# trig

Discussion in 'Carpenters' Talk' started by yozman, Mar 26, 2005.

1. ### Mr. HandyandyScrewfix Select

Andy clearly doesn't realise that the difference between the inside and outside of the wallplate changes the length of the span thus the angles of the pitch, plumb cut, seat cut and length of the rafter.<

Sorry, not having a go at you NOGGIN, but you have obviously missed the point.
Read the posts again. The thickness of the plate makes NO difference to the angle. The pitch will be the same amount higher than the plate, no matter how high the plate is. If the pitch is one metre higher than the top of the plate, it is one metre higher than the top of any thickness plate. Same pitch, same height same angle.

Handyandy - really

2. ### NOGGINNew Member

Hi DD,

Generally you would take the outside of the wall plate to be the point at which the plumb and seat cuts meet, this is generally a point 1/3 the depth of the rafter (I wish I knew how to post a picture). I could the show you how I was taught to work out the dihedral angle on a hip.

3. ### NOGGINNew Member

Andy,

See my last post I think we could be both talking about the same point now (sorry now the penny has dropped).

The outside edge of the wall plate (it doesn't matter about the thickness of the timber below it or the width of the timber between it and the ridge) is the point that must be measured on site to determine any calcs.

I am just wondering if Yozman has found this forum any help or just confused the issue, whilst leaving them firmly in the dark.

4. ### dirtydeedsNew Member

yozman

sorry about the time taken to get back but distracted by ohter posts (festool is VERY important) so is sorting out miscellaneous stuff

here goes

run 2990
rise 1000

so 1000/2990=.33444816 (tangent)

inverse tan = 18.492 degrees (pitch)

18.492 degrees is difficult to cut but 4 in 12 on your rafter square is pretty near at 18.435 degrees

pitch = plumb cut angle

so 90 degrees minus 18.435 degrees = 71.565 degrees = seat cut angle (tangent 2.99999)

on your rafter square this is 12 in 4

5. ### dirtydeedsNew Member

i know my maths is there or there abouts BUT somebody confirm my rafter square set out for goodness sake

6. ### limestone cowboyNew Member

Yes, 4 in 12 is 18.43 degrees.

7. ### dirtydeedsNew Member

yozman, now.

to keep the roof pitch right ( you are going to put a birds mouth at the plate AND THE RIDGE ) the important thing is that the depth of the rafter above the birdsmouth at the plate AND the depth above the birdsmouth at the ridge is the same dimension.

8. ### dirtydeedsNew Member

thanks limestone, i dont use my rafter square much (unless the angles are similar to american pitches)

9. ### limestone cowboyNew Member

I'm still concerned it's such a low pitch.

10. ### NOGGINNew Member

Yozman,
Have any of us managed to help you, because if we didn't then let me apologise on behalf of all.

11. ### NOGGINNew Member

Jeese,

what cufuffle! I was impressed with your calcs DD, I
don't pretend to have that much skill at maths but I
don't doubt you. I can at least agree with your calcs
for the length from plum cut to plum cut.

Yozman, If you don't have a chopshaw with angles
marked out or carry some sort of protractor around
with you try this.

Simply draw (set out) the roof to scale (a piece of
ply,or plasterboard will do) I suggest a scale of
1/3.

Draw a right angled triangle with a rise of 333.33
(the rise 1000 / 3) and a span of 746.66 (the span
2290 minus 50 for ridge /3).

Draw a line from the top of the rise to the outside
of the span and you have you angles and lengths all
nicely displayed in front of you at 1 third scale.

you can set up your sliding bevel for the plumb and
seat cuts, measure the length for the rafter (this
you need to multiply by three).

No rocket science required.

what is the span of this roof ? I have just read earlier postings and now beleive the span to be 2990 <u>not 2290</u>. If this is the caes you will need to adjust the measurement in the above example.

12. ### dirtydeedsNew Member

yozman! wake up!

i dont care what pitch the roof is when im asked to build it BUT limestone's point is VERY important if you are doing it for yourself because the roof covering (type of tiles) work at a minimum pitch. for an extreme example thatched roofs have to be 45 degrees or they let water in AND they rot

13. ### NOGGINNew Member

DD,

are you and me getting crossed wires? I was not posting to comment on the low pitch of this roof but just wanted to correct any duff information I may have posted if the span is 2990 not 2290 if so then my example would be wrong.

14. ### dirtydeedsNew Member

noggin

i belive my last set of cals WERE based on 2990 run (NOT SPAN) because it is a mono pitch or lean to

dd

15. ### dirtydeedsNew Member

nog, yes we may be getting cross wired (but its afer 1am and im off to bed) by y'all

16. ### NOGGINNew Member

DD, fair enough, I should have said run,not span.

17. ### dirtydeedsNew Member

nog, just re read my last nights post, (this morning it looks a bit pedantic) it wasnt meant to be

more a case of anybody reading this post and using rafter tables rather than doing it mathmatically might use the wrong information, i know that some rafter tables work on span others on run

18. ### chappersMember

does anyone on here still use a rafter square, I pretty much use a calculator and a stanley roofing square or occaisonally the ready reckoner and square.

19. ### yozmanNew Member

hi all
just been reading through your comments very good reading anyway decided to re educate myself with the help of bbc gcse website and managed to do calcs myself which i might say are spot on according to your calcs. has for the question of roof covering i intend on matching up tiles with original house, double romans and yes i know
the minimum pitch for these tiles is 22.5 (please correct if wrong)so i might have to raise the height to 1200 which will give me 21.8 near enough. one more question
the head when bolted on to wall will run across my soil pipe (soil pipe will be in the way of continous run)will it be alright to do two separate heads one either side of pipe if you know hat i mean?

20. ### dirtydeedsNew Member

if you go for a pitch under the minimum recommenede and the roof leaked the insurance company (if they checked the pitch)would use it as an reason not to pay

Is there something stopping you raising the roof to 22.5 degrees.

No problem with the head but if a rafter coincides with the soil pipe i would suggest you install a rafter both sides of the pipe