Rafter size for 16ft Span

[nickel plate]

2x8 will come in 20' lengths. Ive seen 2x6 in 20 foot lengths. 2x8 may be pushing it for that span. they will definitely deflect but they wont break/fail. In your area they probably have southern pine which is stronger than our spruce/fir we have on the northeast. Doug fir will also be stronger if you can get your hands on that.

Southern pine/southern yellow pine (SYP) is used primarily in less expensive plywood sheathing veneers. It is far less structural than your spruce /fir and as you said, douglas fir is the most structural North American construction softwood on the market. DF Grades of 2x6 through 2x14 are from least to best: #2, #1, select structural (SEL STR) and SEL STR DENSE. All but the dense grade are generally mixed in a given unit of #2 & BETTER. Special order DF stock will provide the DENSE grade stamp and all grades from a specialty mill are available up to 28'-30' in length. BIG $$$$. This is why the glu-lam and composite joist market was developed.

 

[Egon]

Consider checking out the cost of an engineered truss. You may just be able to get one to approximate your desired shape. :thumbsup:

Also check up on wind loading. Those specifications may have been altered quite recently.

 

[bp fick]

Although a shorter span, the principles apply. I had a disaster of a small barn i inherited with the place. It too was an 18' span, with a 2/12 pitch. It was saggy and leaked no matter what repairs were made. Enough!! We did a tear off of the shingles, sheathing and pulled all the rafters off. I reversed the pitch of the roof. I extended the wall height an additional 2 feet which allowed me to go to 5/12. I reclaimed the old rafters, and cut birdsmouth, upper and lower. I have 9" over hangs with rafter cuts. I faced them with 1 by.

The added pitch and height allowed me to put a small loft at both ends for storage. It is all shingled as of last night. The deck is OSB with good tar paper underneath and drip edge all around.

This week, I will side it with vinyl, mostly because I reclaimed the vinyl that was on it and I won't have to paint, ever. Might have preferred steel, but this project was a re-do and re-build and I have very few dollars in the project.

Hopefully some of these pix might give you a few ideas. Best regards on your new barn/shed.

 

[/pine]

Southern pine/southern yellow pine (SYP) is used primarily in less expensive plywood sheathing veneers. It is far less structural than your spruce /fir and as you said

FWIW...

#1 dense yellow pine is (pound for pound) stronger than steel...

Almost all structural members used in trusses of any kind in the S.E. US are made with #1 dense SYP...

 

[ustmd]

Thanks for the feedback, I knew I could count on you guys :thumbsup:.

I am thinking I am going to go with 2x10 on 24 oc or maybe even 2x12 (for the extra insulation space)--though hauling 85 lbs rafters onto the studs is not exciting me.

I was going to pursue a truss or an engineered rafter, but the nearest lumber yard near me just closed. Since I am not in the tradesm, I am left with big box stores, unless I am willing to go 30-50 miles.

While the 2x8 might work, if the structure of this building has issues, the wife will NEVER let me hear the end of it.

Just to be clear, I am planning on sheathing with radiant foil backed OSB, tar paper, drip edge and metal roofing.

 

[dfkrug]

Don't forget light guage galvanized steel rafters, cut to order. I just
bought 18-ft 2x8x16ga for $22 each. They are very stiff, perfectly
straight, and cut to exactly 18.00' long. Impervious to the weather, too.
This is for my 17'x19' building. I will make trusses and go with a 4:12
pitch steel roof.

 

[WVBill]

Don't disregard engineered trusses just because you're not in the trade.

Search for truss manufacturing companies (google or yellow pages) near you.

I got the trusses for my 24' x 24' studio building from a truss manufacturer. Cost was less than what stick-built would have been because they can engineer a solution to use smaller lumber than you will have to use for non-engineered stick-built.

 

[Temp97]

How about those wood "I" beams for long spans. When my house was built they used these for the floor joists and they covered 14-16' spans. I'm thinking about making my own for my garage/shop project but researching to see what the down side might be...

 

[/pine]

I am planning on sheathing with radiant foil backed OSB,

unless you use 3/4 osb...there will be a good amount of deflection when spanning rafters set at 24" oc...although is is structurally sound using 1/2" sheathing I would not use anything less than 5/8"

Also something to check into...if the metal roofing is laid directly on top of the sheathing (radiant barrier) there may be a condensation issue...

All metal roofs sweat (on the under side) when temperature and humidity conditions are right...

 

[ustmd]

Also something to check into...if the metal roofing is laid directly on top of the sheathing (radiant barrier) there may be a condensation issue...

All metal roofs sweat (on the under side) when temperature and humidity conditions are right...

Good point about the roof sweating--I will need to check into it. I am wondering if a fur it up on some 1x stock to creat an air space with venting is I can get around it.

Due to the urging of several on this thread, I am looking at trusses. I have Don at 84lumber working me up a truss and quote--as well as having calls out to a few other manufacturers. Hope to hear back today.