Any journeymen carpenters/framers here?

[pretendfarmer]

I'm pretty good at figuring things out, but something's got me confounded on my well pump house I'm building. The seat cut or birdsmouth on my common rafters- I've figured out that my total rafter length is 7'2 3/4" (building is exactly 12' 1 1/8" wide outside to outside). I know I need to subtract half the thickness of my ridge board from that, which gives me 7'2" to the outside of my wall, and where the seat cut begins. I will run the boards wild another 15 inches to allow for overhang, and cut them back later. The problem I am having is figuring the depth of the seat cut. I can do it the full 3 1/2" width of the top plate, but it seems that the deeper the cut, the more my rafter wants to drop from flush with the top of the ridge, or if I keep them flush with a full cut then the pitch is thrown off and I have a gap between the bottom of the rafter plumb cut and the bottom of the ridge beam. I am trying to keep the board flush with the ridge so that my sheathing can run to the top. If the ridge is sticking above the rafter, my sheathing cannot run all the way up. What am I missing here? It's a 6/12 pitch (actually a hair less because the ridge is sitting at 47 1/2" instead of 48" above my top plate (I cut the jacks it is sitting on short and didn't want to have to take the whole thing apart after noticing it)). Do small gaps not matter? I like things to be very tight.

 

[mikehaugen]

Would you not add the thickness of the top plate to calculate the height of the ridge, if you are going to do a full 1-1/2" seat cut? Just a guess, but it makes sense to me. Otherwise it seems your calculations are based on the rafter sitting on top of the top plate.

Perhaps try re-calculating based on 49" ridge height.

 

[rtimgray]

I may not be following this at all, so forgive me if I'm heading down the wrong path. I'm confused about the pitch of the roof, because if your run is appoximately 6' (oa width about 12'), and your rise is about 4', then I would get it to be about 8/12 instead of 6/12. Som of my misunderstanding may be because I'm not sure exactly where your 47 1/2" measurement for the ridge board is - top or bottom or where?

Anyway, I tried sketching up what you had listed and I still can't figure out what's going on. Have a look at the drawing I attached and let me know if I'm on the right path. If we can get the correct dimenstions in the right place, I can do a drawing and dimension the rafter so that you can cut it to your liking.

Good luck and take care. Hope this might be on the road to helping, and once again I apologize if I didn't get your initial information correct.

 

[EddieWalker]

I rely on an online calculator to figure out my cuts.

Gable Roof Framing Calculator - Gable Roof Plans - Online

Eddie

 

[rkeith]

I'm assuming you used a framing square... put a level to your framing square to determine your exact roof pitch.

 

[Hiltz]

If your having trouble with your math, for something this small simply lay it out on the floor. Draw out the rafter and simulated top plate and ridge board with with chalk lines. Simple fix.

 

[travlitt]

Good info here.

Yahoo! - 301 Moved Permanently

 

[pretendfarmer]

View attachment 378838

I may not be following this at all, so forgive me if I'm heading down the wrong path. I'm confused about the pitch of the roof, because if your run is approximately 6' (oa width about 12'), and your rise is about 4', then I would get it to be about 8/12 instead of 6/12. Som of my misunderstanding may be because I'm not sure exactly where your 47 1/2" measurement for the ridge board is - top or bottom or where?

Anyway, I tried sketching up what you had listed and I still can't figure out what's going on. Have a look at the drawing I attached and let me know if I'm on the right path. If we can get the correct dimenstions in the right place, I can do a drawing and dimension the rafter so that you can cut it to your liking.

Good luck and take care. Hope this might be on the road to helping, and once again I apologize if I didn't get your initial information correct.

Thank you so much for the detailed response, and I'm sorry I wrote 6/12 and not 8/12. I was very tired when I typed the original post. It IS an 8/12 pitch and that's what I figured my cuts with. My 47 1/2 is to the top of my ridge board, which is a 2x8. It is sandwiched between two vertical 2x4's at each gable end, and sitting on a 3rd 2x4 which I cut 1/2 inch short by accident and did not realize until too late. It was so difficult to get the 16' 2x8 up there that I decided to heck with it, I'll leave it. I could rip a half inch strip of something to make it 48", but I'm not sure that's necessary. A difference of a 1/2" rise over 6 feet shouldn't throw my 8/12 pitch off significantly, should it? I mean, it comes down to less than 1/8 inch per foot of rise.

Anyway, given my dimensions, I am going to cut another test board with a full 3 1/2" seat cut at 7'2" (figured from the outside length, not the inside like you have shown) and at the 7.25/12 pitch and see how it fits. I expected these rafters to be precise because I did the calculation exactly how I was supposed to, but they are not fitting tight. I know the ridge board is not perfectly straight, but it is doug fir and not twisted up hemlock or something. To figure my hypotenuse I used the a[SUP]2[/SUP] + b[SUP]2[/SUP] = c[SUP]2[/SUP], with "a" being the rise, "b" being the run, and "c" being the rafter length.

 

[ace10]

Make sure you account for the sheathing thickness (if any) in your seat cut.


Take a look at this calculator. It has a nice visual.

Rafter Calculator - Animated Visual Rafter Calculator with Large Diagram - Imperial


And be sure to account for your ridge beam thickness.

 

[pretendfarmer]

I think I see where I screwed up. First, my pitch is probably 7.25/12, not 8/12 as tim's drawing indicates, which threw off my ridge connection by a hair. To compound the problem, I was measuring my seat cut from the wrong place. I cut my top plumb line, then the distance to the back of the seat cut was measured from the top of the ridge and not the bottom. My board came up way short. The only reason the length was correct on my second board was because I scribed a line where it should be, but I don't like doing things like that. I like to measure and cut things on a table, then have them fit perfectly. I am particular. I will go cut another board out of some scrap and see if it fits.