Home and Garden | Straighten Basement Girder - Flooring

A common problem in older homes is a sagging first floor due to a sagging basement girder or basement beam. More often than not, the girder itself is still sound and the sag can be easily corrected by jacking up the girder and installing a new adjustable steel post as explained here. If, however, the girder is unsound or if the sagging is on an upper floor, you should consult a professional.

PROJECT BASICS

Project: Difficult
Estimated Project Time: Varies with the amount of work needed
Start Tips: If the sag is minor, it may be better to live with it rather than to repair it. Sometimes, straightening a sagging floor can cause more problems than it eliminates, such as doors no longer swinging freely, cracks in walls or loosened floor tiles.
Safety Tips: If the girder is unsound or if the sagging is on an upper floor, you should consult a professional.
Recommendation: Do-it-Yourself

TOOLS & MATERIALS

8d nails	2x8 pad for jack
String	1/4-inch thick steel pressure plate
Hammer drill (rented)	Long screwdriver
Safety goggles	6x6 post (must be plumb)
Shovel	Wrench
Concrete	4-foot long spirit level
Wheelbarrow	Sledgehammer
Straight piece of lumber	Rubber boots (when working with concrete)
Plastic sheeting	Sturdy work gloves (especially when working with concrete)
12- or 20-ton hydraulic jack

Fig. 1

INSTRUCTIONS

1) String a reference line
At each end of the sagging girder, start an 8d nail 1 inch from the bottom edge. Tie a string from these nails along the girder. Make sure the string doesn't touch the girder. Use this string to guide you in straightening the girder. (Fig. 1)

2) Place a footing
If the girder sags over a post, you will have to replace the old post with an adjustable steel support post. If there is no footing beneath the post, you will have to place one there before installing the new post. This is also true if the girder sags because there simply is no post at all. In this case, you must add a new footing and a post.

On the floor where the new post will be located, mark a footing the size your building code requires. Wearing safety goggles, use a rented hammer drill to break through a concrete floor, and remove all chunks of slab. Dig a hole in the subsoil the depth the building code requires. Dampen the hole, and fill it with concrete (Fig. 2). (Be sure to wear safety goggles, sturdy work gloves and rubber boots when working with concrete — it can burn your skin after prolonged contact.) Release air bubbles by repeatedly thrusting a shovel into the wet concrete. With a straight piece of lumber, level the concrete surface. Cure the concrete for two weeks by keeping the surface wet and covered with plastic sheeting.

Fig. 3

3) Jack up the girder
Position a 12- or 20-ton hydraulic jack on a 2x8 pad close to the footing, set a 1/4-inch thick steel pressure plate on the jack, and use a 6x6 post on the jack to raise the sagging girder. Make sure the jacking post is plumb. Raise the girder until it is straight by checking its position against the reference string (Fig. 3). Then pump the jack one more time to put a slight crown in the girder. Remove any existing post.

Fig. 4

4) Install the new post
With the adjustable steel support post on the footing, unscrew the post's plate to fit snugly under the girder, then insert a long screwdriver into the pivot holes and continue turning the screw until it is very tight. Use a spirit level to check for plumb and a sledgehammer to knock the post into plumb (Fig. 4). Then release the jack and remove it and the jacking post.

Straightening a Basement Girder