Attaching Roof to spars and sealing roof to walls

[carlbergs]

We will admit it right up front, we are newbies. My daughter and I are building a square drop design.
We have watched dozens of videos and read dozens of posts and replies. But there's nothing like getting an answer to your specific situation which I haven't exactly found.
We are using 3/4" plywood for the walls. We planned on using 2" x 2" spars to support the roof, seemed to be a reasonable solution but we haven't done this step yet.
It is a 4' x 8' build.
The floor is a full sheet of plywood. The walls will sit on the floor so that the outside width is still 4'.

I did some tests this weekend with PL3X and it is amazingly strong. And here is where I get unsure and need help.

The plan was to use 3/8" ply for the roof. We were going to do The Mix on the walls and the roof before assembling.

When we have the walls up and our spars fixed in place, when we go to attach the roof can we be confident that beads of the PL3X will be strong enough to hold the roof on or do we need to use screws as well. My tests tell me the PL is all we need but the wrath that will come to me if my wife's baby girl has any issues makes we want to add screws. But that is just asking for water trouble. So that's the first part.

The second question is do I use the same PL3X on the seam where the top of the wall meets the roof or should that be a "sealant" like the PL window and trim? I did see in one post a product called OSI Quad I believe and I saw that at my local store too. But maybe those are better for window to wall and not in the application I am talking about.

And final question for now, should I use The Mix on the top of the wall where it will seal to the roof?

I hope that made sense.

Thank you.

[philpom]

We will admit it right up front, we are newbies. My daughter and I are building a square drop design.
We have watched dozens of videos and read dozens of posts and replies. But there's nothing like getting an answer to your specific situation which I haven't exactly found.
We are using 3/4" plywood for the walls. We planned on using 2" x 2" spars to support the roof, seemed to be a reasonable solution but we haven't done this step yet.
It is a 4' x 8' build.
The floor is a full sheet of plywood. The walls will sit on the floor so that the outside width is still 4'.

I did some tests this weekend with PL3X and it is amazingly strong. And here is where I get unsure and need help.

The plan was to use 3/8" ply for the roof. We were going to do The Mix on the walls and the roof before assembling.

When we have the walls up and our spars fixed in place, when we go to attach the roof can we be confident that beads of the PL3X will be strong enough to hold the roof on or do we need to use screws as well. My tests tell me the PL is all we need but the wrath that will come to me if my wife's baby girl has any issues makes we want to add screws. But that is just asking for water trouble. So that's the first part.

The second question is do I use the same PL3X on the seam where the top of the wall meets the roof or should that be a "sealant" like the PL window and trim? I did see in one post a product called OSI Quad I believe and I saw that at my local store too. But maybe those are better for window to wall and not in the application I am talking about.

And final question for now, should I use The Mix on the top of the wall where it will seal to the roof?

I hope that made sense.

Thank you.

First a couple of questions. Do you plan on putting any sort of skin on the camper like aluminum or PMF etc? I used 3/4" ply for my walls and 2x2 for the roof spars. I used Sikabond (adhesive/sealant that remains elastic) across the tops of the spars and along the edge of the roof but I also used screws to mechanically attach everything. My roof is 1/4" BCX and seems more than sufficient. The reason I ask if you are going to skin it is because after I applied my PMF it covered every screw hole, every seam etc from the top all the way to the bottom so no worries at all about water from these areas. I would personally be concerned about using no fasteners and relying on glue alone. Even if you don't put some kind of skin on the camper I would still screw the roof down and use some sort of membrane at least on the roof. I curved my roof but sounds like you will have hard angles and of course that means you are already going to have seams to deal with.

Good luck!

[carlbergs]

Thanks for the excellent questions philpom.
I just looked at all your photos and your camper is essentially the direction we are going.

We are not opposed to PMF on the roof but we would like to keep the side wood grain. I see from your photos that you slightly sloped the roof toward the back. Can you tell me how tall the wall is at the rear? I could easily do this at this stage and it makes a lot of sense. So with PMF then screws are a logical choice. Did you seal the tops of the screws before the PMF?

Can you tell me your spacing on your spars?
What method did you use to attach them? I was leaning toward a pocket screw and PL3X on the end then butt them against the wall. Again, trying to keep external screws to a minimum so not screwing from the outside of the wall into the spar.

1/4" BCX will certainly save us some weight, yes this is a consideration, and some money considering the price of plywood these days.

Thanks again.

[working on it]

We are using 3/4" plywood for the walls. We planned on using 2" x 2" spars to support the roof, seemed to be a reasonable solution but we haven't done this step yet.
It is a 4' x 8' build.
The floor is a full sheet of plywood. The walls will sit on the floor so that the outside width is still 4'...The plan was to use 3/8" ply for the roof. We were going to do The Mix on the walls and the roof before assembling.

* Mine is 4x8, 3/4" plywood floors/walls/roof, etc. Using zero spars, just Simpson StrongTies angle brackets, and generic steel corner braces from Home Depot and/or Lowes. Plus 10-12 tubes of Loctite PL adhesive. I did use additional 1/2" thick 3" or 4" wide oak boards as inner bracing around my doors, windows, and the interior-mounted A/C (in the rear bulkhead). The roof overlaps the sidewalls and front & rear bulkheads, attached with the aforementioned hardware, using 1/4"-20 stainless carriage bolts, fender washers, and either acorn or nylock nuts, with PL adhesive at every interface, and seamed inside and out with the same. No leaks, no runs, no errors! Every hole drilled was filled with PL, and the entire exterior was coated with the "mix" of polyurethane and thinner (3 coats), then with oil-based Acrylic silo or farm implement paint (two coats).

* After 9 years, there has been no separation, delamination, nor warpage. The roof is as flat as ever, since I clculated that there would be sufficient strength just using 3/4" plywood and the brackets, without spars. I used this http://www.woodbin.com/calcs/sagulator/ to calculate the strength, and or possible warpage (3/8" wasn't strong enough for mine.


* I have no build journal, but my postings and gallery photos tell my story, so if you can glean any info that might help your build, then you're welcome. Good luck.

[Sparksalot]

I used 1x2 poplar for the roof spars, doubled at the fan location and hatch hinge. The roof skin was 1/8" birch, covered by automotive vinyl. I used titebond 3 to adhere the skin to the spars, held during drying by staples I later pulled.

[tony.latham]

when we go to attach the roof can we be confident that beads of the PL3X will be strong enough to hold the roof on or do we need to use screws as well.

I believe you need both glue and some kind of mechanical device (such as screws) to attach the spars to the wall. Even with a 4' span and 3/8" plywood, you'll get some flex while bouncing down the road.

I build using sandwich wall construction which allows me to set the spars on the 3/4" wall lip (after installing the headliner) and screw them down vertically so I'm not a guru when it comes to building walls with a single sheet of plywood. So take it with a grain of salt.





Tony

[frosty]

My spars are oak ¾" x 1½ " and are strong with the ply walls

002.JPGsmall850.jpg (195.75 KiB) Viewed 407 times

[carlbergs]

Thanks for the different ideas. I'll sit down with my daughter tonight and discuss the pros and cons for us. I do like the idea of just sheeting the top with 3/4" ply and keeping it simple.

Thanks again all and I will have more questions down the road.

Shawn

[working on it]

... I do like the idea of just sheeting the top with 3/4" ply and keeping it simple....

* You do understand that the main weight of the roof sheets (mine is 50"x48", the "front slope" sheet is separate) rests atop the top edges of the sidewalls (extending from the "front slope" sheet above the doors, to the open rear (where the hatch door hangs from the rooftop sheet) with the angle and corner braces mainly used as attaching plates. The vertical rear bulkhead is 22" from the rear, and it supports the major portion of the rooftop sheet's weight.

here's how it was bolted & glued together

steel braces, bolts, and PL.jpg (247.44 KiB) Viewed 432 times

roof sheet laying on top, unattached.JPG (137.63 KiB) Viewed 432 times

* I used the Sagulator calculator https://woodbin.com/calcs/sagulator/ to verify whether my 3/4" roof would resist sagging without support beams underneath. it said it was acceptable. 1/2" or 3/8" plywood wasn't. I have tested it by standing on top a couple of times (always pushing the envelope...at 230 lbs), and no sag was felt. I put a straight edge on top each year, just to check, and it's still flat. Here's my calculation sheet:

roof weight-bearing calculation.jpg (87.15 KiB) Viewed 432 times

[philpom]

Thanks for the excellent questions philpom.
I just looked at all your photos and your camper is essentially the direction we are going.

We are not opposed to PMF on the roof but we would like to keep the side wood grain. I see from your photos that you slightly sloped the roof toward the back. Can you tell me how tall the wall is at the rear? I could easily do this at this stage and it makes a lot of sense. So with PMF then screws are a logical choice. Did you seal the tops of the screws before the PMF?

Can you tell me your spacing on your spars?
What method did you use to attach them? I was leaning toward a pocket screw and PL3X on the end then butt them against the wall. Again, trying to keep external screws to a minimum so not screwing from the outside of the wall into the spar.

1/4" BCX will certainly save us some weight, yes this is a consideration, and some money considering the price of plywood these days.

Thanks again.

The slope is actually at the front and from the subfloor it is 42" high, the back is square and 48" high. In hindsight I would have added a bit of slope to the rear also for air flow reasons. The spars are spaced roughly 16" except they are 14" at the point where the fan mounted. Since my camper is 64" wide and has a slope I was unable to use a single sheet of 1/4" ply so actual spacing on the spars landed where I needed them to meet up with the seems on the 3 pieces of ply I used for the roof. I attached the spars using adhesive and #10 deck screws through the outside. The top ply rests on the 3/4" ply walls and the spars (so the spars support the roof span but not the full weight at the roof/wall line. I'm happy with this and it was easy for me to accomplish but the best way would be to notch the walls so they accept the spars being flush along the top edge of the wall.

I put 1/4" birch ply on the underside of the spars to form the ceiling first and trimmed that out with quarter round. Then I cut the pink foam board and pressed it in to the roof cavities between the spars and finally glued and screwed the top 1/4" BCX ply to the top. After that I used a router to go around the outer edge to make it perfectly flush and smooth. I wouldn't dance on the roof but I am sure I could get up there.

I used wood filler on all of the screw heads and sanded flush before I applied my PMF and I used cabinet grade birch throughout the camper because while the outside is protected with PMF the inside is fully visible and this grade of ply if very straight and smooth with no defects (it's also lighter than pine plywood. PMF will show any defect in the wood (my opinion).

Good luck with your build, I feel close and then I realize I still need to do this, that and the other!