# helping working out my pitch 15 degrees

Discussion in 'Carpenters' Talk' started by tiler7777, Jan 26, 2016.

1. thats right you are given 2 values 15 deg and 3.3m. in this case its the sin function as s=opp/hyp

so sin15=x/3300
or 0.258=x/3300
or x= 3300x0.258

= 854

2. This is what was said at the beginning, it's only HA who can't do the maths and then tries to wriggle out of it.

3. ### Mr. HandyandyScrewfix Select I'm still confused actually. Reason being, I can't figure out how the OP knows the length of the joists, but not the height(or the difference) of the two walls. coz change the height of the wall relative to each other, then the joist length will be different.

If however his initial 'length of joist' was wall to wall(3.3m), then the roof joists would be longer and the high wall would be 1100mm for the 15º.

Mr. HandyAndy - Really

4. ### Phil the PaverScrewfix Select

That's very good question, I only worked the 850mm out on the fact the joists were in fact 3.3m in length, if as you say the distance between the walls is 3.3m, then the joists will indeed be longer and to achieve 15° would need the rear wall to be higher than 850mm.

5. If the distance between the walls is 3.3M then the rise will be 884mm and the length 3616mm not including any over lap,
to keep the rise down to 500mm the acute angle is only 8.6 degs.

6. ### Phil the PaverScrewfix Select

Looks like a flat roof is going to be his preferred choice at this rate.  7. Looks like it  8. ### Mr. HandyandyScrewfix Select

The rise will surely be 1100mm?

Mr. HandyAndy - Really

9. ### Phil the PaverScrewfix Select

No, Chippie pretty much on the money.

10. for a difference (rise) of 1100 you can work out from both sin and tan but not cos

sin15 = 1100/joist on an angle (hyp)
or 0.259 =1100/x
or x= 1100/0.259
= 4247mm

tan15 = 1100/joist level wall to wall (adj)
or 0.268 = 1100/x
or x= 1100/0.268
=4104mm

proving with pyhtag to approx figures

4247^2 = (4104^2)+(1100^2)

11. ### Mr. HandyandyScrewfix Select

12. it doesn't work like that..............

the 45 deg is correct but the 1100 is actually 884 and the 2200 is 1900.

this is because the sin tan and cos functions follow a sinusoidal shape ie like a wave form and their mathematical values are not linear. which means when you are working to rule of thumb, you won't be able to expect them to be of predictably nice and easy proportions.

Last edited: Jan 27, 2016
13. The measurement is 3.3meters from the external side of the outerbrickwork.

Ive allready bought the roof tiles off gum tree for a fiver so they are going up. I dont like wasting five pounds. lol

So is 884 the correct measurement or is it more with the joists being more than 3.3meters.

At the momnet the wall is 1.8meters high so if I add the 884 thats 2.64 at the front and 1.8 meters at the back. Then I was going to build up the corners of the front of the building with soil so any measurment comes out 2.5meters

14. Does depend on the tiles used, as most have a minimum pitch to work to.

If you draw it out using a scale of 1:2 and protractor.

Then the height comes to about 650mm

15. Scale should be 1:20

16. If you are having problems there is a trig app, all you have to enter are 2 values and it works out the rest.

17. so is it 650 or 884?

18. If 3300mm is the horizontal distance: Tan 15 * 3300 = 884mm
If 3300mm is the slope length: Sin 15 * 3300 = 854mm

650 does not come into it anywhere

19. I though I would draw it out and see it would work.

I did another drawing with a scale of 1:10, ans it still came out to 650mm The Tan function is the one to use as you have the adj and need to work out the opp; opp/adj