How do I measure to get the pitch of my roof correct?

[kingandy2nd]

I understand the concept of Run and Rise for getting pitch, but there doesn’t seem to be a clear steer on where to actually do the measurements from.

Is the run on the outer edge of wallplate to the other outer edge of the wallplate (ie the inner skin of my wall)?

Is the rise from the bottom of the truss to the top of the top chord??

Sorry if this is obvious to those in the know, but I want to get the pitch and height on my new roof correct to marry at right angles with the existing.

Thanks
Andy

 

[^woody^]

It's all just a right-angled triangle - horizontal line, vertical line and a sloping line. It don't matter where you work things out from as long as you bear that in mind.

But if you are matching anything existing, then you use those features and don't set your roof out to something that may not match the existing , even if it says so on paper.

 

[kingandy2nd]

It's all just a right-angled triangle - horizontal line, vertical line and a sloping line. It don't matter where you work things out from as long as you bear that in mind.

But if you are matching anything existing, then you use those features and don't set your roof out to something that may not match the existing , even if it says so on paper.

Thanks Woody.

I’ve been in the loft today and the height is 985mm from the top of the top chord to the bottom of the bottom chord on the existing roof trusses.

So if I understand correctly, this is the key measurement for the new trusses to match to in order to get the two new roofs to connect correctly?

It means my new roof will be 18 degrees rather than 17.5 as it says on paper, but I guess half a degree makes no difference in the real world.

 

[mrrusty]

but I guess half a degree makes no difference in the real world.

Yes it will matter. You need to be exact if matching.

 

[kingandy2nd]

Yes it will matter. You need to be exact if matching.

ok, so any suggestions then?

my run on the extension has been determined by my plans. rise has been determined by the existing roof height.

the only think I haven’t calculated is the true pitch on the existing roof...I’m just going off it being 17.5 degrees that’s stated on the plans. Even if it’s a different angle, the run and the rise are fixed...

 

[mrrusty]

I’ve been in the loft today and the height is 985mm from the top of the top chord to the bottom of the bottom chord on the existing roof trusses.

So if I understand correctly, this is the key measurement for the new trusses to match to in order to get the two new roofs to connect correctly?

It means my new roof will be 18 degrees rather than 17.5 as it says on paper, but I guess half a degree makes no difference in the real world.

If you are matching up you need to be very precise - the plans may differ very slightly from the reality.

This may help https://www.trussform.co.uk/calculator-tool

Take exact measurements from existing and then calculate the angle from the measurements using this calculator which will make allowance for the rafter size

 

[^woody^]

No, 1/2 a degree probably wont matter.

How much it matters will depend on the length up the slope, as the more it is the more it runs out, but you wont notice it on a typical roof. It just depends what and how you plan to join these.

The problem with tying to measure pitch from an existing rafter is that using you are only measuring a specific potion of a rafter or rafters and each potion could be slightly out for one of several reasons. So you have to take an average or best guess.