If you can see a dip curve or swag on the peak when standing in the street your roof has a problem.
What does a roof jack look like.
Avoid using short pieces as the risk of a length of lumber tipping upward if you should happen to step onto the end of it is greatly increased.
Standard 2 by 10 inch lumber with fit into the channel on most roof jacks and should be available at any lumber yard.
A roof jack will not do you any good without planks of lumber to span them.
Roof jacks provide a stable surface to work increasing safety and making it easier to apply roofing.
What causes a sagging roof line.
You can usually feel sheathing nails which are driven into rafters through the underlayment attach a jack by driving two 16d nails never roofing nails into a rafter.
Ideally locate a rafter below the sheathing.
It might be something.
If it doesn t have a corner and simply looks like the image below follow the second installation procedure.
Cut roof shingles to fit around the roof jack and insert them under the existing shingles.
Also before you start installing your flashing you need to look to see if the wall in question has a corner on the roof face as in the image below.
If it does follow our first installation procedure.