# trig

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

1. ### yozmanNew Member

i know i should have paid attention at school but
can anyone help me with a bit of trig i need to cut some rafters for my monopitch roof and need to know plumb and seat cuts the measurements are 2990mm span x 1000mm rise

2. ### dirtydeedsNew Member

before we can do this what is the thickness of your planted ridge

50mm

4. ### yozmanNew Member

i will be sitting the top of my rafters onto a 100mmx50mm
joist bolted to wall making plump and seat cut 1/3 the thickness of rafter which will be 50mm rafters 150m x 50mm
why do you need ridge thickness?

5. ### dirtydeedsNew Member

because otherwise the length of the rafter will be wrong and as a consequence the angle of the roof will alter.

more on the angles to follow

6. ### dirtydeedsNew Member

This is a mono pitch so you take the length and deduct the total width of the planted "ridge"

***************************
I have assumed that the horizontal measurement is from the wall to the OUTER face of the wall plate. IF IT ISNT AND YOU HAVE MEAUSRED WALL TO INNER FACE OD THE WALL PLATE TESE CALCS WILL HAVE TO BE REDONE
**********************************

So 2290-50= 2240
The rise is 1000

so using the tangent rule opp/adj (so 1000/2240)=.446428
using the inverse tan give the angle = 24.057 degrees.

nobody can work to .057 degrees so 24 degrees is good

The plumb cut is therefore 24 degrees and so the seat cut is 90-24 = 66 degrees.

The rafter length from plumbcut at the ridge to plumb on the birdsmouth is therefore 1/cos 24 x 2240

DONT CUT THE RAFTER THIS LENGTH BECAUSE YOU HAVE TO ALLOW FOR THE FASCIA AND SOFFIT

7. ### Mr. HandyandyScrewfix Select

because otherwise the length of the rafter will be
wrong and as a consequence the angle of the roof will
alter.

more on the angles to follow

How come ?

Handyandy - really

8. ### Mr. HandyandyScrewfix Select

This is a mono pitch so you take the length and
deduct the total width of the planted "ridge"

***************************
I have assumed that the horizontal measurement is
from the wall to the OUTER face of the wall plate. IF
IT ISNT AND YOU HAVE MEAUSRED WALL TO INNER FACE OD
THE WALL PLATE TESE CALCS WILL HAVE TO BE REDONE
**********************************

So 2290-50= 2240
The rise is 1000

so using the tangent rule opp/adj (so
1000/2240)=.446428
using the inverse tan give the angle = 24.057
degrees.

nobody can work to .057 degrees so 24 degrees is
good

The plumb cut is therefore 24 degrees and so the seat
cut is 90-24 = 66 degrees.

The rafter length from plumbcut at the ridge to plumb
on the birdsmouth is therefore 1/cos 24 x 2240

DONT CUT THE RAFTER THIS LENGTH BECAUSE YOU HAVE TO
ALLOW FOR THE FASCIA AND SOFFIT

Did that help ?

Handyandy - really

9. ### yozmanNew Member

sorry just realised i put mono roof should be lean to
your calc sound right to me and yes the measurement was to the outer edge of wall plate, so if i am not using ridge board just 100mm x 50mm timber for a head with common rafters birdsmouthed on underside to sit on top of head will the calcs be the same

10. ### dirtydeedsNew Member

yoz, the words dont matter much. mono pitch is a perfectly correct description for a lean to roof.

by the way handyandy, please check my calculations and tell yozman if they are correct. two people doing a calculation is always better than one

11. ### yozmanNew Member

dirtydeeds just realised you have worked on wrong measurement the span is 2990 you have put 2290 thanks yozman

12. ### chappersMember

if you are going to birdsmouth your rafters over the ridge then the measurements given for the rafter run should be made using the 2990 and not subtracting the thicknes of the ridge plate, your plumb cut at the top should then be 50mm back.

13. ### dirtydeedsNew Member

thats why i asked handyandy (another person) to check my calculations

2990-50 = 2940

1000 height

1000/2940=.340136

inverse tan = 18.78 degrees = roof slope = plumb cut

seat cut = 90 - 18.78 = 71.22degrees

you can use pythogoras therom to get the length of the rafter (plumb cut at ridge to plumb cut at birdsmouth) as i said last night dont cut the rafter this length because you need to allow for the fascia and soffit

you can also use the calculation 1/cos 18.78 degrees x length to get the length between plumb cuts

14. ### dirtydeedsNew Member

thanks chappers, didnt spot the birdsmouth over the ridge.

yoz, redo the calcs as chappers says with no deduction for the ridge

16. ### dirtydeedsNew Member

sid, morning

its no good grinning, im not perfect like mr MDF

im still waiting for him to put his head above the parapet (sorry door)

17. ### yozmanNew Member

sorry to be a pain dd but could you give me the calc
based on 2990

cheers

18. ### Mr. HandyandyScrewfix Select

because otherwise the length of the rafter will be
wrong and as a consequence the angle of the roof
will
alter.

more on the angles to follow

How come ?

Handyandy - really

How come ? Top of plat to ridge is top of plate to ridge whatever height of plat, so same angle, if 1000mm higher than plate. Just curious.

Oh, and on the bater couple of posts back(knew I'd come in useful for something, even if it was as an excuse)

Handyandy - really

19. ### dirtydeedsNew Member

andy,

first i notice that you did not check the calculations for me and went for a p* take instead. it seems to me that possibly you cant do the maths???????????

second reason why i think you cant do the maths is because you didnt help the guy with his problem

There is a third possibility. you can do the maths and you cannot find nothing wrong with the basic principals and have no interest in helping the guy because your raison detre (my french spelling is useless) is to have arguements with people.

anyway i will now rise to the bait. you question the fact that if the rafter length is not correct (longer or shorter) the angles will change. Please STOP and THINK.

use a 3, 4, 5 triangle, the roof pitch would be 36.87degrees approx . Now if you cut the rafter longer at say 5.1 the pitch alters to 38.34 degrees approx

( i have put approximatly so you dont argue the point needlessly)

I rose to your bait BUT ive snapped your line. weep celebrity mdf joiner

20. ### dirtydeedsNew Member

PS andy do YOU use mdf for rafters as well.

Woops! now that was deliberatly provocative.

That was very bad dd. Shush youre lying mouth

DAM andy has got me at his arguement game AND im talking to myself