It is best to do all repairs from the top of the roof which means you will have to tear up shingles and other coverings as well as the roof decking in some cases.
How to take the sag out of a roof.
Push up on the plywood until the foam is touching the underside of the roof decking.
The first step is to the basement first.
Use instructions from my page how to fix a sagging floor before you continue trying to fix the roof sagging issue.
The first step is to find out how many rafters are sagging and if anything else is damaged.
How to straighten sagging roof rafters.
This is intended to form an added protective layer helping to avoid any cracks occurring in the wood.
It may be necessary to tear up shingles to fix a sagging roof.
You can do this by guarding it using an angle iron piece.
Drill holes in angle iron.
You should obtain a piece that measures 1 2 with sides that measure 4 in length and an overall length of the entire piece being a minimum of 2 feet.
Check the ceiling for sagging by attaching a string on one end of the beam and run it along the bottom edge of the beam.
After that use a piece of lumbar strapping to keep the plywood in place until you can get a roofing repair expert to fix it.
Next place your piece of plywood and styrofoam foam side up against the damaged roof decking.
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If the line does not continually follow the bottom of the beam and the bottom of the beam is below the line the beam is sagging.