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Ketchikan
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# Posted: 25 Sep 2013 09:12
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Hello from a newbie to this forum.
I have a 16 x 16 cabin in Alaska.
The walls are 8' high and the roof has a 12/12 pitch.
The original owner who built a loft for half the cabin and left the other half as a cathedral style.
He built the roof using 2 - 2x4x16 nailed together as a ridge board, not a ridge beam, and 2x4's placed on 12" centers as the rafters.
There is a 4x4 post directly in the center of the cabin that goes all of the way to the ceiling for support.
There are 2 - 2x6x16, nailed together, used as a beam which tie the sides of the cabin that the rafters sit on.
What I would like to do is eliminate that 4x4 post but keep the loft and cathedral open.
Does anyone know if I can convert that ridge board to type of ridge beam with support on both ends and maybe use a collar ties?
Thanks for all replies.
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rockies
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# Posted: 25 Sep 2013 19:22
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So the original owner put two 2x4's next to each other and then nailed them together to form a 4x4? Do the rafters sit on top of this or are they butted into the sides of it? I take it the 12/12 pitch roof is sitting on top of the 8 foot wall with no short walls above the loft floor line? (In a section the cabin's shape would look like a triangle loft area on top of a rectangle). The ends of this ridge board must be resting on posts set in the exterior walls. Your roof sound very undersized for the amount of snow you must get in Alaska. 2x4 rafters are really just 3 1/2 inches deep, so a great deal of weight is being supported by that post. The post also keeps a lot of the roofs weight from pushing down and out on your exterior walls. I think you would have to really beef up the rafters to 2x8's and the ridge beam to 2x10's to support the roof without a post, and then you have to put in collar ties at a height of 1/3 down from the ridge (check codes). The collar ties will interfer with your idea of having it look more open and will really reduce the height in your loft because you would have to do the whole roof to stabalize it properly. That gives you about a 5 foot headroom in the loft. I would leave the post and perhaps sister 2x6's at a minimum to the 2x4 rafters to strengthen the roof.
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toofewweekends
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# Posted: 26 Sep 2013 02:14
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Ketchikan,
I'm also in Alaska, and while bigger rafters would be the way to go if you were building, this is an interesting situation. If you're in Southeast (Ketchikan, maybe?) you probably deal with more rain. My place is north of Anchorage and 4+ feet of snow sticking all winter isn't uncommon. That said, we have friends who have had outbuildings with 2x4 roofs with 16 or 24" centers with a decent pitch that held up for years. If they'd been there to shovel in super snow years, the workshop roofs would still be fine. I would think collar ties would help, plus the end supports. And a snow rake.
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Martian
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# Posted: 26 Sep 2013 09:28 - Edited by: Martian
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If it was me, I'd put collar ties roughly 2' below the ridge. The ridge board can't sag if the walls can't move outward. Preventing that is the job of the collar ties, and the support post can then be removed.
Tom
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MtnDon
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# Posted: 26 Sep 2013 10:56 - Edited by: MtnDon
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Quoting: Ketchikan anyone know if I can convert that ridge board to type of ridge beam with support on both ends
No, or at least it is not easy, not cheap to convert a ridge board to a ridge beam.
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The 2x4 rafters may not be as bad sounding as one might think. The AWC calculator indicates that common SPF grade#2 2x4 on 12" centers can actually support 30 psf of snow load up to the 16 ft width of this cabin. So, if toofewweekends is right on his location sleuthing. the roof rafters are barely okay.
Once again I believe there is some confusion about what is a collar tie and what is a rafter tie and what each of those do. Ketchikan, have a look at this thread. As noted there a collat tie is in the upper third and does nothing to prevent wall top spread.
Ketchikan , you make no mention of how the loft is floored. What the loft floor joists are. But we'll assume they are acting as rafter ties for the loft end of the cabin. That leaves the other, cathedral ceiling end. There should be rafter ties across that area too. You could probably get away with using doubled 2x6's every 4 feet across there. As long as there is no loading on top of those, that is no floor on top of those ties 2x6's are strong enough if well nailed to the wall tops and the rafters.
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That leaves the question of what the 4x4 post is actually doing. That is hard to say from a distance. It might not be doing much at all if it stops at the ceiling and does not reach the ridge. I can't be sure from your description. . What does it rest on?
It might be interesting to measure the distance between the side walls at the ends and at the center. Measure across the top. Is there any spread evident in the center? Another interesting exercise could be to measure down an inch from the ridge at each end. Pull a tight string between nails at those points. Is there still one inch under the center of the ridge?
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Ketchikan
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# Posted: 26 Sep 2013 13:08
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Thanks for all of the replies.
This is in Ketchikan, AK so we get very little snow.
The support post actually goes from the floor to below the 2 - 2x6 beams and then there is another 4x4 post from on top of the 2x6 beam up to the ridge board.
After closely looking at the loft unfortunately he ran the loft joists off of the 2x6 beam back to the rear wall of the cabin and they do not act as rafter ties. I will be fixing this by installing 2x12x16 joist to act as both a loft joist and a rafter tie placed on 16" centers.
In the cathedral area do you think that a doubled 2x12 placed at 4' could handle the wall spread?
Thanks to all.
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MtnDon
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# Posted: 26 Sep 2013 13:29
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Quoting: Ketchikan In the cathedral area do you think that a doubled 2x12 placed at 4' could handle the wall spread?
If these have no loads on them, that is they span the walls as a rafter tie and there is no flooring on top the 2x12 is more than what is needed. A 2x6 of any common #2 construction lumber can span 19 feet as a simple rafter tie / ceiling joist.
Quoting: Ketchikan After closely looking at the loft unfortunately he ran the loft joists off of the 2x6 beam back to the rear wall of the cabin and they do not act as rafter ties. I will be fixing this by installing 2x12x16 joist to act as both a loft joist and a rafter tie placed on 16" centers.
That would be an excellent idea.
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Ketchikan
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# Posted: 26 Sep 2013 13:33
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Great, so with the 2x12x16 on half of the cabin and with a 2x6x16 across the cathedral at the 4' mark that should give me enough support to remove the 4x4 post?
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MtnDon
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# Posted: 26 Sep 2013 15:09
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Assuming that these are well attached to the wall and rafter tails it should be safe. Understand that it is not code; code calls for a rafter tie on every rafter. But most folks do not have 12" spacing on rafters, more like 16 or 24". A doubled up pair of 2x6 would be even better. You could box that to appear more beam like for appearance sake.
Use at least 3 - 16D nails for the rafter to rafter tie connection. ((TABLE R802.5.1(9) RAFTER/CEILING JOIST HEEL JOINT CONNECTIONSa, b, c, d, e, f, h )) Those will protrude on one side when nailed through. Clinch them over and that greatly increases the connection strength. Then nail on the doubled one so it is rafter tie - rafter - rafter tie.
With all that done the 4x4 post should not really be doing much.
It would be good to run some strings or take measurements on the ridge before and after just to see what happens.
Understand that with the rafter tails, at the wall top, properly retrained the ridge board is simply something for the rafters tips to be nailed to. The ridge board is not a structural element, but the rafter ties are.
Checking the wall to wall distance as mentioned before would be a good idea too as that will indicate if the walls have any spread before you start any work. That can be tough to pull together if it has spread but might be something to consider. I've used a few "come-a-long" ratchet winches for that.
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rockies
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# Posted: 26 Sep 2013 21:43
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It sounds like the person who built this cabin used the cheapest materials possible in order to frame it. Either that or he was hauling it into the brush piece by piece and couldn't lift very much. I still think 2 x 4 rafters are way too small even if they can support 30 psf of snow. I would be interested in what he used for sheathing on the roof? 1/2 inch plywood? 3/8 inch osb? I wouldn't want to walk around on it shovelling off the "once in a hundred years" snow storm (that now seems to be happening every 10 years). That said, 16 feet is still a fair distance to span without a post, and I'd rather have it there than risk the collapse of the roof from a freak storm. can you take some pictures of the connections?
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MtnDon
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# Posted: 26 Sep 2013 22:22 - Edited by: MtnDon
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Yes, 2x4's is cutting things a little too fine. In fact the roof does come up short on load carrying ability. The ground snow map for Ketcikan does show 55 PSF as the design load. But short of either totally rebuilding the roof, or sistering some 2x6 on every other rafter there is no solution , IMO. IF, the roof is metal that would be better than anything else as it will shed snow more readily under many situations. The good thing is that snow melts or blows away so the load may not be a long term load.
Maybe beefing up the rafters with sistered 2x6 on every other rafter is a good idea. ???
As for the sheathing with 12" OC darn near anything will do. It is the 12" spacing that saves the roof.
Quoting: rockies That said, 16 feet is still a fair distance to span without a post,
the 16 feet of the length or the 16 feet of the width? If it's the 16 foot length, that is immaterial. Rafters, with rafter ties the way they are supposed to be, are self supporting. The cabin could be 100 feet long x 16 feet wide. As long as there are rafter ties in the lower third the roof should withstand the design load. If it's the width that is being questioned... same answer... the rafter ties hold the lower ends of the rafters together to prevent wall spread.
I have a roof on a 16 foot wide cabin; 30 feet long. Design load is 80 PSF snow LL and 10 PSF DL. No posts needed as there is a rafter tie, on the wall top, on every 16" OC rafter pair.
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Truecabin
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# Posted: 28 Sep 2013 12:37
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if you have equipment around like a large excavator
cut holes in the gable and put a 14" log across that the rafters sit on
and then rebuild your gables to take the load down to the wall
thats an alaska cabin
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