Beam Span 12x12

[rocking23nf]

Hi everyone, just got a quote back from a contractor and he advised me to build my cabin extension like this... Any advise, it seems undersupported, but maybe im wrong.

First, use the existing beam on the house, which is 3 - 2x8s and frame the complete 12x12 with 3-2x10s all along the peremiter and have 2 posts, 6x6 on the outside corners. The roof support will be on the joists, so thats why he advised 3-2x10s for the outside joists, effectivily having a beam going the whole way around the extension.

so this will be a 12 foot span between posts, but I can probably cantelever it a bit so 11 feet would the span.

Any thoughts? this will all be Pressure Treated Pine.

[MountainDon]

Can you supply a few more details? 
A drawing would be a great help.
Are there any building codes to be met; inspections to be passed?

Is it correct to assume the addition is being added to a long side of the existing cabin? The existing beam down that side is situated at the existing wall line? The existing cabin is built on beams resting on posts?


Twelve feet is a very long span for 3-2x10's as far as I am concerned. Also hanging the addition onto the side may exceed the design limits of the original structure.

[rocking23nf]

heres a link to a pic i just made in paint.

http://img194.imageshack.us/img194/8250/84698796.png



ED: image inserted for convenience - MD

[rocking23nf]

The existing structure is 24x24, its a A-frame. It will have to pass inspection, and the inspector has told me I can connect to the existing beam on the cabin.  I am going to reinforce the existing beam with a few scraps of 6x6s on concrete sidewalk blocks with some gravel underneath.

[MountainDon]

I still believe 12 feet is too long a span between posts; let's see what some others who actually build for a living think.

As for the idea of adding supports under the existing beam... does your location experience any freezing? If it does and if the existing support posts are dug in properly to have their footings below the expected frost depth, you run the risk of having the add-in posts experience frost heave unless they also have footings below frost depth. That could be unsettling. 

[rocking23nf]

Well thats my whole problem with this cabin, its on blocks, nothing is cemented in the ground. It does heave a bit, but not much, but for new additions, you need to have posts in the ground at 4 feet down.

So if i build the extension completly in the ground, when the cabin moves, the extension will not, and then i get cracks. So the inspecter approved me puting one side in the ground, and the other side on the existing beam.

Since the roof tie-in is to complicated for myself, I decided to pursed a contractor to frame the extension. So far my quote is 4000, 4500 to shingle. thats Canadian. My wife thinks its expensive, but im willing to pay it.

[MountainDon]

Oh dear! One side floating and the other anchored solidly; no matter what you do there will be problems someplace.

If you tie onto the existing beam and then install a solid foundation at the other end of the addition, there will be stress placed someplace on the structure. Maybe at the joint between the old and new, maybe someplace else.

If you were to build the addition on it's own solid foundation then at least you know ahead of time where the problems will likely be located. That would be the joint between new and old. A flexible rubber joint between old and new would be nice.     A flexible coupling like between rail cars?   

Might be worth thinking it over. We'll see what other thoughts the folks here come up with.

[rocking23nf]

Its solid clay and I was told theres very little heaving, the cabin is 40 years old and has been jacked up around 2 inchs in those 40 years.....

[rocking23nf]

update, talked to the inspector today, he said I can use 3 2x10s as long as its cantilevered by 1.5 feet.

Im assuming they make simpson joist hangers for a 3 - 2x10 beam to attach onto an existing beam?

[ScottA]

It should work fine cantilevered. I wouldn't do it with the posts out at the corners.