Common rafter work out

[markblue777]

Hi all,
Today I was doing the roof on my porch, well trying to and then getting massively frustrated as I followed this walk though http://www.thisiscarpentry.com/2012/11/09/common-rafter-framing/ and cut one of the rafters, offered it up and the top of the rafter sat above the top of the ridge plate by like 50mm.

After trial and error and wasting a couple of boards I managed to get it, not pretty though and not how I want to do the rest.

I think I must be missing a step out but this is what I done to calculate the rafter

run 610mm (from out side of wall plate to the side of the ridge plate (where rafter would connect))

rise 490mm (from top of wall plate to the top of the ridge plate))

the length of the rafter i got was 782.4mm

the pitch degree worked out as 38.7

with the plum cut to the rafter being at 51.3.

As i said the once offered up the top of the rafter sits about 50 mm above the ridge plate.

the wall plate it is sitting on is a 2 X 5 (45 x 125) is there something that needs to be done with this?

If anyone can help it would be greatly appreciated, been banging me head against a brick wall all day and in the end i just had to walk away or else I would have done some series damaged to a length of timber with a hammer haha.
Cheers
Mark

 

[rdesign]

the birds mouth notch should go 1/3rd the depth of the rafter measure ur run along this line then do your angles.

so use a marking gauge and mark a line 1/3rd the thickness.

 

[markblue777]

Hi rdesign,
would that be the 782.4mm measured along that line?

cheers
Mark

 

[blackrodd]

An easy way, put a straight piece of 5x2 across the wall plate, edge ways up, where the rafters will run, it would be nice if it were level.
measure the height of the proposed ridge/ rafter line from the bottom of the 5x2 allowing for any sarking board, battens
tiles and finished roof etc.
measure the width of the wall plates, outside to outside, armed with the height and width measurements draw a full
size detail ,from that carefully draw the wall plates and the rafter details for the seat cut or birds mouth,
allow a 4x1 ridge and detail the top rafter plumb cut, just set you're sliding bevel, and make a pattern, keeping the same pattern for all of them
You should have a perfect rafter detail as existing, allowing for any differences in timber sizes,and shape and building line not quite exact etc etc,
Regards Rodders

 

[markblue777]

hi rod,

and issue i have is that my measurements are different for the left side of the roof (when you looking at it) as it has a shallower pitch.
So I am dealing with 2 different pitches.

I did find this walk through that seems like it can help

http://www.builderbill-diy-help.com/rafter-length.html

although I wanted to see where is was going wrong with my calculations before.

Cheers
Mark

 

[blackrodd]

Well do what I said before, but plumb down from where you want the ridge centre to be, and mark the 5x2 as well as the two wall plate edges. draw it out carefully and you will get a long rafter and a short rafter
Regards Rodders

 

[Roughcut]

I found this online calculator helpful in determining roof pitch by calculating length and height.
http://www.blocklayer.com/Roof-Pitch.aspx

Once I had the dimensions and pitch I did a scale drawing which helps to reference from.
And yes as it was my first time cutting rafters I made 1 or 2 bonehead mistakes initially.

 

[markblue777]

Hey rough cut,
Yeah I have used this calculator and this returned the same result that I'm getting.

Cheers
Mark

 

[Roughcut]

Yep your calculations are indeed correct.
Hypotenuse (rafter length) is coming out at 782.43.
So you can't be far now from cracking it.
I would probably now double check the instrument you used to transfer the angle measurement from drawing to rafter, I used a sliding bevel.
And also where exactly you are marking the birds mouth on the rafter.
ps. I would use scrap material if possible rather than good rafter timber until you get it correct.

 

[markblue777]

I was using a framing square todo it. Using the points of 248 and 300.

I am thinking I will change this and use a sliding bevel. Maybe it will be a bit more accurate for me with that.

Am I right in saying that the angles I need to put on are:
For the rafter to the ridge plate the angle is
51.3 then that is transfered down to the length of there to

Then I mark 25mm up and cut to 38.7 degree angle for the birds mouth and 90 degrees to the seat cut.

Cheers
Mark

 

[Roughcut]

I was using a framing square todo it. Using the points of 248 and 300.

I am thinking I will change this and use a sliding bevel. Maybe it will be a bit more accurate for me with that.

Am I right in saying that the angles I need to put on are:
For the rafter to the ridge plate the angle is
51.3 then that is transfered down to the length of there to

Then I mark 25mm up and cut to 38.7 degree angle for the birds mouth and 90 degrees to the seat cut.

Cheers
Mark

Looks right to me Mark.
Although as I said earlier I also found it helpful to do a quick small scaled drawing to reference off of.

 

[rdesign]

Hi rdesign,
would that be the 782.4mm measured along that line?

cheers
Mark

yes and I think that is the only mistake you made in your first attempt.
use a sliding bevel to get your angles and see if you have it right with scrap or do a full size drawing it always helps if you have a sheet of mdf or osb lying around

regards Richard

 

[markblue777]

The more I'm reading it does seem that maybe it is because I went over a 1/3 of the timber. I will use a sliding bevel to as maybe the framing square was off slightly.

I'm lacking a sheet of OSB or MDF but I have a few bits of svrap laying around so will try the cut on that.

Cheers
Mark

 

[HOJ]

I use Goss's Tables to work out my roof cuts:

According to your measurement I get a rise per meter run of 0.803, which, from the tables gives a pitch of 39 degrees.
So the seat cuts are 39 and ridge cuts are 51 degrees.

From the tables the rafter length is given as 722mm, along the top edge of the rafter, from ridge to a plumb line on the back of the seat cut.

This length has factored in the seat cut depth at 0.7 of the depth of the rafter.

You are not far out but the 10mm or so, on your rafter length could be your overshoot.

I use one of these for setting out and as saw guide.

http://www.screwfix.com/p/stanley-adjustable-quick-square-6/1655f

 

[markblue777]

Hi Hoj,
How is that meter run calculated does it explain it within the book?
I have a couple more calculations to do so wondering how I can do this correctly myself.

I have deducted the HAP from the rise and the result it gave me was basically correct (had to take about 20mm off for the seat cut but it did line up after that correctly).
Cheers
Mark

 

[markblue777]

Hi Hoj,

I tried those calcs in sketch up and I get a rafter that sits like so against the ridge

should the 51 degree cut happen on the ridge plate side or should that be cut to 39.

Cheers
Mark

 

[blackrodd]

39 degrees seat cut is the top of the wall plate cut, 51 degrees the ridge plumb cut
If that's to scale, you need a deeper ridge board, 7x1 would be better.
Have to say, the system you are using is all well and fine, in theory,
But when I come along I find that when the wall plates are out of square, and not level, or parallel, increasing and decreasing cuts are a pain
Hopefully, as you have built this you're self, this is not the case.
please keep the pics coming, it's all interesting stuff!
Regards Rodders

 

[markblue777]

Cheers rod. I try these angles and they just don't seem to fit correctly. I have now though got one that works so will template it off that.

Wanted the roof finished this weekend but hopefully will get it sorted next weekend.

Cheers
Mark