Cutting a common rafter

[johnfarris]

Hello Folks

Have been playing around with sketch up this afternoon, and I am trying to get my head around cutting a common rafter to length for a mono roof. I have used an online calculator and got perfect results. What I would like to do is work it out for myself using a scientific calculator

I want to have a 38 degree pitch

So I input 38tan into my calculator and then multiply by the span to get my rise

So
Mono pitch roof

Tan (Roof pitch) 38° x (span)1798

=1404 Rise

I then use pythagoras therom to find out my rafter length

1798 ² + 1404 ²= 5204020

√5204020 = 2281 rafter length from outside edge of wall plate to wall

I then draw the rafter in sketch up
And start by marking my plum cut at 52° I then measure down from the top of the plum cut to 2281 and mark the birdsmouth and cut it 1/3 of the way in, I then go back to my plum cut and take off the thickness of the ridge plate and cut it

I keep getting this result, what am I doing wrong?

 

[blackrodd]

Far too technical for me, also it would be a pain in the butt when you need to cut a roof, on existing where the roof is
totally different level to the pole plate and 6" wider at one end.
looking at you're common rafter, It's too long, possibly because you're seat cut at the wall plate end starts too early.
If the seat cut started at the plumb cut at the birds mouth, I think it would be the right length.
I think you need to check you're calculating formulae, shouldn't you're blue line be plumb?
Regards Rodders

 

[PAC1]

Measure to the green line rather than the blue.

 

[nev]

Rodders and Pac, I think that the green line is the Z axis and not an upright.

 

[Chrispy]

You have measured to the wrong part of the birds mouth, you need to measure to the beak not the root to give them names, also have you allowed for the top wall plate thicknes?

 

[RobinBHM]

It doesnt seem to me that although the trigonometry is correct for a 90 degree triangle of that size, it seems the drawing is showing a mixture of top of plumb cut to birds mouth, which is not the same thing.

In other words the distance from ridge down to top of wall plate is not simply taken from a triangle with the pitch angle.

I design conservatory roofs and I find it easiest to always calculate from the top of the rafter, from plumb cut to plumb cut. Birds mouths or other details can be calculated using a number of 90 deg triangles as required.

 

[MMUK]

You need to deduct the wall plate thickness from your projection.

Assuming a wall plate of 50mm thick (therefore a projection of 1748mm) and an eaves wall thickness of 100mm, at 38 degrees I work out the rise from the top edge of your eaves wall to be 1493mm. The top cut is 52 degrees and assuming a 150mm overhang the rafter top length is 2409mm with a birds mouth of 81mm horizontal cut and 63mm vertical cut. Pitch is 9.38:12

HTH

 

[Deejay]

Morning John

http://www.blocklayer.com/Roof/

Any use to you?

Cheers

Dave

 

[johnfarris]

You need to deduct the wall plate thickness from your projection.

Assuming a wall plate of 50mm thick (therefore a projection of 1748mm) and an eaves wall thickness of 100mm, at 38 degrees I work out the rise from the top edge of your eaves wall to be 1493mm. The top cut is 52 degrees and assuming a 150mm overhang the rafter top length is 2409mm with a birds mouth of 81mm horizontal cut and 63mm vertical cut. Pitch is 9.38:12

HTH

Hello all

Having a rise of 1493mm does actual work Sorry for being a bit thick, but I don't understand how you got there
By having a smaller span to take into account the wall plate, my rise goes down not up :?

Thanks everyone else for the replies to be perfect honest still haven't got my head around it. There's is loads of videos on Youtube in imperial and a lot using roofing squares. May be iI should trying using a roofing square to get my head around it. If anyone has a good link to a metric tutorial please post it.

I have been on the blocklayer website and it's really good, but I would rather understand how to do it for myself.

Edited for clarity and spelling

 

[mseries]

When you used 1404 in your calculation you should first have deducted the width of that piece that's on vertical (ridge pole ?), and deduct the width of the wall plate from your 1798, then you'd calculate the length of the underside of your rafter from the birds mouth to the ridge pole. Using this and your angles you can mark out your rafter. Essentially I see te triangle in the inside not the outside

 

[merlin]

Hello John,

If you get a roofing square go for an I&D Smallwood type number 390, they are now manufactured by a company called G Gibson in Leeds.
They are simple to use (comes with a good instruction book) and are useful to use just as a square.
I have been using mine for over 30 years and they never fail, I tried a Stanley square but found them over complicated then a roofing ready reckoner. (always seemed un professional to me reading a book on a building site)
When I started using the Smallwood I never looked back - a doddle.

If you need a hand getting started with it let me know.

Merlin

 

[Billy Flitch]

John the guys have already pointed you in the right direction. Let me try and explain it to you, all measurement you have in your drawing are correct.
You work your rise from the top of wall plate this is called the HAP height at plate, when working out your sizes on a roof like this it is usual to draw a right angle triangle put in the run and pitch and then work out the rise and the length of the common.
But a triangle on a piece of paper are just lines and Carpenters do not work with just lines the timber that we use has a width and a thickness.

The triangle on the piece of paper sitting on the HAP would represent the underside of the common , so you cut out the birds mouth lets say 5 centimeter and the whole spar drops down bodily the 5 centimeter, you then measure on the plumb line on the birds mouth from the HAP to the top of the spar, this measurement is then added to your rise and now you are working on the top of the spar and you have the true rise to set your ridge plate to.

Just one word and you can ignore it if you so want but technically you are correct when you use the term span because it goes from the front to the back of the building. I would use the term run and I have heard it called half span if you keep things simple they usually fall into place, hope this is of some use.

 

[johnfarris]

Thanks for the help folks, penny has dropped now and its working out onto working out how to cut a Hip rafter now, but before I go on to that I do have one more question regarding Common rafters.

When cutting common rafters what happens if the wall plate is not parallel with the ridge plate, do you have to measure and cut all individually?

Thanks in advance

 

[blackrodd]

Yep! I just use the first , and then the previously cut rafter, as a pattern and add on the "shortfall" measurement, before fixing.
The angles are all OK, the rafter is just short.
Regards Rodders