There’s been discussions before about when doing an addition on a house and your new ridge has to match the height of the existing ridge and the new addition has wall heights given.
There’s many different situations that we run into but the big question is what is the pitch of the rafter or how do you figure out the rafter for this?
I’ve tried several ways over the years like nailing the new addition ridge in place and scribing the rafters which works. I’ve also extended my top plates past because the ridge has a fly rafter on the gable end so when you nail the ridge in with it sticking past whatever the overhang is so now scribing it on extended top plate makes it easy. I’ve aslo nailed a 2×4 up plumb and braced in the center of the gable cut to the exact height of the fixed ridge and then scribe the rafter and that works.
The way I do it now doesn’t matter what size rafter you use or what size walls you use whether the walls are 2×4, 2×6, 2×8 or the rafters are 2×6, 2×8, 2×10, 2×12 it doesn’t matter.
What I do is just take the ridge height that is already there and use that as my rise from the top of the plate and then figure the run from the INSIDE of the top plate and to the back of the ridge which is already deducting half the thickness of the ridge.
Doing it this way doesn’t matter what size rafter, ridge you use or what size walls you use whether the walls are 2×4, 2×6, 2×8 or the rafters are 2×6, 2×8, 2×10, 2×12 it doesn’t matter.
Once I have the run and rise I figure the Hypotenuse and pitch and do all this from the bench not even going near the roof but that wont be the end result pitch. There’s a thread at Joe Fusco’s forum where a guy had a 16′ span with 2×6 walls with 2×12 rafters and a 1-3/4″ lvl ridge with a fixed ridge height of 145-7/8″ from the deck but you want the ridge height from the top plate.
This is what I posted to him using his numbers and my drawings are from his numbers.
“You gave a number of 145-7/8″ from the deck to the fixed ridge height but not the number from the top plate to the fixed ridge height which is the one you need but I’ll just give an example as if you had 8′ wall using 92-5/8″ precuts which gives you 97-1/8″ with your 2 top plates and your bottom plate and a 16′ span with 1-3/4″ lvl ridge. Also you said that the rafter was higher that the ridge and lower than the ridge which still doesn’t matter because the TOP of the rafter still shouldn’t be higher than the other fixed ridge that your following.”
16 – 1-3/4″ = 15’10-1/4″ / 2 = 7’11-1/8″
7’11-1/8″ – 5-1/2″ (2×6 wall) = 7’5-5/8″ (run from inside plate to back of ridge)
145-7/8″ – 97-1/8″ = 48-3/4″ (Rise)
7’5-5/8″ [run]
48-3/4″ [rise]
[diag] = 102″
[pitch] 28.54°
1) Now lay your 2×12 dowm flat on the top of your horses crown facing up and start from the right side and hook your tape at the TOP of the rafter and measure down to the BOTTOM of the rafter and mark 102″.
2) Take your speedsquare and hook it at the TOP of the rafter where you hooked your tape and now slide your speedsquare until it reads 28.54° at the chalk line and hold it there and mark the right side of the speedsquare and that’s your plumbcut and look at the speedsquare where it hits the top of the 2×12 and whatever that ANGLE says, that’s your PITCH.
3) Go down to the bottom of the rafter where the 102″ chalk line hits and lay out your birdsmouth and overhang with seatcut of 5-1/2″ using a speedsquare or framing square with the pitch that you were given and your done.
I hope this is clear because this takes about 2 minutes to do and is very accurate.
The bigger the rafter the smaller the pitch is. Sometimes you don’t have a fixed wall height but you have a fixed ridge height and the rafters are 2×12 on the new addition and the rafters on existing are 2×6 but you have to have the same fascia height and overhang so therefore your walls are shortened.
The way I do it works for that to but you use the existing overhang measurement from the top of the rafter at the back of the fascia which is the end of the overhang and get that measurement to the plate line whether it’s below or above that’s all right because your just working backwards now no matter what size rafter you’ll use and lay it out from the inside of the plate.
Here’s a couple of drawings for the measurements above.
Replies
TDRucker,
Is this the thread that you're talking about?
Yup, That was it. This time I printed them out. In this world of trusses it is sometimes necessary to do a little review work. Thanks again.
T.
I do a lot of additions and tying into and matching existing roof lines and fascia line is very important around here. This works out great doing it like this and making adjustments in the field is always an issue. It saves a lot of time.Glad I could help.Joe Carola