do put it in. For me will I go half inch or three quarters and I am leaning three quarters and the front face half inch with aluminum diamond plating for the lower half of it.

[Boanerges4]

Trailer will be 6x10 with one partition wall for shower and porta potty. I think it might be in my best interest to stay with 1x1 spars every 12 inches and use 1/4 inch luan for roof. Trailer will have a 22 1/2 degree slant on front and from there to the rear will be flat roof. Please chime in because I want this thing to last. I dont want to have a soup bowl in 6 months. Im thinking a couple of coats of marine varnish and then paint for a finish. Thanks for the input, Dennis

[Moho]

I would definately install spars. The ply across a 6 foot span is gauranteed to not support it's own weight over time.

To help with the situation Bob Henry is facing on Chubby, when I installed my spars I paid alot of attention to grain direction when cutting and selecting the lumber to rip for them. I installed them with the original bow direction (before ripping) facing the outer skin surface. It will not stop 100% of the bowing over time but shoud help.

Lumber almost always has a bow to it due to the casehardening from lumber mills pushing the drying process using kilns. The center of the wood has a higher mositure content that the outer surfaces, resulting in internal stress. For example to properly dry wood for lumber purposes is to let it sit for 1 year for every 1 square inch of dimension (ie: 1x1 = one year, 2x2 = 2 years). This is unrealistic with todays lumber needs. In order to produce the quantites need for consumption, kilns are used which results in casehardening. This is why lumber from the chain stores is always so twisted, cupped and warped. They need the most quantity which results in the mills pushing the drying process to make it dry faster for them.

[48Rob]

Trailer will be 6x10 with one partition wall for shower and porta potty. I think it might be in my best interest to stay with 1x1 spars every 12 inches and use 1/4 inch luan for roof. Trailer will have a 22 1/2 degree slant on front and from there to the rear will be flat roof. Please chime in because I want this thing to last. I dont want to have a soup bowl in 6 months. Im thinking a couple of coats of marine varnish and then paint for a finish. Thanks for the input, Dennis

Hi Dennis,

Though I am not an engineer like some of the good people here, my experience with wood would suggest that 1"x1" roof supports will not support your roof very well, unless they are steel.

1.5" tall x 3/4" wide will work if spaced closely (12" or so) but when you account for the lost strength when you drill holes for wiring, 1.5 x 1.5 as a minimum is something I'd consider.

I used 2" x 3/4" Oak roof supports on 12" centers with two layers of 1/8" ply for the roof.
It is very strong, but there are no holes drilled...
On other trailers, I've used the same, or 1.5 x 1.5 supports.

1.5 X 1.5 was a pretty common size used in the old trailer world.
They were drilled often to carry wiring, and though some sagged over time, most held up okay.

Considering that insulation in the ceiling is far more critical that the walls, you might consider going with 2" roof supports for strength and better insulation/comfort.

As for the marine varnish and paint, sealing the wood with varnish isn't a bad idea, but if you are going to paint anyway, thinning the first coat or two of oil based paint for good penetration prior to applying the top coats of paint will deliver the same results with perhaps less expense.

Though it is not my intent to start yet another debate... caution/research/investigation is in order before you nail/glue/screw on Luan ply for the roof.
Plywood is plywood? Luan is plywood?
There are as in all aspects of merchandise, different grades/qualities of products.
Some believe that Luan plywood is a very cheaply made, prone to failure version of higher quality plywood.
Saving money is great, just be careful that you save money in areas/on parts of the trailer that don't require dismantling most of it to replace failed products...

Rob

[bobhenry]

I would definately install spars. The ply across a 6 foot span is gauranteed to not support it's own weight over time.

To help with the situation Bob Henry is facing on Chubby, when I installed my spars I paid alot of attention to grain direction when cutting and selecting the lumber to rip for them. I installed them with the original bow direction (before ripping) facing the outer skin surface. It will not stop 100% of the bowing over time but shoud help.

All of the spars were cut from #1 SYP and the 2x12 from which they were cut were selected as center cut boards only. When cutting I cut 6-8 extra and cured them for 4-5 weeks. In that time a few went south with twisting and excessive bowing. The remaining "good" spars were culled for minor bowing and the bow went up as your post suggests. I think the biggest single factor that has aggrivated the problem is the inside temp of the clear top trailer has in the past reached 140 F while stored in the sun.

(Talk about kiln dried)

Well all possible precautions were taken and the distortion has not yet created an actual leak problem I still think if I were to do it again I would have cut the spars in a very very shallow arch to prevent the sinking roof syndrome.

[jonw]

I agree with others that 1x1 for a 6 foot span will sag. I'd go with a minimum of 1x2 (3/4 x 1-1/5 actual), which would then also give you a 1-1/2 space for insulation. I'd also recommend using a harder wood than pine for these, like poplar or oak.

Another alternative is to have a longitudinal rib/spine down the center which would allow you to use shorter horizontal joists and go a long way to addressing any future sagging.

Good luck!

[48Rob]

Another alternative is to have a longitudinal rib/spine down the center which would allow you to use shorter horizontal joists and go a long way to addressing any future sagging.

Jon,

Great thought!

Though not the same, some manufacturers really thought long and hard, and came up with some very practical ways to reduce spans, which of course reduces sag...

Here is one of them.

Rob

<img src="http://img.photobucket.com/albums/v284/1948rob/CABIN%20CAR%20PROTECTED%20IMAGES/Fleet_Trailer_with_Harry_-_1946jpgp.jpg?t=1235229197">

[Moho]

Well all possible precautions were taken and the distortion has not yet created an actual leak problem I still think if I were to do it again I would have cut the spars in a very very shallow arch to prevent the sinking roof syndrome.

Now ya have me thinking of removing my spars and doing just that, lol

[JeepThing]

On my build I did use 1.5" spars 12-14" apart at the roof vent I used 2"w x1.5 cross braces, covered with 1/4" door skin glued and screwed. the ceiling also has 1/8" door skin glued and stapled to it plus 1.5" ridgit styrofoam glued with PL prem. alltogether they become one piece and are incredibly strong I can stand on my roof with no signs of sagging. Now we'll have to see what a year or two will show but for now it is still perfectly true. Without spars I would think for the few pounds you would save it would not be worth it. It also gives a place to screw cabinets into and maybe other things to attach too. Good luck

A pic of my roof spars w/ insulation


P.S. Rob - I realy like that old photo very cool to see them being build in the ol' days

[rowerwet]

Another alternative is to have a longitudinal rib/spine down the center which would allow you to use shorter horizontal joists and go a long way to addressing any future sagging.

Jon,

Great thought!

Though not the same, some manufacturers really thought long and hard, and came up with some very practical ways to reduce spans, which of course reduces sag...

Here is one of them.

Rob

<img src="http://img.photobucket.com/albums/v284/1948rob/CABIN%20CAR%20PROTECTED%20IMAGES/Fleet_Trailer_with_Harry_-_1946jpgp.jpg?t=1235229197">

look at aluminum boat makers, those long straight ribs on the bottom aren't just for directional stability, they also keep the panels from getting beaten in by the pounding over waves and beaches. You could do the same thing with your roof a couple of ways. 1x2's going across inside and 1x2's going across on top to make a sort of I beam set up, just make sure your glue is really tough stuff. Or 1x2 or 1x3's going length wise with the trailer roof glued or epoxied on would do the same job and work a a roof rack of sorts also, without causing to much drag. make them go all the way from the front edge over the top and to the bottom of the rear and they would really look different but cool.