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teachertoby
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# Posted: 25 Sep 2019 06:11pm - Edited by: teachertoby
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I am in the midst of building a small structure on my woodland in upstate NY--it's an 8 by 12 platform (built on grade on concrete blocks with 2x6 pt joists covered with pt plywood). I erected the two longer side walls and built them in an unusual manner--36 inches spaces between studs and some studs are turned 90 degrees from normal orientation. This is to allow for large 4 ft by 3 ft openings for mosquito screen and the bottom 2 feet are covered by tar paper and will get pine shiplap siding.
Looking at the pics, is this way underbuilt?
The roof will be made of triangle trusses (5 ft long 2x4 rafters) spaced 24 inches apart that will sit on a triple top plate (I need the extra height). The trusses do not line up with the stud work. I intend to use clear polycarb plastic sheets for the roof. I plan to build front and back walls out of 2x3s with screens and door on front wall and shiplap siding on rear wall. I plan to tie the side walls together with single 2x4 crossbeams at the very front and back as well as with the crossties at each truss.
I have attached some photos--is this completely underbuilt? There will be no insulation or interior coverings--this is really a glorified gazebo or covered tent platform to be used in the summers only.
Any and all advice or feedback or help is welcome--I don't want to build something that will collapse!
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teachertoby
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# Posted: 25 Sep 2019 06:13pm
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Apologies--I don't know why the photos are upside down. Will try to upload a couple more here.
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ICC
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# Posted: 25 Sep 2019 06:28pm - Edited by: ICC
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do a google search on the photo orientation thing..Happens a lot with phone cameras
this might help https://www.howtogeek.com/254830/why-your-photos-dont-always-appear-correctly-rotated /
basically if you have an image editing program you can correct the issue
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teachertoby
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# Posted: 25 Sep 2019 06:47pm - Edited by: teachertoby
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Thanks for the advice on the pics.
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ICC
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# Posted: 25 Sep 2019 06:58pm
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NY state has snow last I knew. My first concern is about the polycarb sheets on the rafters. Seems a little iffy with rafters at 24" as the only supports for the somewhat flexible polycarbonate sheets.
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teachertoby
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# Posted: 25 Sep 2019 07:17pm
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Thanks--you are right about the snow! the polycarb actually is drilled into 2x3 purlins that run perpendicular to the rafters and I can space those out more closely.
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moneypitfeeder
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# Posted: 25 Sep 2019 08:18pm
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[I'm no expert by any means...] Maybe ICC will see your post, but I wonder if your rafters would better tie the system together if they were bird's mouth joints connected to the supporting walls.
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ICC
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# Posted: 25 Sep 2019 08:37pm - Edited by: ICC
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rotated
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ICC
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# Posted: 25 Sep 2019 08:49pm - Edited by: ICC
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As moneypitfeeder stated, it might have been better to use standard simple rafters with rafter ties at wall tops. They are proven and can be tied to the wall with metal rafter ties quite easily and provide a good connection.
The home-made trusses may work. Their biggest flaw is potentially the use of what appear to be deck screws. Those usually have poor shear strength, that is they can snap or shear off. I have broken them at times when I screw them into a denser section and the drill motor just snaps them before I stop the drill motor. The other potential problem with them is that if the wood decides to split at the screw, it loses all strength. It would have been better to angle cut them so the members were all in the same plane and then cut exterior grade gussets from 3/4 plywood and nail them. Nails bend and distort rather than fracture.
Assuming the poly-c does not break there can be hundreds of pounds of snow load. Snow might mostly slide off, but it might also stick if it is kinda wet when it falls and the temperature drops.
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ICC
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# Posted: 25 Sep 2019 08:53pm - Edited by: ICC
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Rafters and trusses should always fall over wall studs. If you do triple the top plate that will help distribute the load.
Do ypu have any idea what the local ground snow load is? That is the figure used to calculate spans. You should be able to find info on that online. You might find the snow map at http://design.medeek.com/resources/resources.html useful. Be aware that you only get one chance per day to check the load on a location you click on the map.
The wall stud load path should also be continuous from rafter down the stud all the way to the beams that support the floor framing.
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ICC
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# Posted: 25 Sep 2019 09:02pm - Edited by: ICC
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The 8 foot span for the 2x6 floor joists on 24" centers are at the max. if that was a cabin floor. Should be okay for your purpose. The PT plywood is needed to head off rot but I don't particularly like the idea of sleeping on top of it. I might worry too much about chemicals in stuff so take that as just one man's opinion.
In one photo I see that one of the concrete blocks used to support the 4x4 beam is laid on its side. Those blocks are not meant to be used in that orientation. I've seen them crack and break when used sideways under a small shed. If you worry a bit about things you could either jack it up and redo the block with the internal voids placed vertically, or stuff concrete mix into the cavity as solid as you can.
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Brettny
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# Posted: 26 Sep 2019 09:28am
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Im also in NY and see many problems. Where in in "upstate ny" are you?
First your roof and wall stud spacing is to far appart, alomg 12ft you only have 3 wall studs actualy foing down to the floor. 2x4s are to small for even a 16in spacing on the roof and your collar tie ( the horizontal board near the peak) is to far up even for 2x6 at 16in spacing.
Also theres no support for side to side racking. That plastic roofing really sucks and gets holes or tears real easily. Metal would be a whole lot better and hopefully snow would slide off it still.
With the way your structure is built now i dont see it lasting a winter with a 2ft snow fall.
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teachertoby
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# Posted: 26 Sep 2019 09:50am
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Thanks. I am near Hobart, NY. I will add more studs that go from top plate to botttom plate. I will also add more rafters and put the collar tie across the top plate instead of so being so high up. I thought 2x4s were sufficient for a 5 ft rafter span.
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fitzpatt
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# Posted: 26 Sep 2019 12:04pm
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You may also consider glued and nailed additional rafter support. Some scrap plywood, glue and nails can add considerable strength.
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ICC
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# Posted: 26 Sep 2019 02:25pm - Edited by: ICC
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Quoting: teachertoby I thought 2x4s were sufficient for a 5 ft rafter span.
Rafter span is measured horizontally. On a cabin that is 8 feet wide the rafter span is 4 feet.
Using the AWC calc we see that #2 S-P-F can safely span 4 ft 5 in when placed 24" OC and support up to an 80 PSF snow load. That span will vary depending on the species used. No issue with the 2x4 material, IMO. However the rafter truss should be modified as mentioned above.
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Brettny
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# Posted: 27 Sep 2019 09:58am
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But once you cut a birds mouth your not left with much meat to hold the eve on.
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