Keeping a flat roof from sagging

[JamesDixonLR]

I'm building a lifting roof camper with ideas borrowed from the Squidget II and the Compact III. The roof, also known as "the Lid", is 8 foot long by 7 foot wide. At this point in time, there is only one layer of .25" plywood but I intend to add a layer of foamular and a second layer of plywood.
It sags a little now so I'm thinking about how to support it. Angle iron and angle aluminum are both flexible at that length if turned flat like an L. I've cut some strips of plywood to act as ribs but they are no more rigid.
My best solution to date is to go back to the aluminum angle and turn it so that it is oriented like a V or A. That would put both sides into play to give rigidity. Kind of like a hollow core door.
Square aluminum tube would also be a consideration but the size may not be right.
Any other suggestions?
Thanks

[working on it]

• Here's a little tool, The Sagulator, I used to see if my un-supported roof, of 3/4" plywood, could be self-supporting even if I got up there with some tools, or install a roof rack there, in the future. I used Simpson Strong-ties gusseted angle braces to secure it to the walls, front slope, and inner rear bulkhead (all, also of 3/4" plywood), with the 51.5" x 48" plywood roof section resting on top of 3/4" ply, all-around. The galley roof was also supported the same way, but although it was actually part of the roof section, I removed its' 22" x 48" size from the calculations (the roof piece is actually 73.5 x 48"), since it has basically its' own structural supports, and really won't ever sag. http://www.woodbin.com/calcs/sagulator/
• Also, here's my roof sag, that I confirms that my small, but thick, roof didn't need support beams underneath it, though I've since added a 4x4 cross beam inside, with 1" space between it and the ceiling, between the sidewalls (as a lift/grab-handle to help me in and out...I'm old), which I can use to fit in wedges to lift the roof, if ever necessary.

• 

  calculated acceptable sag for unsupported roof (my TTT).JPG (64.85 KiB) Viewed 1895 times

• Also, I ran a preliminary calculation sample, using your data (what I know of it), which shows that the 1/4 roof you have will definitely sag without spars holding it up, an it becomes borderline holding only 6.9 lbs of loading (rain pooling, snow cover?). I also tried calculating just adding a 3/4" plywood sheet underneath the 1/4", and it doesn't show appreciable/borderline sag until 398 lbs of total loading is applied. I may not have all the pertinent data, or really envision your layout, but it is probably the way I would've gone on my tank-like TTT, and it at least shows what sorta ballpark you're in.

• 

  sag calculation shows max load of 6.9 lbs is approaching overload.JPG (64.76 KiB) Viewed 1895 times

• 

  one inch (nominal) thickness eliminates sag..JPG (62.08 KiB) Viewed 1895 times

[JamesDixonLR]

Thank you for the "sagulator" post.
After I sent the original post, I kept thinking to myself "like a hollow core door". Think of my roof as a large shoe box lid where the four lift up wall sections will fit inside the rim. I was thinking of adding these supports, foam, and thin plywood as the ceiling, With "door" in mind I realized a basic interior door would be only a little thicker than what I had in mind, cost about the same, admittedly weight more (74 lbs versus 50), but ought to be very rigid. If I use a lot of glue, covering every inch with a squeegee, to secure the required three doors which would fit without cutting to the plywood, the doors would serve as ceiling, insulation, and support.
Is this crazy talk or have others used doors this way?

[OP827]

I personally think that 1/4 skin is an overkill extra weight. All you need is a strong skin attached to foamular. Good quality 1/8" Baltic birch ply covered outside by 1 layer of fiberglass cloth and epoxy and just epoxy coating on inside skin should do a good job. For rigidity, a LOT depends of how thick is the Foamular panel core. 7' feet is a relatively big span for a trailer roof. You may also consider doing some slight curvature in the roof to help with water runoff.

[JamesDixonLR]

Going back to your Sagulator post, how is the shelf load determined? If total weight divided by square feet it should be about 2.
My original plan was a foam sandwich with 1/4" exterior plywood on top, 1" Foamular, and 5mm interior plywood as the ceiling. This should weigh about 92 pounds and be about 1.45" thick. I'f I'm using the sagulator correctly that results in zero sag. I'm wondering if I actually need the interior plywood for the ceiling although its not a sandwich if it only has one piece of bread.
thanks

[JamesDixonLR]

I personally think that 1/4 skin is an overkill extra weight. All you need is a strong skin attached to foamular. Good quality 1/8" Baltic birch ply covered outside by 1 layer of fiberglass cloth and epoxy and just epoxy coating on inside skin should do a good job. For rigidity, a LOT depends of how thick is the Foamular panel core. 7' feet is a relatively big span for a trailer roof. You may also consider doing some slight curvature in the roof to help with water runoff.

I like your idea of curvature. At this point, that would require a do over but I will keep it in mind. If I took a sheet of 3/4 plywood and made a few curved spars, I could lay the top plywood over it. The kerf some foamular so that I could bend it and place that between the spars and then the ceiling plywood.
Since the roof is going to be detachable from the rest of the body if I make a mistake with it, or simply change my mind, it won't be as big a project as it might otherwise be.
thanks

[QueticoBill]

I'm not sure where you are at but a stressed skin panel - like a hollow core door - would easily span the 7'. 1/8" ply on either side of foam - but I would look at 1" or 1 1/2". It requires a good adhesive with no slippage. Portable platforms for rock concert stages are built the same way - 4 x 8 with 1/4 or 3/16" skins and a honeycomb paper core - cheaper than foam for their purposes - and they support people jumping up and down in unison. No framing ("spars").

A slight bow would certainly help with strengh and drainage. It could bow on either axis. I'd estimate in the 12-16 foot radius would do it. I'd like maybe 3-4" rise. Should not have to kerf foam for that large of radius - possibly for 1 1/2" but pretty sure 1" would do. Quick no-engineering calculation: the thickness of the sandwich is probably half as strong as solid wood. So 1" with 1/4" ply skins both sides is in the ballpark of half as strong as 1 1/2" plywood.

The ply joint should parallel with the span - probaby across the trailer unless you bow it the long way - and I would splice the ply joint with the ply cutoff and use thinner foam in that area.

[OP827]

I personally think that 1/4 skin is an overkill extra weight. All you need is a strong skin attached to foamular. Good quality 1/8" Baltic birch ply covered outside by 1 layer of fiberglass cloth and epoxy and just epoxy coating on inside skin should do a good job. For rigidity, a LOT depends of how thick is the Foamular panel core. 7' feet is a relatively big span for a trailer roof. You may also consider doing some slight curvature in the roof to help with water runoff.

I like your idea of curvature. At this point, that would require a do over but I will keep it in mind. If I took a sheet of 3/4 plywood and made a few curved spars, I could lay the top plywood over it. The kerf some foamular so that I could bend it and place that between the spars and then the ceiling plywood.
Since the roof is going to be detachable from the rest of the body if I make a mistake with it, or simply change my mind, it won't be as big a project as it might otherwise be.
thanks

I am curious how are you lifting your roof and what is its estimated weight? Thicker plywood adds weight...

[JamesDixonLR]

I will be using fold up panels on piano hinges to lift the roof. Like the Compact III below but with four sides rather than three.
http://tnttt.com/Design_Library/Compact ... ofanim.gif

I went with 1/4" plywood as it was the thinnest exterior grade plywood I could find. I haven't actually weighed what I have yet but I believe the 8'x7' roof will weigh around 85 lbs when complete. I do plan to skin the entire camper in canvas and Glidden Gripper so that will add some weight too.

[bobhenry]

Let me find the picture but I had plywood scrap from my caboose build that I threw away with regret. It would have made a nice box beam slightly curved roof.

The box beam roof supports for the caboose....

Here is the scrap from the interior curve .....

I thing a pair of these every 24" would offer a nice support and a layer or two of 1/8 luan or 1/4" plywood would make a hell for stout roof without fear of water puddling.

[Pmullen503]

Have you thought of foregoing the plywood and making that roof section just canvas covered foam? With a few ribs you could introduce a bit of curve to shed rain and it should be considerably lighter to lift.

[JamesDixonLR]

In my defense, I had already started down this road before asking for help. The exterior grade 1/4" plywood was already attached to the 1x4 edges of the "lid". My choices were to scrap it now or take a chance and perhaps scrap it later. I already had the materials and decided to take a chance. The "sagulator" indicated that once I added the foam and cieling plywood it would be rigid enough. For what it's worth I think it was correct, the lid is pretty string now. However, I underestimated the weight in part because I didn't take the weight of the paint used as glue into consideration. What I thought was going to be about 90 lbs turned into about 120 lbs. We got it on the trailer but when I tried to raise one side so that I could flip up that wall it was pretty hard and determined to slide at the opposite end. I could anchor it at one end, like the "Compact 3", but I want the whole tall enough that I don't have to stoop except when setting up or taking down. So, I'm going to scrap it and start over.

This opens up avenues for other ideas. Like the bowed foam that Bob Henry suggested and just foam and canvas that PMULLEN503 suggested. One of the parameters of the project was that it would be low enough to put in my garage. That does not leave a lot of room for height which is why I went with the flip up walls. I bought the lowest profile vent I could find. Even though the lid is a bust, the walls worked out and I've come to realize that if the lid is light enough I can take it off before rolling the camper into the garage. Then back on or just lean it somewhere out of the way. That means the "lid" can be taller than originally planned, perhaps bowed foam with canvas to keep water from puddling and keep the weight down.

[Tigris99]

the sag issue is something I was concerned with on my first build. Your build obvious is a dif different than standard but this idea my be useful to you and others.

I did 2x roof spars (work with 1x of whatever) and intentionally warped them basically.

Laid out a black tarp and set a short piece of pipe in the middle of it. laid my spares so the pipe was at the center of each. Then placed concrete blocks on both ends of all the spars.

once everything was perfect so all were going to be warped evenly to match each other, I poured a bunch of water on everything and so the tarp itself had a small pool of water it was holding under the spars. wrapped the tarp up around them all and left in the hot sun for about a week or so. I did check on them a couple times and didnt move to next step until I could life the blocks and there was little to none of the end lifting up off the ground anymore.

Once there I just opened the tarp all the way and let the spars dry out fully in the hot sun (all blocks and pipe still in place) for a few days.

And done, perfectly bent roof spars just enough that roof would never sag and rain etc would simply run right off without pooling on the roof at all.

I had about 5/8" height increase at center vs edges. Almost invisible unless looking closely.

Sent from my SM-G950U using Tapatalk

[JamesDixonLR]

Since I am frugal, I hang on to scrap material for a while. Several weeks ago I took a wide 8' long strip of 1/4" plywood left over from the lid and cut it into 8 1" wide strips and then glued 2 each together with the idea of using them as spars. Turns out they are too flexible so I put my new 1/2" by 1" by 8' strips in the corner. After my last post I remembered them and thought about turning them on their side and using them as Roman arches to support the curved foam. I cut one to my guess of the right length and fitted to the existing lid and the arch looks really good. I bowed some 1" Foamular scrap to and it bowed easily to the form. If I use a 1x8, or plywood, as the front and back edges of the lid I can cut those to the same curve and have six supports across the 8' length that take up little space and also weigh little. I showed my wife my new idea and she said "gypsy wagon". My original plan would result in a ceiling that people as tall as 6' could walk around in without stooping, this will add 4 to 6 inches to that depending on how much arch I leave in it.

[GPW]

… and then again , if you’re using Foam for the roof , you can always Heat bend it into a slight arch and Bob’s your uncle … We now know that can be done …