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Rafter & Roof question

Started by SwingOak, February 24, 2009, 11:25:49 PM

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SwingOak

Hi folks,

It's been a while since I've been here, but hey life is like that sometimes. Anyway, I'm back to the drawing board on my shed design (computer crash and my weekly backups tuned out to be NFG  :( ) and have a question concerning the rafters and roof structure. Once again, please bear with me and my silly questions as I'm new at this. In the Sobon book, he recommends the step-lapped rafter seat (pg 122-123), but I can't seem to find where he says how the rafters should be secured to the plates. Gravity is generally pretty reliable, but I don't imagine it would be in a high wind condition.

I'm thinking of either cedar shakes (less likely) or a standing-seam metal roof (more likely). If I go with shakes or shingles, then I presume I just follow standard installation recommendations for purlin spacing. Any suggestions on purlin dimensions & spacing for the metal roof? Also, is it better to just nail the purlins to the top face of the rafter, or set them flush with a half lap or full inlet?

Thanks!

zopi

I read that chapter the other night, and wondered the same thing..I'd likely go with trunnels perpendicular to the rafter down through the plates and cut flush at top

the purlin spacing for a steel roof depends on the snow/wind load....I just put a steel roof on
my workshop, and nailed it 18" OC with neoprene gasketed nails, but the roof is boarded over..

happy medium...12" purlin spacing?
Got Wood?
LT-15G GO chassis added.
WM sharpener and setter
And lots of junk.

routestep

I used a 60 penny nail (pretty sure it was 60d). Went through the rafter and into the plate at the lap. The one I'm now working on will use timloks. I had my rafters stand about 1.5 inches proud of the plate. I wanted a thicker eve. I had two foot spacing of the rafters so I didn't need purlins, I used rough cut boards and shingles on them. I think steel is better if you collect the runoff for your garden.

Jim_Rogers

Quote from: Lurcherman on February 24, 2009, 11:25:49 PM
...he recommends the step-lapped rafter seat (pg 122-123), but I can't seem to find where he says how the rafters should be secured to the plates.

We were taught by Jack to use 6" ring shank nails and to place two of them into the rafter to secure it to the plate.
You really don't want to use a peg as there is a tenon from a post under some of the rafters.
As shown here:



QuoteAny suggestions on purlin dimensions & spacing for the metal roof?
Follow the metal roofing manufacturers recommendation.

Quote
Also, is it better to just nail the purlins to the top face of the rafter, or set them flush with a half lap or full inlet?
You don't want to be cutting the top surface of the rafter as this will greatly weaken it. Most likely you should attach the purlins over the rafters.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

SwingOak

Thanks guys - as usual, great info!

shinnlinger

Lurch,

How much snow do you get? How steep is your roof?  and what are you building? I see you said shed, but what is your intent for it?

I try to put my purlins 24"oc if I know I am only screwing ribbed roofing to it, but if it is 10/12 or steeper I have gone 32"  Held up fine.  But if it is a heated living space or even alot of livestock, you will probably  be dealing with boards of some sort and a vapor barrier so you can  space the purlins more then depending on what substrate you are using.

If the rafters have some beef 6x6 or larger I would  cut pockets to accept the purlins so they sit flush and then squirrel cut the purlins, but that is more of an appearance issue than anything else.  Securing rafters to me depends on overhang.  You can have the rafters die into your eave-plate(no overhang) and cut a small notch if you wish to help register it and then some nails(Or screws or lags)  will do fine or you  can use an eve tie if you want  overhang and put the rafter on top of a post (say in a high posted cape) and then you can tenon the top of the post and mortise the bottom of the rafter and peg it. 
Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home

SwingOak

I'm in NJ, so we do get snow but these days not too much, and not too often. My design more or less follows the Sobon garden shed model, so my plan is for 4" x 6" rafters 2' OC.

I like the idea of collecting roof water for the garden, so I'll probably go with metal.

moonhill

Jack Sobon has my highest respect, but a 6" ring shank nail is a problem.  This is 2009 and we now have in our tool boxes cordless impact drivers and those sweet lag screws.  It makes disassembly a breeze, it will happen.  When you move you will take this shed with you.  Screw everything, but follow the book.  Get an impact driver, there is a thread on them here as well. 

Tim
This is a test, please stand by...

Jim_Rogers

I said we were taught to use 6" ring shank nails, but I too use hex head screws for future disassembly. I get the brand name timberlok screws. Each box comes with a hex head socket driver... nice extra feature....

Shinnlinger:
I have to really disagree with your comments about cutting the top of the rafter for inserting purlins. It isn't the best and I believe it's against code. You'll weaken the timber. If you have planned on doing this and have oversized your rafter because you intend to cut out a purlin notch that's one thing, but taking a standard 4x6 and cutting out a notch makes it a lot less of rafter..... spacing and loads will have to be carefully reviewed to see if the rafter can support the load after the notch has been removed....

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

shinnlinger

JIm,

I agree, you do need to size the rafter to accept the purlin pockets.  I suppose another option would be metal hangers. 
Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home

SwingOak

Quote from: moonhill on February 26, 2009, 06:52:59 AMGet an impact driver, there is a thread on them here as well. 

Got one! I have a Makita that I bought a few years ago after I read a review of 6 or 7 brands of cordless impact drivers in Fine Homebuilding, and they rated it the best of the bunch. I use it more than any other power tool I own.

I never thought about taking the shed with me when/if I move, but that's an interesting idea.

moonhill

My Makita has fallen off the roof too many times.  I fixed it with J B Weld and fiberglass cloth, it worked great.  I like the power to weight ratio.   I use the GRK lag screws, They use a T-30 bit.  We recently moved a variation of the Sobon shed.  It was my brothers and he lived in a trailer park.  They are just like furniture, the sheds.

Tim
This is a test, please stand by...

Jim_Rogers

The sheds we build here at my sawmill yard are assembled and set up for customers to view.

When it is sold we just push the pegs out and disassemble it and transport it to the customer's site.

Having done this many times it isn't hard to do if you have a good "peg pusher" to use. Most pegs will push out, but if damaged you may need to replace them with newer pegs after moving the shed.

Taking apart and moving barns and sheds when relocating has been done for hundreds of years. And many companies make their entire business out of doing this.

Good luck with your project.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

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