# trig

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

1. ### dirtydeedsNew Member

serious question andy, are you (were you) sponsered by dewalt when you were on tv (bad spelling again)

2. ### limestone cowboyNew Member

Calm down DD, it's only a forum. I've checked your calculations and they are spot on.

Yoz, 18.78 (call it 19) degrees is a low pitch. Out of interest, what roofing material are you using?

3. ### Mr. HandyandyScrewfix Select

Whoops dirtydeeds. You're wrong again. Sorry.

The part I am questioning(I might not have made it clear)
was the part where the thickness of the plate made a difference.
If the 1000mm height of the ridge is measured from the top of the plate, the ridge will be 1000mm above the top of the plate, even if the plate was 10m high. The ridge would be 11m high. And the angle would be exactly the same. There's my question.

However, I don't know if the measurement is taken from the top of the plate, so if it isn't that invalidates my question, and everything is back hunkydory. Are you with me ?

Handyandy - really

4. ### dirtydeedsNew Member

sorry all. was away doing important things on festool, back with you in a minute

5. ### DewyNew Member

Too remember simple trig:-

Some People Have
Curly Black Hair
Too Plastered Back

S=P/H
C=B/H
T=P/B

where:-
S=Sine
C=Cosine
T=Tangent
P=Perpendicular
B=Base
H=Hypotenuse.

6. ### dirtydeedsNew Member

andy, i think we have an understanding at last. im just going to ponder on your question a little longer. but you are right about the starting level (10m) being irrelevant

PS im always wrong, clare says so, so it must be true

7. ### dirtydeedsNew Member

dewy, the one i was taught was

SOH
CAH
TOA

sine=opp/adj
cos=adj/hyp
tan=opp/adj

8. ### dirtydeedsNew Member

ps dewy

my hair was black and curly and sometimes im plastered

9. ### bathstyleActive Member

Was taught the SOH CAH TOA thing too!
We were told it was an old Indian prayer or something!
Amazing what you remember from school

10. ### limestone cowboyNew Member

We were taught:

Silly Old Horses
Clumsy And Heavy
Trod On Albert

Nonsense, but it works.

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.

12. ### DewyNew Member

The mathematics teacher I had taught us that so we would never forget it.
For him trig was a matter of life or death for many.
He was a Lancaster bomber navigator so his trig kept the crew on course and was needed to get to the correct target.
He is one of the few teachers I can remember because of his love of maths and the way he taught it.

13. ### dirtydeedsNew Member

andy, the thickness of the plate makes no difference in the same way as the staring height (10m) makes no difference.

(not teaching ducks) but to raise a roof we need only 3 things, the span (which HAS to be measured on site because brickies cant build buildings square or vertical [despite protestations]) the thickness of the ridge and the pitch (given by the architect/engineer)

becasue we were both being agumentative (this post is however NOT however a love in) the point i was deliberatly making was that if you cut the rafter the wrong length you end up with a pitch that is NOT the correct angle required by the architect/engineer. result the angles are wrong.

hope this explanation isnt clear as mud

14. ### dirtydeedsNew Member

limestone, PML, but, when i stopped found it was the same thing

I WILL get back to redoing the calcs, but its too much fun and nobody i know can pitch a roof in the dark

15. ### Mr. HandyandyScrewfix Select

andy, the thickness of the plate makes no difference
in the same way as the staring height (10m) makes no
difference.

(not teaching ducks) but to raise a roof we need only
3 things, the span (which HAS to be measured on site
because brickies cant build buildings square or
vertical [despite protestations]) the thickness of
the ridge and the pitch (given by the
architect/engineer)

becasue we were both being agumentative (this post is
however NOT however a love in) the point i was
deliberatly making was that if you cut the rafter the
wrong length you end up with a pitch that is NOT the
correct angle required by the architect/engineer.
result the angles are wrong.

hope this explanation isnt clear as mud

Backtracking, beep beep beep
Yes your explanation wasn't as clear as mud, thankyou

Handyandy - really

16. ### dirtydeedsNew Member

dewy, i wish i had had a maths teacher with that much love of his job. the guy made it sound as if learning maths was a interesting relevant AND of life or death. (it clearly was)

17. ### dirtydeedsNew Member

andy

OUCH i thought it made perfect sense.

18. ### dirtydeedsNew Member

OUCH. OUCH. OUCH

that was painful, i just read your post (properly) again

19. ### NOGGINNew Member

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.

I refer to my earlier post, I would do this, cut a patten and offer it up to check all was well before using it to mark out the other rafters.

PS I wish we could have had a decent maths teacher at school, ours was a lifeless bully who would hold anyone up for public humiliation for any mistake, but never give an explanation for where you went wrong.

20. ### dirtydeedsNew Member

noggin, STRICTLY speaking as long as you adapt the maths and layout to suit, wether (cant spell it to save my life) you measure the run to the outer edge or inner edge of the plate it doesnt matter. some people do the internal face BUT most do the external face of the plate because you can hook your tape on it