Sistering or Joining Joists

Do you need more space on your deck and want to know how to make it bigger through an upgraded deck design? Does your deck have a joist that is bouncy or soft, or one that is starting to rot? Sistering deck joists could be an easy way to solve both problems.


"Sistering" joists means to put two joists on top of each other across a beam by a certain distance to make a longer joist. This isn't often done when building a new deck, and it would only be needed for very long spans that are longer than your longest boards.


Sistering joists is often done when repairing or adding on to an existing deck. In these cases, the overlapped joists help hold the frame together by connecting the rigid members over a beam. The connection is stronger the farther apart the overlaps are. At least a couple of feet should hang over each side of the beam. The joists that are next to each other should be nailed together in a pattern similar to how boards are put together to make a beam. If you want to add onto a deck, you need to make sure the footings are big enough to hold the extra weight.


Read more: How to Angle Corners and Joists


Why Do You Need to Sister Deck Joists?

Sistering is when two or more joists are put together to fix or strengthen an existing joist or to make it longer or wider. There are many ways you might need to connect two deck joists together.


One obvious reason to use sister joists is to add on to an existing deck. By putting new joists on top of old joists where they cross a support beam or wall, the new deck will look like it has always been there. It uses a support structure that is already there, but the other end of the new deck may need to be supported.


If longer boards aren't available or won't fit, you can also make a joist longer by putting it between two lengths of the same material. This will allow it to span acceptable distances. Usually, bolts or lag screws are used to connect the sandwiching boards to the joists, especially if the joint is not supported.


A common reason to join two joists together is to fix something. A joist may sag, get spongy, crack, or break over time. Lifting and levelling the damaged part and then putting wood of the same size next to it fixes and strengthens the problem. The new piece could be the same length as the old one, or it could go two to four feet past the weak spot. Some joists rot and fall apart much faster than others. The damage may only be on the surface or where nails or screws went in, leaving most of the plank in good shape. Cut out the damage and put a board with the same width and thickness next to it or along the whole length. Use approved fasteners that won't cause rot to reattach the deck boards. Two other reasons for sistering are to strengthen the joists and add support to the structure. When joists are doubled from one support to the next, they can carry more weight. This is especially important if the deck will have to hold something heavy, like a spa or hot tub. Sistering is also used to fix a joist that has been damaged by a notch or hole made to let something pass through it.


How to Properly Sister a Deck Joist

How you should join two deck joists together depends on what you are joining and why. Sistering joists to make a deck bigger is not the same as sistering a joist to fix it or make it stronger. So, what and why make a difference in how.


Depending on the type and grade of wood, plank dimensions, and joist spacing, end joists can cantilever. Check your local ordinances for cantilevering restrictions. Cantilevered deck 1 to 3 feet over the beam. Posts and beams may be needed to support the outer joist ends.

Rim joist removal exposes joist ends. Cantilever extensions are possible if they don't exceed the beam. A joist's supported span can extend 40% past the beam.

A 2x12 joist with a 10-foot span can cantilever 4 feet. Depending on wood type and joist spacing, 3'-1" to 4'-6" Wood kind and spacing determine how far a 2x8 may cantilever.

Removing the rim joist and sliding the new joist next to the previous one sisters cantilever joists. Inserted and sistered sections should be double the cantilever's length. Depending on local codes, a 2-foot addition will be 4-feet sistered to the existing joist with 3" nails in a 'W' pattern every 12" or 12" carriage bolts in a tandem pattern every 16".

Extending a Deck

Extending a deck by sistering joists connects the old and new portions and is similar to cantilevering in that it requires additional supports to carry the extension. To expose the existing joists and beam, remove the rim joist.

Get the joist dimension (2"x?") to determine the span possibility for the new support beam location. This assumes you'll be using the same dimensional timber. At 16" centers, an SPF No. 2 grade 26 can span 9 feet, a 28 11'-10", and a 210 15'-2"; however, check your local code for spans.

Install the posts and beam at the proper spacing. Insert the new joist into the existing beam and connect it to another joist using appropriate fasteners. Because the new joist will be supported on both ends, the sistering overlap is limited to the width of the beam plus any existing cantilever.

Repairing or Strengthening a Joist

If a deck part is sagging or mushy, inspect the joists. What is wrong can be identified using a string-line, level, or standard eye-balling. A rotted or cracked joist could be replaced or sistered with equal dimensions lumber. A brief section pairing may suffice, or a full sistering or doubling-up may be required.

Remove as much rot as possible; the more rot there is, the more expensive the repair. Reinforce the existing joists along the whole length of the rot removal area, as well as at least two feet beyond both ends of the rot. To add the new board, you'll need to remove and replace any blocking or bracing.

Back-out fasteners from deck boards above the damaged section, remove it, and replace it with equivalent dimensional wood. Sandwich the replacement between two equal-dimensional lumber sisters that extend two or more feet past each end of the cutout. Temporary supports may be required to hold and align the cut joist. Depending on the extent of the damage, it may be preferable to replace the affected joist.

Sister Joist Nailing Pattern

The nailing pattern for sistering is determined by the type of sistering and the amount of working area available. Depending on joist spacing and ground clearance, swinging a hammer between joists may be challenging. Nails are also more difficult to drive into ancient wood.

Three 16d common nails every 16 inches are sufficient for a full sister or doubled joist. It should be noted that hammering adds extra stress to the component being sistered. Instead, predrill and use two 3-inch lag screws or 4-inch bolts every 24-inches.

The ends of the new board are not supported underneath when sistering a damaged or weak joist with a piece shorter than full length. To increase shear strength, double or triple the nail pattern at the ends. Instead of nails, lag screws or bolts can be used in a tighter pattern than a doubled joist.

The nail pattern for double sistering or sandwiching a joist is similar to that of sistering a damaged joist, with the exception that it should be done from both sides. To connect the tripled part together, use two 4" or 412" lag screws in a 'W' configuration, screwing from both sides. Depending on the length of the repair, two 5-inch bolts could be used instead of nails or lag screws, with two every 12-inches or fewer.


Sistering a board of identical width and thickness for part or all of its length is a typical procedure for repairing or strengthening a deck joist. It is also used to increase an existing deck area by cantilevering sistered joists over an end beam or to connect new and old deck construction. Always check the local building code before installing sistered joists.

Back to blog