Sistering is the process of adding material to a damaged joist to bolster it. If the cause of your sagging floors is damage to support joists, sistering is a potential solution that many DIY blogs will suggest. Of course, we do not suggest that you try to implement a DIY fix at all, but sistering is a particularly inefficient solution for damaged joists.
Sistering is a Delay Method
Choosing to sister damaged joists in your crawl space will provide a little extra support to the floor above, but it really amounts to a method of buying time (assuming it is done well). Even the best sistering will only delay the breakdown of a damaged joist; over time, the sister material is likely to succumb to the same pressure, or else be rendered ineffective when the joist breaks down entirely.
If the process of sistering is unsuccessful or undertaken poorly, it could actually speed up the breakdown of the joist as a whole. This generally happens because of the way the joist must be moved during the process. This weakens them and can even leave them more vulnerable to mold and mildew growth as well as pest infestation.
Your Overall Repair Costs Will Increase
Whether the process of sistering is successful or not, you will face a higher overall repair cost when you choose to sister a cracked or damaged joist. The joist is still likely to break down and require replacement at some point. This means you are paying for the sister materials as well as the eventual replacement joist.
As such, it is far more cost-effective to invest in replacement joists or heavy-duty jack posts as soon as you see sagging in your floor. This will prevent damage from taking root in your flooring as a whole, but will also keep your repair costs low.