Sagging basement girders can be a major problem for homes that are more than a few decades old. If the girder itself is still in good shape, correcting a minor sag with an adjustable steel beam or adjustable steel post can be an easy DIY fix. However, an extreme realignment of the house's structure may cause other issues such as cracked walls and floor tiles. You may also find that some doors no longer open or close quite as easily. If the girder is sagging severely, is rotting or cracking, or is located on an upper floor, contact a professional contractor as soon as possible.
Safety Recommendations: We strongly recommend that you wear wear sturdy work gloves and safety goggles during this project. Also, always wear rubber boots when working with concrete.
Things you'll need
  • 8d nails
  • String
  • Hammer drill (can be rented)
  • Shovel
  • Concrete
  • Wheel barrow
  • Straight section of lumbar
  • Plastic sheeting
  • 12 of 20 foot hydraulic jack
  • 2 x 8 pad for jack
  • 1/4 inch steel pressure plate
  • Long screwdriver
  • 6 x 6 post
  • Wrench
  • 4 foot long spirit level
  • Sledgehammer

How to Straighten a Basement Girder

1. Hang a guide line by attaching an 8d nail to each end of the sagging girder and attaching a string between the nails. The string shouldn't touch the girder.
2. Replace or add a footing for the new post. If the girder is sagging over a post, you will need to swap out the existing post with an adjustable steel support post. If there is no post or footing, you will need to add these. If you are adding a post, you will need to mark the required post size as per building code on the floor above. With a hammer drill, break through the concrete in the basement floor. Be sure to wear safety goggles. Remove all the resulting debris and dig a hole in the subsoil as deep as the building code requires. Then, dampen the hole and fill with concrete.  The concrete will contain air bubbles, which you can pop with a few thrusts of a shovel.  Now, you must wait for the cement to harden for two weeks, keeping the concrete wet by covering it with plastic sheeting.
3. Raise the girder using a 12- to 20-ton hydraulic jack on a 2 x 8 pad which you have placed close to the footing. With the jacking post plumb, lift the girder until it is straight. Check the girder's new position against your reference string from step 1 before you pump the jack one more time to allow a slight crown in the girder. Now, you can get rid of the old post.
4. Add a new steel post. Place the adjustable steel support post on the footing. Now, unscrew the plate of the post so the post fits snugly under the the girder. Use your spirit level to make sure the post is plumb. Once the post is snug and tight against the ground and the girder, you can knock it into perfect plumb with a sledgehammer. The new post is now supporting the weight of the girder, so you can remove the hydraulic jack and jacking post.

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