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DavidH
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# Posted: 1 May 2014 11:37am
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Hey there, I'm in the design stage of a 18' x 26' timber constructed cabin. My area has a bylaw for maximum height.
Can anybody tell me based on personal experience, how high off the ground the beams (sitting on piers) should be above grade to help prevent decay, and to allow access underneath if necessary to (at a later date) spray foam insulation under the floor?
Based on the design and local bylaw on height restriction, the bottom of the beams is currently expected to be 18" above grade.
Is this enough clearance to help prevent decay? is this not enough?
Thanks
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MarkG
Member
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# Posted: 1 May 2014 03:12pm
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The 2006 IBC madates grade minimums of less than 18 inches requiring the use of approved grade PT wood. So, I would say 18" or more you should be good!
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MarkG
Member
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# Posted: 1 May 2014 03:15pm
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Wanted to add that having a vapor barrier topped with gravel under the structure, and obviously good drainage would go a long way to prevent decay.
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bldginsp
Member
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# Posted: 1 May 2014 03:28pm
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Building codes allow 12 inches under floor for girders, 18 inches for joists. Log builders say keep the sill logs 18-24 inches off the ground. As stated by MarkG, your 18 inches is plenty high, but you could still have problems if you let the brush grow up to it, holding moisture, or if they are uncovered on the outside such that water dripping down the walls can get onto the beams.
But if you plan to get under there to spray foam, you might need more. I wonder just how far from the surface the spray wand has to be held, and how long the wand itself is.
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leonk
Member
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# Posted: 1 May 2014 05:03pm
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depends where you are. it was common in places with a lot of snow to build high off the ground, say 3-4 ft
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MtnDon
Member
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# Posted: 1 May 2014 06:38pm
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Bottom edge of girder less than 12" above grade = PT wood needed.
Bottom edge of joist less than 18" above grade = PT wood needed.
Those are still tight to crawl under and do any serious work should that need arise. But taller than those makes for a less and less stable pier and piers are not all that stable to begin with.
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Don_P
Member
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# Posted: 2 May 2014 07:05am
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If the "bylaw" is for an accessory structure, it normally reads 18" to the top of the finish floor. This relates to Mt Don's comment on pier stability. In that case it works better to build the floor system out of treated and then switch above.
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mongo1
Member
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# Posted: 3 May 2014 05:03pm
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I built my cabin on 6x6,s 3 feet above grade.I have gutters installed and direct the water from the roof 4 feet away from the building.It,s great for putting stuff under.Most of the firewood I used this winter was stored under the cabin and stayed bone dry.I use my cabin a lot and while up this winter I always shovel any snow away from the piers mainly because i need a path around the cabin to run my 4-wheeler.I keep extra lumber-ladders-firewood-etc. under the cabin.I tried keeping plastic totes with firewood kindling under the cabin but the dang bears like to play with the totes.have a super day!!!Mongo
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DavidH
Member
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# Posted: 7 May 2014 04:05pm
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Thanks everyone for the responses.
The plan was to use pressure treated lumber for the beams and joists. I'll take Mark's advice and put some plastic under it, covered with a bit of gravel on top. I'm guessing this is as sun can hit the portions not shaded by the structure (around the perimeter) the ground will warm up, pick up moisture from the ground, and rise up to the bottom of the structure (which is shaded).
My max height seems restricted based on the size of the concrete columns, at twice the width of the columns.
Not sure what to do about the subfloor that will sit on the joists; perhaps painting before installing?
David
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