cutting rafters, what is the formula?
 

Cutting rafters, really interesting video but a bit I dont understand.
watch this video from 13.00 https://www.youtube.com/watch?v=gz-Ej0qY1pQ

He uses a calculator to work out the length of part of the rafter but what is the formula behind the answer?

Also, from 15.00, when he draws the plum line, wont the angel change if the rafter is not, as i'll say, wide ? How will it stay accurate?

Is the answer in his left hand at 21.349? was I looking in the wrong place for his 7 inch measurement?

If I could remember all of what he said I could build a roof. An excellent video

macafee2

I thought the basic rule was don't cut the one you are standing on ?

found another video, not all the information was the same but also very good.
I have made a list of tools that were in the videos, I sold one of them last year not thinking I would ever use it. Not too expensive for the tools but is my skill up to it?
I'd like to have a go at doing a single garage roof. I would use osb and felt but would like proper tiles and felt. Who knows, may be.

macafee2

Rafters
 

Hi Mac,
I'm afraid I almost lost the will to live in that video.
He's using that "building calculator" to avoid doing/explaining a bit of geometry and good old Pythagoras for calculating right-angle triangle sides.


For what it's worth here's my twopenn'orth.
Firstly dealing with the plumb cut where the rafter will meet the ledger board, wall plate or whatever one wishes to call it (and for eventually also for cutting the plumb cut at the bottom). I've mocked up a model here to illustrate the idea of creating a cutting template.



1) Get hold of a square/rectangular piece of card (or thin ply if you want it to be more robust).
Make sure the corners are 90 degrees.
2) For his "rises 7" in 12" then as per pic 1 starting from the lower RH corner mark off along the bottom edge 12"and then mark off 7" up the RHS edge from that bottom corner.
3) Join as pic2 and cut off the resulting triangle and keep the rest.
4) Place this remaining piece of card/board on your rafter as shown in pic 3 and draw along edge I've marked as CD.
There is your plumb cut line for the top as per pic4 (excuse the line thickness pen not cooperating!)
Will continue with the rest in a new reply if you're interested
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yes please pcj-the very interested. Thank you for what you have done, it seems easy.:eek:
I had no idea how anyone worked out how to cut rafters and so I found the videos very helpful but long.

Is building or plans your thing?

macafee2

Hello Mac,
Building/plans etc not actually my thing, but like many folks have had to get to grips with it.
Semi-retired engineer here and the Frau and I are into heavy DIY as we are renovating our "tin chapel" retirement place whilst living in it.


We won't mention other tasks like the mighty "Bagheera". A Jaguar V12 saloon currently hiding under cover waiting for me to get back to work on her, once the million and other jobs are done. I will have to build her a garage eventually, hence my interest in all things lean-to and raftery!


Also thus my membership of this club, having purchased a Rover 75 tourer as temporary wheels pending restoration of Baggie.


Now back to the plot.


2nd part:
1) His building calculator wouldn’t win any prizes.
If the rafter rises 7” for every 12” of horizontal travel (horizontal travel here = his adjusted run of 35”) then we have:
total rise = 35 x 7/12 = 245/12 = 20” 5/12 and NOT the 20” 7/16 as produced by his calculator.!


You don’t need a specialised calculator. Any simple one that can do + - x / and squares and square roots is fine.


O.K. with our first plumb cut marked (and cut?) at the top end of the rafter we need to know how far down to cut the bird’s mouth.
Note in pic1 we are interested in distance GH.



Now we come to the Pythagoras bit he was avoiding:( with apologies if I’m teaching my granny to suck eggs)
In a right angle triangle of sides a, b, and c, where c= the hypotenuse (longest side) we have:


c x c = (a x a) + (b x b)
e.g. for a 3, 4, 5 triangle:
5 x 5 = (3 x3) + (4x4)
viz 25 = 25
so c = square root of 25 = 5 (as we expected here).



For our rafter distance GH x GH = (rise x rise) + (adjusted run x adjusted run)
Now 20”5/12 = 20.42 to 2 dec places so using the other figures in his example we have
= (20.42 x 20.42) + (35 x 35)
= (416.98) + (1225)
= (1642) approx


So GH = square root of 1642 = 40.52”
So if we mark off 40.52” down the top edge of the rafter from pt G to get point H (pic2) we will be in a position to begin marking out the bird’s mouth.



With H marked, take the card template and positioning edge CD as per pic 3 mark out this edge across the rafter as per pic 4. Call this new line HI. Mark on the line HI ptX which is up 1/3rd the length of HI from I.


As 1/3rd = 0.333 in decimal then calculating IX distance is easy = HI x 0.333


More to come in next bit

ok mind blown. I was fine upto 2nd part:D

I understand A squared + B squared etc but anything more is doubtful but dont let that put you off telling me again.
This is going to have to wait for tomorrow or Sunday when I will be more awake.
viz25, what is that? I take it it is C squared but what does viz mean?

thank you pcj-the, this is going to take me some time to understand

macafee2

Hi Mac,
Apologies firstly I was struggling to find an elegant way to layout "a squared" etc in text mode (a formula layout option would be nice) so I'm pretty sure some of the text is a bit of a pig to get your head round.
Try making up a cardboard template using any old "rise" and then following the steps marking out a scrap bit of wood as I did for the pictures.


Secondly apologies for the "engineer slang" speak:
Viz originates here: videlicet
The abbreviation viz. (or viz without a full stop) is short for the Latin videlicet, which itself is a contraction of the Latin phrase videre licet, meaning "it is permitted to see". It is used as a synonym for "namely", "that is to say", "to wit", "which is", or "as follows".


Cheers, will now get back to pt3::}
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Now take the card template again and position it as shown in pic5 with edge AB passing through ptX.


Draw in the line BX and as per pic 6 you now have your bird’s mouth.


Move the template as per pic7 so point B lays over point I.


Draw in the line AB for a total length of your desired overhang (remember to allow for the fascia thickness). This will give you a result as per pic 8.


Move the template again so its CD edge passes through the end of the desired overhang line as per pic 9.


Draw in the line on edge CD and you now have you bottom end plumb cut marked out as per pic 10.


One post to go: working out the ridge board position.



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Pt4, the final frontier!

Ridge board positioning.
Simple really.
Top edge of ridgeboard will be at a height of “total rise” + “height above plate”

Bottom edge of will be at a height of “total rise” + “height above plate” - width of ridgeboard


Let’s use the videoman’s figures for an example:
He had total rise at 20”7/16, height above plate of 4”3/16 and a ridgeboard width of 8” but actually 7”1/4 finished size


So we get top of ridge at 24”5/8th (“total rise” + “height above plate”)


Bottom of ridge at 24”5/8 – 7”1/4 = 17”1/8 (hence the size of the temporary block he used to prop up the end of the ledger).


I'm going to lay down now.

:getmecoat:


P.S. If there are any chippy's (working or retired) who know a quicker/simpler method please let us amateurs into the secret.

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