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I am constructing a timber outbuilding with a shed roof that

is cantilevered towards the peak...
I am constructing a timber outbuilding with a shed roof that is cantilevered towards the peak. The main beam will support about 600 sq ft, I am keeping it a free span of 30' with no interior posts, the roof is a 6/12 pitch. I have trees that I will cut into beams onsite but I think the largest beam I can cut is 10" or 12" wide X 30" high. Can you shed any light if I am in the ballpark of making this work?
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Answered in 1 minute by:
11/10/2011
StructuralEng
StructuralEng, Consultant
Category: Structural Engineering
Satisfied Customers: 7,482
Experience: Structural Engineer
Verified

StructuralEng :

Hi, I can hellp

StructuralEng :

Give me a minute to read the question completely

StructuralEng :

What do you mean by cantilevered toward the peak?

Customer:

Post on lower eave wall and in the middle of the building (about 2/3rds across on the side walls) and the rafters free hanging over the cant beam as they rise towards the peak, there will be no enclosed walls on this structure.

StructuralEng :

The rafters cantilever past the ridge beam?

StructuralEng :

The ridge beam spans 30'? And that is the beam you need designed?

Customer:

No ridge beam, they free hang towards the peak, rafters will be 8" X 12" with purlins on top to attach roofing. Think of a boyscout emergency shelter built with a shed roof only on a much larger scale

StructuralEng :

Is it a monoslope roof?

Customer:

yes

StructuralEng :

Ok. So it starts low at the eave and slopes up to the other wall?

StructuralEng :

Now I'm with you.

StructuralEng :

and the span is 30'?

Customer:

you might call it a ridge beam but it is canted over by the rafters

StructuralEng :

It's a wall, though, not a beam, right?

Customer:

Yes 30' wide. Thats the width of slope and building

StructuralEng :

Ok, where are you located, so I can look at snow loading.

Customer:

No wall just open working space to drive into with forklift to move products

Customer:

Washington state

StructuralEng :

Do you have a spacing in mind or do you want me to work that out?

StructuralEng :

Do you want to use purlins between these or just span the sheathing directly over it between adjacent beams?

Customer:

I figured 600-700 sq ft of roof will be carried by the beam. the other 200 or so sq ft of roof will be carried by the lower eave posts and beams

StructuralEng :

Ok, so just one beam? Give me a few minutes to run some numbers

StructuralEng :

What type of trees do you have to cut?

Customer:

Just the beam size, rafters are about 8 ft center and 4"x4" purlins that will have metal roof fastened to

Customer:

Douglas fir, should make number 1 or better. I went to grading school

StructuralEng :

If you can put it as No. 1 Doug-Fir, then you can cut it 13" x 26", assuming dressed and dried dimensions of 12.5" x 25"

StructuralEng :

If you've found my answer helpful, please "accept" it. I'll be happy to answer additional questions on the topic. If you could provide feedback, I would appreciate it.

StructuralEng :

That keeps deflection to span/300.

StructuralEng :

Not terribly stiff, but it's already a very big beam.

Customer:

No, I would expect to have alot of bounce in it

StructuralEng :

It's just a roof, right?

StructuralEng :

It works for strength and is within acceptable limits for deflection. If you want it more stringent, let me know what kind of deflection you are looking for and I'll size it for that.

Customer:

Yes, an oversized machine shed type structure. It will be open workspace to build timber frame trusses for my home, thats why its free span

Customer:

This is a great service, just lets me know I am not totally unrealistic before I get complete engineering for the structure. Thanks alot

StructuralEng
StructuralEng, Consultant
Category: Structural Engineering
Satisfied Customers: 7,482
Experience: Structural Engineer
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