Post and beam roof over trailer question
Hi, folks.
I’m going to put a roof over my trailer this Spring, and am just looking for clarification on one point. Here’s the details on it:
Roof will be 24′ wide by 42′ long, end gable style.
I’m using a ridge beam and outer beams supported by 6×6 posts every 7′ – 8′ (depending on where windows in the trailer fall) but a max. span of 8′.
I’m using 2×8 rafters on 16″ centres fastened to the ridge beam with rafter hangers and birdsmouth cuts at the outer ends with hurricane ties.
I’ll be putting 1×3 strapping at 2′ centres and covering with a metal roof. No ceiling, no rafter or collar ties (need the clearance to get the trailer in and out)
I’m hoping to use a 4/12 pitch, but can go to a 5/12 if need be (want to keep the height of the peak as low as possible for aesthetic purposes)
I’d like the outer edges to be 8′ off my deck, and the centre to be 12′ (or 13′ if I have to go 5/12).
My question is, will my beams be sufficient if I’m using a triple ply 2×10 and laminating them as follows:
16-16-10
10-16-16
16-16-10
Snow load in my area is rated at 31.5 PSF. I will be running 3 beams (2 outer and a centre) of the same construction, with 7′ centred posts of 6×6 on the outside “walls”, and as previously mentioned, centre will have the posts adjusted to fit for windows/doors, with a max. of 8′. If needed, I can put in plywood triangles at the tops of the rafters coming down a foot, but beyond that, it will interfere with the roof of the trailer.
If I’ve not been clear on a point, or if you need any more information, please ask.
Appreciate any help on this!
Cheers,
BA.
Replies
How will you support the center beam?
Hi, Florida.
The centre beam will be supported by 6x6 posts every 7' set into 12" builders tubes in concrete at 42" deep.
Okay, I was thinking in terms of a double-wide. Your beams should be fine but I'd still be concerned with the lack of joists or collar ties. Maybe you could use some cables and turnbuckles that could be removed as the need arose.
The ridge beam will keep the walls from spreading. Still not a bad idea to have this engineered, though. I'd be very worried about wind loading with an open structure.
Curious about the shape of the trailer if it precludes old fashioned collar ties, 1/3 of the way down from the top - 16" in this case.
Engineered trusses would be a lot cheaper than stick-framing this. Even cheaper would be an engineered steel building - just get the walls taken off the BOM to reduce the price further.
In this area, (professional) engineering is required for such a structure. I'm not sure what means you are planning to stop the sides from spreading, how you are planning to effect shear walls, or stop the posts from spreading at the bottom, but if it's all OK, the engineer would stamp it. Before investing in the materials, I'd suggest talking to an engineer.
Hi, berferdt.
It's a 41' park model trailer with 2- 14'6" x 4' tip outs on the outer side. I've looked into engineered trusses and the cost is around 8 times the amount of doing it with 2x8 rafters and beams. I can come down a maximum of around 12" or so, and then the rooftop AC unit and the vents/chimney would clip the collar ties if the trailer were to be pulled out. The lot is slightly slanted front to back. Also, it has a higher roof in the livingroom area at the front of the trailer.
As for keeping it from spreading, I would have thought by having each beam carried on 6x6 posts every 7' from end to end, each would support the weight and since the centre beam is fully supported in the same manner, the hangers for the rafters on either side of the ridge beam would prevent spreading. But, as I said, I'm able (and willing) to put in plywood triangles at the tops of each set of rafters on both sides, glued and screwed, if need be, for added rigidity.
I've looked into LVLs for it, but have been told I'd have to get 2- 42' x 6" LVLs per beam but can cut them into lengths of 14' and offset them (14-14-14, 7-14-14-7) and use steel plates for connectors. The "seams", if you will, would be sitting directly on posts with those spacings. The costs, however, was around $1450 CDN plus taxes for the 6 beams, vs. around $550 plus taxes for doing the triple laminate 2x10 beams.
While a pre-fab steel structure would solve a lot of problems, the park owner is against it. His solution is to build a simple gable end roof over the trailer, 12' wide, then attach a roof to the one side to cover the deck. Unfortunately, (as I've discovered on a few in the park already) the roof over the deck area is WAY too low, and it seems like you're in a cave suffering from claustrophobia! The way I'm wanting to do it, the roof over the deck would start at 12' up, and end at the outer edge at 8' up, so lots of airy space over the deck, as I'll be building an outdoor kitchen/bar under it, and a screened in entertainment room.
I hope this clears up any questions you had.
Basically, from what I've researched, this would work, but my question is about the size of the beam, as in, would a triple ply 2x10 supported every 7' on 6x6 posts in concrete be strong enough to support the structure with the aforementioned snow loads.
Cheers,
BA.
* What's preventing you from raising the whole roof up two or four more feet so you can use rafter ties?
* If there's any question of load, why not go from 2x10 to 2x12? 2x12 is still quite inexpensive compared to the cheapest I-joist or LVL, and will hold very roughly twice as much load as 2x10. I would be plugging 2x14 or 2x16 if it existed at most lumberyards.
* If you're laminating lumber together into a beam longer than any one piece of lumber, try to do it so that no two breaks occur at the same place, and you get maximum separation between breaks.
* Laminating lumber together and rating it is the domain of a certified structural engineer.
* Those center posts - how are you going to achieve the height? Do you have availability on a 6x6x16 or a 6x6x20 pressure treated post? If you're going to be laminating things already, I would consider replacing the monolithic 6x6 posts with 4-ply 2x8 posts, which you can interleave at junctions with 2-ply of meat bolted together; This style of joinery seems a lot more likely to me to hold up than your traditional birdsmouth-notch rafters; Remember that each beam is taking the force of 8ft of tributary snow load, on the order of 3000lbs each spread between ridge and outer posts. (Although I'm not sure how well this will tend to hold up to weather)
* Those steel nailer plates you buy off the shelf for hurricane ties are allegedly not rated for the same stresses as the ones you use in truss plates. If you do go with laminating beams, go nuts with the construction adhesive and big nails/screws/lagbolts/lagscrews.
* You're spanning 7-8ft at 30psf between beams. 1x3 strapping at 2' centers is not sufficient for that. 2x4 strapping is not sufficient for that. 2x4 purlins (on their end) are only borderline. Ideally you should use 2x6 purlins on end (16" OC for some extra strength maybe?), and I'm unclear on how you should hold them on there - probably some random Simpson hangar, with extra 2x6's riding on top of the beam to provide nailing surfaces, and plywood to tie the 2x6's to the 2x12's.
* 7ft spacing is going to play havoc with your 8' plywood sheathing, all of whose edges should be secured to dimensional lumber blocking, and whose edges should ideally avoid butt joints (4 panel edges meeting). 3/4" sheet goods are probably not optional here, and T&G would likely help a lot if some of that blocking is missing.
* You're building a bunch of trapezoids. These will hold up to gravity just fine. What's going to happen when the wind gets ahold of them, though? Is the only racking strength in this structure comprised of a bunch of 42" chunks of concrete foundation plug straining against soil under the force of an 8-12ft lever arm?
The whole project gives me a feeling of overkill. You've put a fair bit of thought into this, but I think you could get better suggestions if you describe the project from the start. Is this structure meant to enclose the mobile unit permanently? Why the limitation on height, just cost alone? what are you doing to prevent the structure from racking. A drawing would be very helpful. With proper bracing you can get the size of the lumber down and still be plenty strong or are you a fan of big timbers?