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Sistering or Joining Joists

The term “sistering” joists is the practice of overlapping two joists by a certain distance across a beam to create an extended joist. This is unusual in new deck construction and would only be necessary for very long spans that exceed the length of your longest boards.

Sistering joists is commonly used when adding onto an existing deck or repairing a deck. In these cases, the overlapped joists help tie the framing together by combining the rigid members over a beam. The longer the overlap distance, the stronger the connection. You should overhang at least a couple of feet on each side of the beam. The side-by-side joists should be attached together by a nailing pattern similar to combining boards to form a beam. If you are adding onto a deck, you must make sure the footings are large enough to support the added loads.

Sistering Joists

Balconies are defined as small decks built without support posts usually constructed with projected joists that are run back into the house. This type of deck uses sistered joist to project a cantilevered effect. A structural engineer should be consulted anytime an unsupported balcony is being constructed to ensure the structural integrity of the design.


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Create a free account and get instant & exclusive access to all that Decks.com has to offer:

Checkmark 1,000+ How-To articles
Checkmark 80+ Free Deck Plans
Checkmark Deck Planning Calculators
Checkmark Free & Simple Deck Design Tool
Deck Framing

Sistering or Joining Joists

The term “sistering” joists is the practice of overlapping two joists by a certain distance across a beam to create an extended joist. This is unusual in new deck construction and would only be necessary for very long spans that exceed the length of your longest boards.

Sistering joists is commonly used when adding onto an existing deck or repairing a deck. In these cases, the overlapped joists help tie the framing together by combining the rigid members over a beam. The longer the overlap distance, the stronger the connection. You should overhang at least a couple of feet on each side of the beam. The side-by-side joists should be attached together by a nailing pattern similar to combining boards to form a beam. If you are adding onto a deck, you must make sure the footings are large enough to support the added loads.



Balconies are defined as small decks built without support posts usually constructed with projected joists that are run back into the house. This type of deck uses sistered joist to project a cantilevered effect. A structural engineer should be consulted anytime an unsupported balcony is being constructed to ensure the structural integrity of the design.


Popular Framing Articles
Deck Support Columns

How to Decorate Deck Support Columns

Browse some examples of decorative deck support posts for tall decks.

Deck Support Beams

How to Install Deck Girder, Cantilever or Drop Beams

Learn about the pros and cons of installing a cantilever, girder or drop beam to support your deck joists. This method allows for a notched post-to-beam connection.

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Learn what factors determine the maximum joist cantilever overhang that is allowed.

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Decking Patterns

Installing your decking at a 45-degree angle is a popular style for decks. This can be done to reduce decking seams or just for appearance.

Decking Installation

Introduction to Decking

Spacing between deck boards provides several critical functions, such as drainage & drying. Learn how to space and stagger deck boards at Decks.com.

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Decking over decking

No; this is not a recommended practice.

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