rafter question

diyfrank · May 06, 2009, 10:22:40 PM

This from the blocklayers calculator.

Can someone explain the circled items.
As simple as it looks, it doesn't have arrows from point to point on the measurements given.
And HAP

, Short for... Happy??

And part two of my question. When using a ridge beam, what would be the way to attach the rafters.

1 Same as a ridge board, nailed to the sides.
2 nail rafters together on top of beam.
3 Other

I need to cut what I can here before I cross over.

PEG688 · May 06, 2009, 10:38:57 PM

Quote from: diyfrank on May 06, 2009, 10:22:40 PM
This from the blocklayers calculator.

Can someone explain the circled items.
As simple as it looks, it doesn't have arrows from point to point on the measurements given.
And HAP , Short for... Happy??

And part two of my question. When using a ridge beam, what would be the way to attach the rafters.

1 Same as a ridge board, nailed to the sides.
2 nail rafters together on top of beam.
3 Other

I need to cut what I can here before I cross over.

The circled items are lengths , the top most from the peak cut to the birds mouth "plumb line cut"

is 11' 1 5/8" strong.

HAP = Height At Plate , 8 15/16"

15/32 , 1/2" weak

6 5/16" Must be the length of the birds mouth after the cut.

IT all so of odd using 32nd of a inch , and the things they "made important"

You do know your looking at a miniature rafter right the green back grounded thing?

At least thats my guess if you laided out a rafter full scale you'd get those #'s.

G/L PEG

May 06, 2009, 10:57:30 PM

I used that calculator for my cabin rafters. I rounded off the 32nds. It worked well enough. And yes, the green thing is a not to scale rafter.

I'm not sure that PEG saw the second part of the question; (ridge beam); did you edit that in? I have no idea without researching it.

I'd second think cutting everything before taking it to the site. I cut one pair of rafters as a trial and had to make slight adjustments to cut the finals to fit perfectly. I built plywood jigs/guides and used my Ryobi 18V circular saw to cut everything on site. It allowed me to make nice straight non-wavy cuts.

Maybe you are aware, but just to be sure... when you cut out the birdsmouths do not overcut with the circular saw. Cut to the guideline with the circular saw and then finish by hand or use a jig or reciprocating saw.

diyfrank · May 06, 2009, 11:28:11 PM

Pegg,
Yes I do do know I'm looking at a miniature rafter,and its not to scale.
I'm just used to seeing lines and arrows with measurements. and abbreviations like (hap) will usually have an asterisk with a note below. "Height at plate" is still a little fuzzy though.
top of plate to top of rafter in a vertical line? from what point to what point? I'm sure its quite simple, I want to know what points to measure from to know where I'm at. The rest of the measurements make sense now.

Don,
I didn't edit. I've seen it done both ways on here and I'm sure there are other ways to secure a rafter to a beam for different reasons. Do you think cutting everything except the birds mouth would be safe? yes, I know to finish my cuts with a hand saw, but I'm not a carpenter so all tips are appreciated.

I have a 6 hour trip and a 3/4 ton truck to carry everything so I'm trying to eliminate unnecessary cargo. Maybe 2 or 3 days a month to pound nails.

MountainDon · May 06, 2009, 11:42:08 PM

Quote from: diyfrank on May 06, 2009, 11:28:11 PM
"Height at plate" is still a little fuzzy though.
top of plate to top of rafter in a vertical line? from what point to what point?

The HAP is measured right along that line that goes from the cutout to the top side of the rafter. When the rafter is installed that will be a vertical line.

May 06, 2009, 11:47:15 PM

Re: the cutting off site...

Call me cautious... conservative... When I installed my ridge board it ended up slightly higher than what I had calculated for. That threw the dimensions off a little. SOOOOO, if you knew for absolute certain that your actual ridge was going to be placed as drawn, then you might be okay to cut the plumb cut at the ridge. Maybe.

You should also consider cutting the tails on site, after the rafters are all up. I have done it both ways and the tails I cut off after the rafters were up are more even. It helps to have a light weight saw when doing that.

PEG688 · May 07, 2009, 01:32:48 AM

I missed answering the "how to attach" part.

It's a depends thing, you could toe nail them to the side but IF your using a beam for a ridge why not let it show in the finished product.

On the cut every thing at home a hope it fits , I'd suggest cutting one set , don't cut the tails those are best cut after you've set all the rafters . There are exceptions to that like if you can't get the saw in to cut the flat / level / soffit cut due to the saw hitting the building before the cut is completed.

I would suggest IF your going to cut any thing at home to cut the birds mouth and just make sure you leave a few inches extra in length up at the ridge. Mark the ridge cut when you mark the birds mouth but don't make that cut , you can do that on site once your sure everything is good to go.

Once you check the fit of the first two rafters you can cut all the others , and you'll have all those end cuts for camp fire wood on the site!

You do know to crown your rafters first right?

Set up one of the best straightest rafters as a pattern use it to mark all the others.

You may think your saving time by cutting all the rafters by doing the math at home , BUT all you'd need to do is trip a number as screw up all your stock . Then you'd have to go to a lower pitch roof to make things work out . Sounds like a recipe for disaster to me.

Why risk it.

G/L PEG

diyfrank · May 07, 2009, 07:20:57 AM

Thanks Don & peg for your suggestions. I'll cut the ridge plumb on site.
Nice example on attaching to the beam. I'll be sure to crown.

MikeT · May 07, 2009, 07:34:03 AM

Chiming in a bit late here, but another way of dealing with the rafter cuts at the top of the ridge beam is to scissor the rafters. You cut your bird's mouth but don't worry about the plumb cut and lay the opposing rafters next to one another and the top laps. Then you can cut off the excess on each side and nail the rafters to one another and to the top of the ridge beam. I believe there is an illustration of this in Wagner's book on framing.

mt

May 07, 2009, 07:54:58 AM

Quote from: diyfrank on May 07, 2009, 07:20:57 AM

Thanks Don & peg for your suggestions. I'll cut the ridge plumb on site.
Nice example on attaching to the beam. I'll be sure to crown.

Your welcome! I'm happy to see you understood what I was saying. When I write these things I wonder if I'm getting my point across. A lot of things a guy does out of habit and remembering to write down the proper sequence and getting the right words that a laymen / non-carpenter will be able to decifer can be a challenge!

G/L PEG

Don_P · May 07, 2009, 08:32:07 AM

Typically with a ridge beam as opposed to a ridge board you need to think of the rafter as hanging from the ridge. The spec I've had on plans several times was the simpson framing angles, the longest that can fit on the plumb cut, usually the 7 or 9" on one side of each rafter only. I think there is a skewable hanger as well. I'll look more tonight if nobody links to it, gotta run her down the mountain to a specialist, I hate doctorin.

Oh, for us HAP is height above plate, same difference
we call the 11' 1-5/8" the "plumb to plumb" measure so I would call down "133-5/8 plumb to plumb"
The HAP is the measure up along the plumb mark, the vertical height above the plate at the outer framing line.
the shy 1/2" is the vertical portion of the bird, the 6-5/16" is the length of the level seat cut of the bird.
PEG said all that just trying to say it another way in redneck in case anyone got lost.

MountainDon · May 07, 2009, 10:17:08 AM

Quote from: Don_P on May 07, 2009, 08:32:07 AM
I think there is a skewable hanger as well.

here... http://www.strongtie.com/products/connectors/SUR-SUL-HSUR-HSUL.asp

and also while we're skewing things...

here... http://www.strongtie.com/products/connectors/LSU-LSSU.asp

davidj · May 07, 2009, 11:41:58 AM

Quote from: MikeT on May 07, 2009, 07:34:03 AM
Chiming in a bit late here, but another way of dealing with the rafter cuts at the top of the ridge beam is to scissor the rafters. You cut your bird's mouth but don't worry about the plumb cut and lay the opposing rafters next to one another and the top laps. Then you can cut off the excess on each side and nail the rafters to one another and to the top of the ridge beam. I believe there is an illustration of this in Wagner's book on framing.

mt

This is what we're doing on our 20x30 with 6x18 ridge beam. The minor downside is that with 2x12's and a decent overhang, you end up with rafters that are slightly over 16'. And I think you need blocking (at least our drawings have it). So you're buying maybe 15% more rafter than the other way, but no hardware (which I'm sure they'd insist on in earthquake/snowload country). And you do get to look at your very expensive glulam beam once the cabin is built, as it ends up below the ceiling.

Don_P · May 09, 2009, 11:06:45 PM

Ooops

Something must not have felt right.
The 15/32 and the 6-5/16 are not what I said

.
In the inputs you must have entered 4-15/32" for the foot cut (what I call the seat or level cut). There seems to be a glitch in the graphic output of the program. If you enter 4-7/16" for the foot cut in the inputs it will read 4-7/16" where the 15/32 is now. So it should read 4-15/32", the length of the foot cut.

It took a little trig to see what the 6-5/16" is. It's the length along the edge of the removed part of the birdsmouth, the hypotenuse of the triangle that's removed. It is not the plumb or level cut of the birdsmouth. The cosine of 45 degrees is .7071 Multiply 6.3125(6-5/16) x .7071 and you'll get 4.4635, or about 4-15/32" (4.46875"). The plumb and level cuts on the birdsmouth of this rafter would both be 4-15/32".

I double checked the HAP after MD's comment about a slight misfit but it seems to check. 9.5/.7071=13.435 That would be the length of a 12/12 plumb cut across the 9-1/2" wide board. 13.435-4.46875=8.966 or about 8-31/32".

Finally one possible correction, a 2x10 is actually 1-1/2" x 9-1/4".
Up through 2x6 lumber is 1/2" narrower than nominal. From 2x8 up it is 3/4" narrower.

May 10, 2009, 09:00:28 AM

Lots of good technical advice has been given already to your questions about cutting rafters. Instead of using programs and calculations to do my cabin rafters, I just used a much simpler approach that worked fine. I first attached temp posts (2 x 4's), to the end walls, with scraps nailed to the sides, and extending above the top ends to form saddles. Of course, the length of the temp posts needs to be determined to give the desired height of the ridge after the ridge board has been placed vertically up in the saddles. I used a 20', 2 x 10 for the ridge board, and installed it by walking each end up tall step ladders a few steps at a time, until placing it in position in the saddles. I then selected the straightest piece of rafter stock (2 x 8's), and clamped one end to the end of the ridge board, and the lower end to the outside end of the wall. Then just marked the rafter at the intersection with the ridge board, and at the intersection with the end of the wall (for birdsmouth). After making the cuts, I checked the fit, and then used it as a pattern to cut all the other rafters. They all fit fine, and the tails lined up. After all the rafters were up, I installed 4 collar ties (every-other rafter) to keep the sidewalls from spreading. Personally, I wouldn't make any of the rafter cuts before having the materials on-site. If you're going to do an actual ridge beam, it's going to be too heavy to place by yourself. I can give you some help, if we can coordinate being over there at the same time. I'll be headed over for my second trip the middle of next week. Will be at the cabin about a week/month thru the fall.

PEG688 · May 10, 2009, 10:12:38 AM

Then of course there is the 'step it off' method .

Using square buttons , one clamped on at 12" and one at the pitch of the roof.

Also on a good steel / carpenters / framing square on the body you'll find a list of tables telling what the run per foot (length of the rafter per foot ) for all the common pitches.

Here's a article about this ,

http://www.carpentry-pro-framer.com/framing-square.html

Note to Mt. D , Glenn or John.

That last link is a good one , it has a lot of stuff folks here could use , if we don't have it linked direct you may want to add it some where .

Thanks

G/L PEG

glenn kangiser · May 10, 2009, 10:46:49 AM

Thanks. PEG. I'll copy it to referral links.

diyfrank · May 10, 2009, 11:27:10 AM

Thanks for the double check on the math Don_p. The 9 1/2 was my mistake and would have screwed things up for sure. All the measurements now make sense.

I'm still thinking about using hangers or joining the rafters above.

The picture above looks like a nice way to go.

Woodswalker, I appreciate the offer for help on the beam. I have 2 1/2 sons I'll bring for the heavy work. I'm heading over the 16th and memorial weekend to dig holes and start the floor. I'll try and look you up.

Peg, The square method may be a little advanced for me. I may play with it a little but It may turn into a boomerang. The 16' x 20' with 1 wall and 12 /12 roof I'm doing was designed using the "KISS" method. The calculator is smarter than me when it comes the math.

PEG688 · May 10, 2009, 11:58:17 AM

Quote from: diyfrank on May 10, 2009, 11:27:10 AM

Peg, The square method may be a little advanced for me. I may play with it a little but It may turn into a boomerang. The 16' x 20' with 1 wall and 12 /12 roof I'm doing was designed using the "KISS" method. The calculator is smarter than me when it comes the math.

The step-off method is the ultimate KISS way , no math , no tape , just a carpenter and a couple of his simplest tools . The square and a pencil.

diyfrank · May 10, 2009, 12:08:36 PM

Well I'm all for that, I'll take a look.

May 10, 2009, 01:10:31 PM

One more quick way I learned is called the "line length ratio". For a 12/12 the LLR is 1.414, if you multiply the horizontal span times the LLR it gives the plumb to plumb measure along the rafter.

Example: a 16' wide building with a 3" thick ridge.192"-3"=189". The rafter span would be 189"/2 or 94.5". Multiply 94.5 x 1.414=133.623 or 11' 1-5/8"

I can put up a table of line length ratios but if you have a calculator with trig functions here's where that comes from. 1/cos(pitch angle) so for this it is 1/.7071=1.414

When stepping off I use my knife. As the number of steps adds up you can add a bit if the lines are fat.

Don_P · May 10, 2009, 10:16:45 PM

Found the ridge height "error" on the blocklayer calc. It depends on how you look at it but it is giving the height to an imaginary point formed by the two roof planes above the ridge. I'm not sure that I'd call it an error but you can't set ridge height by just deducting ridge depth from that number. With a 3" wide ridge on a 12/12 that point is 1-1/2" above the ridge.

I build saddles like Woodswalker described (we call em yokes) and set the ridge in those then attach the rafters to the ridge. I had made this calc to tell me how tall to build the yokes. If you're setting the saddle on the plate enter 0 for height from top of wall to floor.
http://windyhilllogworks.com/Calcs/ridgehgtclc.htm

There is also no shame in building the temp yokes an inch short in the pocket and putting a tapered spacer in that can be tuned. Mark the "correct" height and try to hit it but a slight dip or wave can make things vary a little.