Joist Sistering: The Key to Repairing Floor Joists

Written by Bay Crawlspace and Foundation Repair on . Posted in .

Sagging floors add character to an old house—right? Well, character is one thing. Structural damage to a house; on the other hand, is a whole other layer of problems no person should let go unchecked. 

Having sagging, moving, or shifting floors may indicate that there’s a bigger problem underneath. In fact, it most likely points to a major issue with the floor joists that are supporting the floor.

If your floors are sagging, then it might be time to make some repairs. Read on to find out how professionals use joist sistering to repair old or damaged floor joists.

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Purpose of a Floor Joist

Floor joists are structural members used in the framing of a floor structure to support a space from beam to beam. Most people understand that without floor joists, your floors wouldn’t have any structural support and would eventually collapse from the weight. Not only do they support your floors, but they also carry the weight of the walls, ceilings, furniture, appliances, and the people that walk on them. 

Reasons Floor Joists Get Damaged

In residential homes, most floor joists come from natural or engineered wood. These joists are sometimes doubled or even tripled to give extra load capacity depending on where in the house and what the building codes require. So why do they fail sometimes?

There are 3 main reasons for floor joists failing. The first is the loss of integral strength due to crawl space moisture. Oftentimes, crawl spaces contain moisture from exposed damp ground, groundwater flooding, and outside humidity. This dampens and weakens the wood, leading to fungus and wood rot.

Another reason joists fail is due to water damage. If a house has a plumbing leak, it can cause wood to deteriorate more rapidly. And to compound it, if a joist already sags, water will naturally expedite the damage and movement.

The last common reason for floor joist failure is termites. Termite infestations in homes can cause significant damage to the structure of homes, leading to sagging floors. 

Signs That a House Might Have Structural Damage 

A sagging floor is the biggest telltale sign that there are issues within the structure of your home. Doors that stick or drag on the floor, cracked walls or ceilings, beams that bow, and cracks in the foundation are also signs that can indicate structural damage within a house.

Expert Advice

Depending on how bad your floors are sagging or moving, it’s wise to get the advice of multiple structural expert before going ahead with any repairs. You’ll want advice from foundation repair experts and possibly structural engineers if the problem seems complex. In the case of structural repairs, it’s best to hire a professional with years of experience to do the job.

Professionals Fix Structural Damage With Joist Sistering

Joist sistering is adding an extra identical floor joist, to a damaged or inadueqate floor joist, and tieing the two together with screws or nails. It is a very effective way of adding the additional strength needed to hold up a sagging floor. Let’s take a closer look at how this was accomplished.

Leveling the Floors

Damaged joists usually mean sagging floors—right.  Well, to make a floor level again, it needs lifting.

In order to lift a sagging floor, install a screw jack and beam underneath to push the damaged sagging joist(s) up. A screw jack is essentially a metal support post that raises or lowers by the turning of a threaded steel pipe that’s inside of it (hydraulic jacks are often used for this purpose as well).

If there are significant sags in multiple joists spanning the floor, then professionals will use multiple jacks to do the heavy lifting. Holding a 4ft level up to the joists will show how much a floor could need to be jacked up. 

Lifting a house always include the risk of damage. Lifting floor joists can cause cosmetic damage to paint, drywall, flooring, counters, trim, etc. It’s important to lift carefully but some damage cannot be avoided if lift is necessary.

Sistering the Joists

To repair floor joists, the damaged joist is sistered together with a new piece of wood. Usually, the new piece of lumber spans the length of the old one. At times when this isn’t possible, shorter pieces are used instead (this is sometimes referred to as scabbing). 

When a joist is weight-bearing, meaning it holds up a wall or has extra weight attached to it, it’s sometimes necessary to sister multiple pieces of new material to the old one. This is usually done by “sandwiching” the old joist in the middle of the two new ones. Doing this gives additional support to the joist.

Doing Joist Repairs Yourself 

Working on your house yourself can save you money as well as give you a sense of pride and ownership. If you have the tools and the time, you might be tempted to take on the job and try floor joist repair sistering yourself. Below are some issues you might run into along the way.

  • Incorrectly Notching Floor Joists: Maximum notching at the end of the joist cannot exceed 1/4 of the joist depth. Maximum notching in the outer third of the joists is 1/6 of the joist depth, and no notching in the middle third of the joist. Incorrect notching at any length of the board can seriously affect the integrity of a joist.
  • Failing to Level Joists: Leveling a floor is a process that sometimes takes days. This is because jacking up a joist may cause the rest of the house to settle in ways that weren’t expected. 
  • Sister Board Not Level: Sistering a new board to an old joist often takes multiple people to hold up in order to get level. Not leveling the new board to the old joist will defeat the purpose of the job.
  • Failure to Properly Secure Boards Together: Not knowing how to secure sistering joists together can cause structural problems in the future.
  • Insufficient Over-lapping: Knowing weight loads is important when figuring out how much to over-lap boards. Failure to over-lap boards enough may cause future sagging to reoccur.
  • Not Using Enough Support: This happens a lot in the case of load-bearing walls and other unique circumstances when more support is needed to hold up a joist.

Building code for sistering joists is also important to know when making joist repairs. Not knowing these will lead to problems if you ever plan to sell your house. Sometimes building permits are needed as well.

Other Repairs Needed

Another reason to hire a trusted professional when making joist repairs is that other issues often arise from damaged floor joists. Usually, water damage also occurs in subfloors and floors and sometimes need replacing.

A professional will spot where moisture is coming and help prevent future problems from occurring. They will also determine if beams need replacing and/or additional support beams placed.

Length of Time it Takes For Repairs

Jacking up a joist and raising a sagging floor can be a relatively quick process. An expert repair technician will know the size board to install and how to perform the repair efficiently. 

If lifting the floors more than 1/2″ is your goal, it should should be done over time to avoid damage. Sometimes this means that a screw jack is only raised once or twice a month, moving up only 1/4″ at a time. If your sag is 1″ or 2″, then you can see how this process would take a little longer. Expect new cracks to form on walls and ceilings after they’ve been jacked up.

Call in The Professionals

At BAY, our experts have years of experience in identifying the causes of structural damage within your home. If you have concerns about the integrity of your foundation, we offer free no-obligation inspections and estimates. When we inspect your home, we explain the problems, solutions, and explore multiple options to find the best solution for you—joist sistering is only one of our expertise.

If you’ve found this article useful but still have questions about your foundation, check out our BAY’s Foundation Repair Q&A.

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