I am cutting the rafters for my addition but they are not coming out correct. My existing roof is supposed to be a 4/12 pitch hip. It is really about 4 1/2/12 pitch so I figured for that pitch. My rafter book says for a span of 26′-5 3/8″ span (I used 5 1/4 instead, no eigths in book) I should come up with 14′ 5/8″ from plumb mark to plumb mark on rafter. When I cut them at this length my plumb cut on the birds mouth sticks out about 1/2″ past the top plate on both walls. I deducted the 3/4″ for the ridge on my calculations. What am I doing wrong?
Note. I am using an old rafter as my birds mouth template in order to keep the same hap as before. Could this be the problem?
Thanks
Replies
I can't tell you why they're wrong, but the fix should be pretty easy. If you have a bit of extra length at the bottom end of the rafter, why not just cut off the 1/2" from the top, and use that new measurement on all the rest?
After all, altough it would be nice to find out WHY they're wrong, now you know what they should be, so just make them that way.
I can think of a few places where you may have introduced some variation into your calculations; fudging the slope, or possibly the ridge isn't as high as you think...walls not quite plumb...
You are probably right Geoff, I probably have many errors. I think i'll trim the top and keep going.
In situations like you have, I like to take and existing rafter down as a template!
Is your ridge totally parallel with the top plates? Are the plates totally square & parallel with one another?
Make sure of that first, then do what Frammer said. With a template, it's hard to go wrong.
"It is really about 4 1/2/12 pitch so I figured for that pitch"
The key word in that statement is "about". You are figuring and cutting rafters that are "exactly" some pitch....not "about" some pitch. Therefore, when you use a table that demands an exact pitch and your existing pitch is "about" 4.25/12, you'll end up with a half inch deviation somewhere.
You are much better off calculating your remodeling rafters using total rise squared, plus, total run squared, then take the square root of that sum. It will ALWAYS be exact. To find your cuts, take the total rise and divide that number by 12. Take your total run and divide that number by 12. Use the answers for your numbers on the square and everything will be perfect every time.
I decided last night to use the pythagorean theorem. Thanks
Jim what do I do about the hap? Using a2+b2=c2 does not include the hap in the calculation.
What do you do about the HAP? What do you think you should do? The easy answer is this. Ignore it for the purposes of calculating the length of the rafter. When you are calculating the ridge height (if you do), add the HAP to the "Total Rise".
I figured it out Jim. Thanks
You must subtract 1/2 the thickness of the ridge, per side?
Thanks W. I did that but they still were not coming out. I think it is because as Jim stated that the existing pitch is not exact so neither are my calculations. I'm going to use a2+b2=c2
You've got a construction master?
I can't do much w/o one. Yes, Jim's rec. on finding your exact pitch ought to do it.
FHB article by Rick Arnold on remodel Dormers will have good info there for you. Search it above.
Thanks W.
No matter how careful you are theres almost always a difference if the walls are not plumb or parallel, Even when i framed i would test the rafter pattern in the middle and both ends.
For example , i would start cutting rafters and test them on the shoe plate before the walls went up, they could be perfect there but if someone read there level wrong just a little it would be different on the top plate
As others have said, you need to make sure you are building with straight walls.
Beer was created so carpenters wouldn't rule the world.
True Id but even if there were not straight I should be able to pull the span for a section and come up with a rafter length for the one section.
I guess I should have elaborated and said walls are straight and the correct distance apart, and the ridge is the right height above the plates and centered.
Most of the time when something wasn't working out as expected it was do to something not being in the right place.
If your ridge is where you want it to get the slope and your walls are where you want them, maybe it doesn't matter where the error is, just adjust your template and keep building.
Best of luck
Beer was created so carpenters wouldn't rule the world.
Danger Will Robinson!!!
Where I am, NO BIRDS MOUTH NOTCHES ALLOWED by current code. Metal, metal...
Notched rafters are specifically prohibitted by 2006 UBC except in very rare circumstances.
Why do I know this...It makes no one happy....
skipj
Interesting. Where do you frame at?
Seattle area.
Do you know why that is not allowed?
Around here, you won't pass code without a notch LOL.
I believe you! It makes no sense. Every municipality seems to have little quirks. My favorite is sheathing, 'Those edges are too tight.' 'You need to get these edges tighter'. WTF? It's like every inspector has a different take.
As regards the notching, a couple of years ago, all my structural engineering started coming back with Simpson metal specs and the admonishment: NO BIRDSMOUTH NOTCHES! Which I could see in New Orleans but Seattle ain't exactly hurricane/tornado central; I assumed that it was new 2006 Code, but maybe Mr. Simpson needed to increase his take on every house to 4k instead of 3k.
It's a mystery, wrapped in an enigma, shrouded in the Simpson company paying for thousands of building department officials (and their wives) to attend 'seminars' in fabulous Las Vegas. Hey, presto! We need (approved) metal here, here and here.
Anyway, best I could do on short notice.
skipj
Ok, the no-birds-mouth thing has me thinking. That's new to me. It seems to me that with no birds mouth, the bottom edge of the rafter or truss just bears on the outside edge of the top plate? And you use one of these things http://www.lowes.com/lowes/lkn?action=productDetail&productId=59055-72913-HS24&lpage=none to hook it in?So all the weight is bearing on that little surface area where the edge of the rafter touches the plate vs. having the weight distributed over the bottom of the birds-mouth?If uplift is a concern why not use BOTH birdsmouth AND rafter tie?
Hey,
Pulled a set of plans and it's a Simpson VPA Seat each rafter, also A35's each bird block and H3's every other rafter.
best,
skipj
I had to look up the VPA you were refering to...never had that called out. Where do you work mostly? I live in Edmonds and work mostly Mercer Is. north and have yet to be called on that. Mostly remodel these days, but do all the framing and just had a Seattle inspector out, thought he might frown on the shallow haps I had on my 2X12's ( they were only spanning 11' and had to keep them low so I could squeek a 3/12 under the second story windows) just asked if I was going to finish puttting on the H1s.
Hers's one from Mercer Island, bird mouths every where.
hey ryder,
called and engineered in Issquah. Also, no H1's at all, H2, H3 and A35's every block. Thank god for palm nailers!
skipj
That's the silliest thing I've heard in a while. On a properly framed house, with walls strung straight,ceiling joists adequately attached, and rafters cut identically, it's the bird's mouth that sets the ridge and holds the ridge straight. What do they expect you to do? Set the ridge on legs, string it, and nail bottom to ceiling joist only, counting on the metal connector to keep it from pushing out, since the rafter is now not directly attached to the wall at all? If in fact you have this straight ( the code in your area ) I'd like to know who's responsible.....Cause it's plain dumb :)
Bing