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I am building an 8'6" x 20' tiny house on wheels and am trying to size up the ridge beam. The 2x6 rafters are placed 24"oc and are mounted to beam with Simpson Strong-Tie hangers. The beam rests on 4x4 end posts. The roof will be vaulted on the interior, so no collar ties. Here are some specs:

Currently, I have a 4x10' and I understand the best option would be to consult a structural engineer, but wanted to get some more input from others.

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Note: you'll notice I've tried two different type of gable ends, one with 45deg support ends.

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Massive overkill.

If you are content to give up the vertical height this monster takes, simply extend the line of its bottom out to 1/2" from the top face of the rafter and cut a lot of (looks like 24) 1/2" plywood triangles.

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Glue and screw to the face of the rafters (which you'll cut to meet in the middle.) You can cut a scrap of 2X to fill the bottom void, but it will make little difference for this scale.

You don't need a ridge beam at all. You just need to keep the ends of your rafters from spreading, and the gusset will do that nicely for a house this tiny. If you want to cut the points off and use a 1x4 ridge board on top, sure, fine, structurally irrelevant.

This is particularly important since you are putting it on a trailer, so excess, wasteful weight is a detriment.

Which might be a good reason to talk to a licensed engineer (I'm merely a vernacular builder, but I have sheds bigger than that with less roof structure that have taken snow loads for 20+ years) about how to minimize the structural overkill to suit the scale of the house. You've already given building codes for houses the brush-off by putting it on a trailer - perhaps you should learn more about how trailers are efficiently built before you give up a lot of interior volume and add a lot of weight for structure you don't actually need.

If time is not too much of the essence, you could potentially get some engineering class interested in taking it on as a class project.

If you are open to a change in roof style, the traditional mobile tiny house (the Gypsy caravan) features a stylish arched roof structure (you might need to consider altering the traditional exterior detailing in light of what speeds you'll tow at.)

  • Hi Ecnerwal, this is the type of information I am seeking. I am very keen on keeping unnecessary weight down — this is one of the reasons why I chose to place studs 24oc. You have a great point about the gussets not taking up much more space, which is something I can consider now that I had not thought about. Great idea, thank you. I have recently taken to the idea of steel rod collar ties as they may fit in aesthetically to the overall design. Do you have any experience with these? Anyhow, thank you for taking the time to respond thoughtfully! – Elsw Jun 26 '17 at 12:43
  • Steel rods make lovely collar ties, as they are in tension and steel is great (efficient) in tension, so they can be quite small - many Victorian era timber-framed building use them. You could also almost certainly use 2x3 rather than 2x6 for the rafters, here - given that the rafter span is 4 feet. Then again, you could use Structural Insulated Panels for your walls and roof and gain a lot of insulation and strength while losing a lot of framing lumber. – Ecnerwal Jun 26 '17 at 13:54
  • Unfortunately, SIPs weren't a convenient/cost-effective option for my area; we're using 2x6 to make up for insulation. Thanks again for your input Ecnerwal! – Elsw Jun 26 '17 at 14:53
  • 2x4 with cavity fill combined with 2" of rigid foam will be a lot better insulation than 2x6 with cavity fill... – Ecnerwal Jun 26 '17 at 17:50
  • Interesting, what rigid foam are you thinking of? XPS is around R-5 per inch. and Polyiso is around R~6-8 per inch, making a max of R-16 with Polyiso. The 2x6 insulation I'm using is R-23. EDIT: just realized you wrote cavity fill in addition. Disregard above. – Elsw Jun 26 '17 at 21:35
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In case you're still wondering, your original question about the 4x10, yes, it will work as a ridge beam, but don't forget the post connectors...like Simpson's ACE post caps.

A major concern is keeping the two halves of the structure together as it moves on the trailer. Once the structure starts vibrating, the structural considerations change from static construction. For the ridge beam, you'll need to consider withdrawal of fasteners from joists hangers. For gusset construction, you'll need to consider edge nailing (into gusset and into beam,) nail spacing, etc. (Nail size, spacing, etc. wasn't given, but is critical due to vibration and movement.) Spacing is also important for developing adequate resistance without having the nails too close together triggering a split in the gusset or joist.

  • Great idea on the post caps for the ridge beam. Thanks for pointing out the edge nailing. Luckily this particular structure won't be moving all too often (maybe a 3 times short distances in it's life) but still should be done properly. Thanks again! I feel like the better direction is using a ridge board with steel rod rafter ties now for the lateral tension. Thoughts? – Elsw Jun 27 '17 at 16:06

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