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WILL1E
Moderator
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# Posted: 20 Sep 2021 08:58am
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If you haven't read my build thread, the ground at my build site slopes more than i had thought. The front of the cabin(left side of photo) is at (2) 8" blocks high all the way across. The backside of the cabin (right side of photos) varies. The two pads in the far back right corner in the photo are (1) 4" block high each, next two pads are (1) 8" block high and the pad closest to the front right of the photo is (1) 8" block and (1) 4" block.
My built up beams will be 3 layers thick of PT 2x10's. My floor joist will be 2x10's.
My plan was to use PT lumber for my rim board and the joists would be regular lumber. Since I did not want to go another row of block higher to avoid looking like the place was on stilts, the underside of the joist in that back right corner is going to be around 12-13" from the gravel.
So the question is, do i need to use PT lumber for my joist because of that? If so, do i need to use PT for all of the joist or can i just use it on the joist at that end of the cabin and the switch to regular lumber as i get further away from that corner?
This is the best photo i have of that back right corner. The UNDERSIDE of the batter board is where my strings ran but then the batter boards on the left side of this photo the strings were on top. So basically it drops about 12" going from right to left.
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gcrank1
Member
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# Posted: 20 Sep 2021 10:28am
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Seems I remember 12" from ground or less is best done PT. Reg's, I dont know.
So, If Im getting this, your box frame on the PT beams will be that beams 10" (nom.) higher, well above ground moisture. No need for Pt for box frame or joists.
You are going to not have the box framing exposed, right? Exposure bad.
And have enough eve to keep rain runoff away from the base.
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WILL1E
Moderator
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# Posted: 20 Sep 2021 10:50am
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Bottom of the box frame will be around 12-13" from the ground at the closest point.
Quoting: gcrank1 You are going to not have the box framing exposed, right?
It will be exposed on the underside. Ill eventually do insulation under there.
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gcrank1
Member
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# Posted: 20 Sep 2021 11:23am
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I was getting at the exposed like we see people do where the siding doesnt extend down past the floor deck (bad) because they didnt factor in the depth of the b-frame when thinking siding sizes, especially when using sheet products.
You could just paint those undersides nearest the ground with a 'treatment', even solvent thinned (gas?) motor-oil for peace of mind.
Fwiw, I couldnt bear the thought of having to crawl under my deck (not unlike the pitch of the ground you have) on Cabin I to insulate. We put ledger strips on the joists and cut/fit in blue DOW styro, then the floor deck. I didnt want to squeeze the styro between the floor and joist tops, but I guess it works ok too.
It worked Great! Id do it again.
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BRADISH
Member
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# Posted: 20 Sep 2021 12:52pm
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I think you're at the make it/break it point for using PT.
In my mind it may come down to how well you channel the weather away from the cabin - For example, do you plan on putting rain gutters on the place? Reducing all the splashing water within ~1' of the foundation would go a long way in keeping non-PT boards protected.
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ICC
Member
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# Posted: 20 Sep 2021 02:33pm
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Code compliance in most areas calls fot PT beams when less than 12" from beam bottom to ground. Floor joists is 18"
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WILL1E
Moderator
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# Posted: 21 Sep 2021 08:10am
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While i'm going through the proper permitting, i doubt the BI will actually look underneath...heck, i'll be shocked if she even comes onsite based on what i've heard from others.
So, any downsides to using PT joists for 1/3-1/2 of the total floor joist?
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Nate R
Member
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# Posted: 21 Sep 2021 08:44am
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Downsides? Only that they might shrink more than the regular joists as they dry....But I wouldn't let that stop me.
Make SURE you're using PT approved fasteners and connectors.....I've seen AMAZING results with how fast the ACQ stuff can eat them.
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ICC
Member
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# Posted: 21 Sep 2021 09:27am
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Extra cost, but you'd have the protection. Like Nate R stated be certain to use the proper fasteners and any metal that contacts the PT wood.
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travellerw
Member
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# Posted: 21 Sep 2021 11:49am
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I don't have the time or access to look it up.. BUT I believe PT wood has different span tables (less). The treatment reduces some of the strength... I would suggest you do some research before going that direction, and upsize if required.
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WILL1E
Moderator
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# Posted: 21 Sep 2021 01:48pm
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You are right...might have to use regular lumber and just do some skirting in that corner to keep things (fluffy white stuff) from blowing under there and piling up.
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Brettny
Member
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# Posted: 21 Sep 2021 08:20pm
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Well right now PT is cheaper than kiln dried so if your buying it now, yes you want PT. I did a outbuilding rafters in PT because it saved me about 20%.
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WILL1E
Moderator
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# Posted: 22 Sep 2021 08:02am
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PT for me is about 25% more right now on those 16'.
But as travellerw pointed out, PT beams have less span capability. #2 2x10" SPF has a span of 14'6"....i'm at 14'3" span between my beams.
My local Lowes has "fir" which is listed as coming out of the northwest of north america. Not sure which fir to select on the span calc, but looks like all the firs get better span ratings for the non-PT stuff.
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Just
Member
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# Posted: 22 Sep 2021 10:22am
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The last one I built I covered the whole bottom with pt plywood 3/8 in..I used pt. Peers and rim the rest was untreated. It was only 9 x12 so built it upside down to ease work underneath and then with the help of 3 friends flipped it over on the beams .You would need to build it in sections or nail it from under neath . Makes for easy installation of insulation, it makes a solid bottom to keep out pest also .not sure if it would work for you
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