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roof pitch and rafter strength question

Started by DSam3, May 29, 2016, 09:51:28 AM

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DSam3

I'll try to be succinct...I'm rebuilding two sections of an 1830s house and trying to do it historically.  Most of the frame is white oak, but the rafters of section I are walnut (OK, maybe not exactly historical) 4x6, 2 ½' O.C.  I'm following the plans for Sobon's Garden Toolshed but a little longer and narrower.
The rafters sit on a top plate 8x8 with step-lapped rafter seats with an overhang.  Here's the problem:  I am stuck with a 4 or 5 /12 roof pitch.  Laying out the joint with a 2" thick tail requires a very wide top plate.  To fit the joint on my 8x8 means I must shrink the tail and "seat" to 1 ½" thick.  Is this thick enough for walnut rafters?
Or should I use a bird's mouth (and peg) joint with no mortises on the top plate? (The peak is an open mortise and tenon joint with one peg)  Are bird's mouths really sufficient resistance to wind forces on the exposed gable end?
I'm collecting opinions, and don't really expect a clear cut answer.

Roger Nair

Sam can you present a drawing of what you are trying to do.  The vast majority of tf barns in my area have roof slopes between 8/12 to 12/12, so I don't understand why you think steeper slopes are not practical.   Also I don't know why your tails have to be so minimal.  Rafter feet do not require pegs to fasten securely, pegs will weaken the tail, consider using timber screws. 
An optimist believes this is the best of all possible worlds, the pessimist fears that the optimist is correct.--James Branch Cabell

Brian_Weekley

DSam3, Welcome to the Forum.

In the example below, I used stepped-lapped rafter seats with 2 inch thick rafter tails on the shed addition to my barn (8x8 top plates).  In this case, the angle was 25 degrees (just shy of a 6/12 pitch).  Note: my rafters were 4x7s, spaced 24 inches OC.  I also used timberlok screws through the tails into the top plate to secure the rafters.



I just tried drawing it with the rafter at 22.5 degrees (5/12 pitch) instead:



However, like Roger asked, is there a reason you are limited to such a shallow pitch?
e aho laula

Heartwood

You could just move the step lap outboard a few inches, thus raising the tail up. The top of the rafter doesn't need to be flush to the arris of the plate, although you'll probably want to add a piece between rafters to close off the gap from the outside.
Also, you could use a ridge beam and forget the step lap entirely - just use a birdsmouth and timber screw - since there would probably be no thrust. If no ridge I wouldn't rely on screw or peg shear alone to resist thrust on such a low pitch.

DSam3

Thanks for the responses to my rafter strength question.  It just did not occur to me that the underside of the rafter could come in below the top of the plate.  Sorry my reply is so untimely.  My only defense is that I also have a small dairy farm, so I may get a few hours to work on the house, then none for a month while making hay or silage or whatever.  That's why I set up a hoop building over the whole worksite, to keep the weather off while slow poke plays with chisels and a square. 
     Roger and Brian both asked why I am limited to such a shallow pitch.  I attached a bird's eye view of the house. 
All three sections were built in the early 1800s.  We live in section 3.  I tore down 1 and 2, and am rebuilding  them with a sill on a basement instead of on the ground.  The dotted lines show the direction of the roof peaks.  Section 3 is a 7/12 pitch, but the others were shallower...and the section 2 peak tucks right under the eaves of section 3.  So, to keep the ceiling of sec. 1 and 2 high enough to live in...5/12 will put me just inches under the eaves of sec. 3.
      The span of sec 1 is only 11 ft., so I'm following the tradition out here of forgoing a ridge beam.  The span for sec. 2 however, is 28 ft., and I think it was crazy that they had no ridge beam originally.  The walls on that section slowly kicked out. 
       Again, thanks for your time.  As a "newbie", I appreciate any comments you might have.

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