What Is the Building Code For Sistering Joists and Why You Should Know It

If your floor is sagging or your joists are weak, sistering them—attaching a new joist alongside the old one—can provide the strength your home needs. But before grabbing a new board and some nails, it’s important to understand the building codes and best practices for a secure, long-lasting repair.

While most building codes don’t specifically mention sistering joists, they do cover the structural principles behind it. Improperly sistered joists can lead to uneven floors, structural instability, and even failed home inspections. The right approach ensures your floor stays level and your home remains safe.

In this guide, we’ll explain the key rules for sistering joists, including how to properly attach them, where to place fasteners, and what to avoid when cutting notches or drilling holes.

What Does Sistering a Joist Mean?

Sistering a joist reinforces the structure, much like adding a brace to a weakened beam. Over time, joists can sag under the weight of the floor above, causing gaps between walls and ceilings. Adding a sister joist restores strength and helps keep the floor level.

While a girder is a large support beam used in construction, sistering a joist is different—it strengthens an existing joist rather than acting as a primary support. By securing a new joist alongside the old one, you double its strength and prevent further sagging or structural issues.

Will Sistering a Floor Joist Fix My Sagging Floor?

Yes, sistering a floor joist can fix a sagging floor, but it must be done correctly. If you attach a sister to a joist that has already sagged, you’ll lock in the sag rather than fixing it.

Before installing the sister, you need to lift the original joist back into position. Professionals use telescoping support jacks, which allow for gradual lifting without causing damage.

These jacks have a threaded rod that extends as you turn it, making them both a lifting tool and a temporary support. Once the joist is level, the sister can be securely attached to reinforce it.

What Are the Rules When Installing a Sister Floor Joist?

For a sister joist to work properly, it needs to be installed correctly. Think of it like splinting a broken bone—you must first straighten the joist before reinforcing it.

Professional carpenters follow a few key rules to ensure the joist is structurally sound and meets code requirements. These include proper bearing support, correct fastener placement, and avoiding weak points like oversized holes or notches. Let’s go over these critical guidelines to make sure your sister joist does its job.

Make Sure the Sister Joist Has Bearing Support

Before installing a sister joist, make sure it has proper bearing support—meaning it rests on a solid structure like a girder, beam, or concrete post. If the sistered joist doesn’t have support at both ends, it won’t add strength and could lead to further issues.

To avoid extra supports, try to match the full length of the existing joist. If your sister joist must be shorter, ensure that any joints land directly on a beam or post for stability.

Always Drill Holes and Cut Notches In the Correct Place

Cutting notches or drilling holes in the wrong spot can weaken the joist and lead to structural failure. Follow these key rules to maintain strength:

• Keep Notches Small and in the Right Spot: Never cut a notch deeper than 1/6th of the joist’s width. For example, in a 2” x 8” joist, your notch should be no deeper than 1 inch.
• Avoid the Middle Third: Never place a notch in the middle third of the joist’s span. If your joist is 60 inches long, keep notches within the first or last 20 inches.
• Drill Holes Away from the Edges: Holes should be at least 2 inches from the edge to prevent weakening the joist.
• Limit Hole Size: Never drill a hole larger than ⅓ the width of the joist. For a 2” x 8” joist, keep holes 2 ½ inches or smaller.

Following these guidelines ensures your sister joist stays strong while allowing for necessary wiring or plumbing.

Install the Correct Fasteners For the Sister

Using the right fasteners ensures your sister joist stays secure and structurally sound. Here’s what to use and what to avoid:

• Best Option: Carriage bolts with washers and nuts provide the tightest, most secure connection, especially if using multiple sister joists. Space them in a staggered pattern (e.g., 12”, 36”, 60” on one side; 24”, 48”, 72” on the other) to distribute the load.
• Avoid Lag Bolts: They can strip easily and don’t create a tight enough bond.
• Nails & Screws: If using only one sister joist, #12 or #16 nails or 3” galvanized screws work well. Square drive or Torx screws provide better grip than Phillips-head screws.

A properly fastened sister joist won’t move or separate over time, keeping your floor level and stable.

Orient the Sister Crown Up

When choosing a sister joist, look for one with a slight upward curve (crown). The crown should always face upward because:

• It counters natural sagging over time.
• It strengthens the joist in the right direction, as floors put downward pressure on the joist.
• It prevents future dips in the floor.

A slight crown of about ¼ inch is ideal. Anything more may cause an uneven floor, while a completely flat board could start sagging faster.

It might interest you to read our guide on reinforcing floor joists. It provides helpful insights about floor joist reinforcement.