Putting the straps on top of the surface will strengthen the door, but the best way is to chisel out a recess for them to lie in.
If you have a drooping panel door that doesn't fit nicely in its frame, the typical fix is to remove some material from the door.
To actually square up a panel door and strengthen it from sagging again, you'll need to first measure the two long edges, or the stiles, of the door.
If the measurements are unequal, then the door has been trimmed before and you need to mark the length of the short side on the longer side of the door. Put on your safety glasses.
To remove the door, open it perpendicular to the frame and shim the bottom tightly using folded cardboard. If you have help, one person should hold the door straight while the other backs out the screws.
An electric drill with a Phillips or slot screwdriver tip makes this much easier, but watch that you don't go too fast and strip out the head of the screw.
You should also make sure that the tip of the driver is the same size as the screw slot; if it's smaller you won't get as much leverage on the screw, and you're also more likely to strip out the head when the driver slips out of the slot.
It's easiest to back out all but one screw in each hinge, then, going bottom up, take the last screws out. The tropical hardwood used to make most doors and shutters here is very heavy, so be ready when it comes free.
Measure from corner to corner - or from the short side mark to the corner. The longer of the two measurements indicates the corners that need to be worked on. Mark these corners with an X.
With the edge of the door against a wall or some other solid object that will stop it moving, push the long corner in until the door changes its shape from warped parallelogram back into the rectangular door it began its life as.
Measure corner to corner again and repeat until they measure out equally. Lay the door on a flat, stable surface: Resting it on four paint buckets, one on each corner, works well, or lay down some cardboard if you need to lay it flat on the ground.
Buy four flat L straps to hold the door in its new shape. The bigger they are, the stronger the door will be. Both silver and yellow zinc-coated brackets are available, so if the door's not going to be painted, find your preferred colour.
Putting the straps on top of the surface will strengthen the door, but the best way is to chisel out a recess for them to lie in so that the entire L strap is helping to keep the door square, not just the screws.
Drill out a pilot hole on each side of the strap and attach two screws to snugly secure the L strap down. Using a utility knife, cut a pattern around the edge of the strap. Make a few passes to get a good, clean line. Remove the screws and L strap.
Using a chisel as close to the L strap size as you can find, remove the wood to the depth of the L strap, about 2 millimetres. It's best to use a plastic or wood hammer to drive the chisel, as a metal hammer will smash up the handle. I like to use a wooden pestle.
Strap them down
When the L straps all fit flush with the surface of the door, drill pilot holes and screw the straps into place.
With some cardboard lifting it to its proper height, place the door in its frame to see if it fits with space around the entire perimetre. If there are any spots that still rub against the frame, mark the spots with a pencil and file those areas down a couple of millimetres.
When you've achieved a nice fit, sand out the file marks, apply finish, allow to dry and re-hang the door. It should fit like new and stay that way for years to come.
Thanks to the carpenters at 794 Street 474 for demonstrating some straightening skills.
If you have any questions about keeping
your house in order or fixing the damage
of daily life, email Jet at firstname.lastname@example.org