Strengthen Joists

You wouldn’t believe the types of questions I get about how to strengthen joists.

So, this means we’re talking about basements, crawlspaces, and attics mainly–accessible places where you can get to the joists without ripping apart half of your house.

Now, none of my advice about how to strengthen joists applies to houses that have severe problems.  Things like:

• Deep cracks in the joists.
• Sags lower than a couple of inches.

Method 1 – Wood Shims

Using the wood shim method is a stop-gap fix.  Better to sister with a new joist, if possible.

Underneath, you hammer in wood shims between the floor joist and the bottom of the floorboards or subfloor.  “Whoa!” you say.  That’s not strengthening the floor joists; that’s something else!

Indeed.  This is a quick-and-dirty method.  What this does is help force a gap between the joist and floor, raising the floor somewhat.  This works for slight sags.

Method 2 – Sister with New Joist

So, this is a bit harder, but it is more effective.

• Buy a floor joist the same length as the sagging or weak joist.
• Lay new floor joist alongside the old one.  You will not be removing the old joist.
• Make sure new joist is resting next to the same contact points as the old joist.
• Option A:  Raise up old joist with a 40 ton house jack until it is level or close to level; OR,
• Option B:  Do not raise up old joist.  Keep the sag in place, and merely strengthen instead of level out the joist.
• Attach the new floor joist to the old joist with 2 1/2″ screws, driven every 8 inches.