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Project: 3m by 6m weekend house, roof 3m high, walls 20cm insulation.

Question: I want build it from lighter/thinner materials, and i want to calculate from how thick materials i can build this I Joist (Picture One) So it would be same strength as 200mm by 50mm timber stud. I Joist will be 3m high.

Is there way to calculate thickness of materials for OSB and four Timber studs and how much load it can handle.


Picture 2

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I believe I got your title correct, but if you're trying to construct your own I joists, feel free to correct it. – BMitch Apr 9 '13 at 11:39
I don't think they are meant to handle a compression load in that axis. – Evil Elf Apr 9 '13 at 12:14
@Evil Elf there is a wall joists too, homepower.com/sites/default/files/styles/article_image_main/… or search google Steico Wall – Edmhs Apr 9 '13 at 12:25
I joists for walls in timber framed houses are becoming more common in the UK, as they reduce the level of cold bridging. Some of the companies the sell timer framed walls use them. – Walker Apr 9 '13 at 14:58
Engineered joists are engineered to act as joists. Sounds like you need engineered studs instead. – DA01 Apr 11 '13 at 17:59

This is a bad idea to try and build your own I joists. Often the building codes will defer to the manufacturers specifications for engineered materials like I joists, engineered beams and also engineered OSB board. This is done because every manufacturer is different and because these manufacturers generally have actual accredited engineers that are certified and vetted who test these things in a methodical and proper way.

A small shed, on a residential property, is still considered a seperate structure, and as such is subject to your municipalities building codes, zoning requirements, by-laws and building permits. A review of your plan or an inspection likely will not pass and code enforcement may require you to tear down the structure if they feel it is unsafe. Inability to comply with code enforcement can result in loss of your occupancy permit for the residence.

Code aside, I think even using engineered I joists for studs wouldn't be the best idea. While I joists are okay for hanging drywall on a ceiling, a wall tends to have a lot more nails, screws and anchors run into it for everything from pictures, TV's hanging on lags, etc... With drywall, the screws will be run into the sides of the I joist where they will not damage the OSB in the middle. Running a nail or screw into the middle of an I joist greatly damages the integrity of the joist, and there would always be the possibility that somebody would attempt this and damage the stud. For a shed I am thinking about possibly trying to hang pegboard on the wall as an example.

Furthermore I can't even imagine how one can safely frame for a window or door with I joists. What is the proper way to nail the jack and king stud together? What if the wall is load bearing? Engineered I joists are tested when they are laid out on a span, not supporting load vertically.

Just use dimensional lumber, or purchase pre fabricated engineered I joists for supporting the load on a floor per the manufacturers specifications.

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1) it will be small shed, that can be lifted with crane, and moved anywhere you want to, and container/module houses under 25 square meters dont need any plans. All i need is dimensions and location where it will be, to get permit 2) there is companies that sell wall joists, for example "Steico Wall" 3) for window i will use lumber studs, – Edmhs Apr 9 '13 at 12:36
@Eddsstudio It sounds like you already have the answer then, but I still doubt it will pass a code inspection if you build the I joists yourself. At the very least I am glad that you are using dimensional lumber for window and door frames. – maple_shaft Apr 9 '13 at 12:40
But is there a way to calculate how thick OSB and timber studs to use? OSB(6mm, 8mm, 12mm, 15mm, 18mm, 25mm) studs (20mmx20mm, 30x20, 40x20, 50x20, 50x50) – Edmhs Apr 9 '13 at 12:44
@Eddsstudio Yes there is but I am not a mechanical engineer, and clearly neither are you if you are asking me such a question. This is a Q&A site for DIY advice so I imagine engineering questions are off topic. The only point I am trying to make is that even if you can scientifically prove it is more than capable of handling the load that it won't matter to an inspector because you are not a licensed engineer and you are not making a manufactured product with appropriate quality assurance checks. – maple_shaft Apr 9 '13 at 12:58

I am not familiar with metric tables or codes in the UK, however there are lots of free online span/size reference tables for timber/type of wood, LAMS, and fabricated I-beams. I do urge caution if you are considering making your own I-beams. Fabricated beams have to meet specific standards for the type of woods, glues and fasteners. There are often made with a process of epoxies, pressure and metal inserts. We never try to fabricate our own beams for joists or trusses. I guess the application would be an important factor as to whether self made beams would be safe and strong enough.

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Im not building 3story wooden frame house, its a small "shed". I need 20cm(7.9 inch) insulation, so using 20cm (7.9 inch) by 5cm(2 inch) timber studs every 20-30 inches is overkill. I found I joist and i borrowed idea, these joists will be used for walls only, and they are lighter then timber stud. – Edmhs Apr 9 '13 at 10:04
It is rather unique to use fab beams as studs, but practically speaking, I don't see any reason not to use them for a shed. The "timber" portion of your beams adds up to enough to use as a stud. Not sure of the spacing however. Here we use 16" to 24" OC studs depending on roof load etc. Insulation comes in 14/23" widths for this stud spacing. Guess it might be slightly different for metric measurements. The top plates should be solid wood for your rafters to nest to. – shirlock homes Apr 9 '13 at 10:36

If I understand correctly, you are trying to fill i-joist bays with insulation. If that's so, then you should do a simple 2x4 stick frame 24" OC (this will be your structure), sheathe with 1/2" (or 5/8) plywood, air barrier, then on the outside attach your i-joists vertically to be filled with insulation. Your joist will then be similar to a larsen truss (google it).

If you are dead set on using your trusses as such, then you should replace the two 2x4 on the bottom with a single piece (2x6) perpendicular to the OSB/plywood and with a cutout similar to the commercial i-joists.

Take a look here at this Building Science article for an example similar to what you're suggesting. In this case, the 2x4 interior wall is the load bearing structure. The 2x3 and plywood/OSB joiners are only in place to hold the insulation.

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I don't see any mention of insulation in the question. This is a very confusing attempt at an answer at least. Flagging it as low-quality. – ShoeMaker Apr 11 '13 at 10:47
Actually if you read through the answers you will see why he wants to make his own i-joists. "I need 20cm(7.9 inch) insulation, so using 20cm (7.9 inch) by 5cm(2 inch) timber studs every 20-30 inches is overkill. I found I joist and i borrowed idea, these joists will be used for walls only, and they are lighter then timber stud" – jonnycowboy Apr 11 '13 at 16:13

Trus-Joist Corporation has an informative .pdf that addresses some of these problems. It's here: www.woodbywy.com/document/tb-821/ (I found this link by Googling the phrase 'I-joists as wall studs'.) One of the main problems I noticed in the document is securely attaching the studs to the floor. What they describe is to add stiffeners to the OSB web at the bottom end, then use metal connectors to attach that combination to the floor. If I were doing your project, I'd have no problem doing that, but for a commercial builder, it's some extra materials expense, and quite a lot of extra labor. So I guess builders who are concerned -- as you and I are -- with decreasing the amount of timber used in building, while obtaining the most effective wall insulation, will want to do a cost trade-off between the extra cost of the structure, and the energy cost savings over its lifetime.

When my wife and I built our straw-bale house in southern Oregon, we used conventional 8-inch stud construction for the gables, insulating them with 8" R-30 fiberglass batts. We love the house, which took us 2 years to build.

Hope this has been some help -- Jerry Brown

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