Question on hanging new door in concrete block wall

dave11

My house was built in 1951, and the door leading to the garage from the basement is cut through the concrete blocks. The original carpenter attached jambs to the ends of the block walls, then hung a door between them. It's time to replace the door, and I'm a little unsure of the proper way to approach replacing with a pre-hung door.

The old door is still in place, but as best I can tell, the opening in the block walls is 39 1/4 inches. The typical 36 inch door needs a 38 inch opening, leaving 1 1/4 inches for rough framing. This would leave room for 5/8 framing on either side, though this sounds too thin to me. But to skip the rough frame would require nailing through 5/8 of shim on each side, which sounds wrong as well.

Am I missing something? Or is it not correct to replace this door with pre-hung?

Thanks.

bob22

Could you use a narrower door if you want a thicker rough frame?

dave11

I guess I could, though the original door is 36 inches, and some things in the basement were a close fit getting through it.

Bob Mariani

build a custom jam set with azek. This way you have no concerns with the direct contact to the block wall. 3/4" pressure treated will warp too much.

RegeSullivan

Why not widen the opening???

Dairylander

This is a standard application for a pre-hung door, so you're on the right track. Building a custom jamb would get you the perfect size for your opening, but unless you have a router and a hinge template for mortising the jamb, that project is kind of a pain. So if time is money, go with the pre-hung.
Widening the doorway would require renting a masonry saw and it's a loud and dusty operation. Cutting the block may also compromise the structure, so I would stick with reducing rather than expanding your rough opening.
If you haven't done this already, pull off the trim from the around the door and you should be able to measure your rough opening exactly, and purchase the biggest door that will fit.
If it is indeed 39 1/4, that's not enough for a 38" door, so you'll have to stick with the 36" door.
So now all you need to do is reduce your opening from 39 1/4 to approximately 38 inches, which as you mentioned would be 1&1/4 inches.
You can do 5/8 material on both sides, or do 3/4 on the hinge side and 1/2 on the knob side.
You'll need a hammer drill and masonry screws to attach the rough lumber.
Another thing I would suggest is that once you have the door in place, pull off the wood piece of door stop from the hinge side of the jamb. In that space, put some masonry screws through the jamb, through the rough lumber and all the way into the cement block. Counter-sink these screws and cover them with the door stop. This will give your door some serious structure to hang off of, and will prevent any racking.

Maintenance 6

You could always forego the pre-hung and just replace the door in the existing jamb.

joan518

O.K. I havent seen any door jams 1" thick that come with prehung doors. You are lucky if you get 1/2 inch which puts you at 37". If you predrill and tapcon screw 5/4 treated deck boards (which are 1" thick) to mortar joints on sides of block, that puts you at 39", which leaves 1/4 for adjusting, with careful measuring you just might make it. good luck!