NYC Townhouse Structure Sistering Joists

by | Last updated Jul 8, 2023 | Construction Practices, Renovations

When renovating a townhouse in NYC, you can find hidden or unknown structural problems requiring structural repairs, such as sistering joists and installing new joists, headers, and trimmers.


What is Sistering a Joist?

Sistering Joists is a technique of structural reinforcement where you install a second joist next to the original one and bolt them together. Sistering is a common structural repair method in old homes with wood joist floors. Most old townhouses will require structural repair, especially if you make major changes. 

I am Jorge Fontan, an architect in New York and owner of Fontan Architecture, a Manhattan-based architecture firm. In our office, we work on various project types, including townhouse renovations. We often find these old townhouses can have a variety of structural problems that need fixing. In this post, we will look at the most common structural repair we do, sistering joists.


Sistering Joist For Structural Repair

There are two common reasons we sister joists. The first is because an existing joist is damaged in some way, and we want to reinforce it. The second is because we are removing a bearing wall and strengthening the joist with sisters.

Existing joists in an old townhouse can have damage for many reasons. There can be water damage causing rot which is common. The wood joists can develop splits over time. Somebody could have cut through the joist for some reason on a previous renovation. If the joist is compromised but salvageable, we may choose to sister it for reinforcement. This situation is quite common in old townhouse renovations.

Sistering Joists

Sistering Joists


Sistering Joist For Structural Reinforcement

Sometimes we want to remove an interior bearing wall to open up a space. There are multiple ways to do this. One is to add sisters, the other is to add a header, and yet a third is to reframe the floor above completely. Sistering may be the easiest option, depending on the existing conditions and proposed design.


Sistering Joists for New Openings

One reason you may need to sister joists is to accommodate a new floor opening for various reasons.

The photo below shows an HVAC unit installed in the ceiling. We had to cut one of the joists to fit the unit. Our structural engineer designed a detail to sister the joists and created a new box-out with joist hangers and bolts. This new cavity allowed us to fully recess the unit when it did not fit in the original joist spacing.


Permits For Sistering Joists

In New York, we have stringent regulations and protocols. All our work is filed with the Department of Buildings, approved by a plan examiner, and then the General Contractor can pull permits. We file a structural plan and details if a project has structural work. The structural application also triggers the requirements for special inspections. Our structural engineers design the system and perform all necessary inspections. At the end of the job, we submit Technical Reports to the DOB for signoff.

We have another post on Removing a Wall in a Townhouse if you are interested.


NYC Townhouse Structural Repair Work

If you want to read more about Townhouse and Brownstone renovations, we have another post covering a broad range of topics for NYC Townhouse Renovations.

In this article, we reviewed a few examples regarding structural work in NYC Townhouses. This post does not assume to cover every possible issue or condition but provides a general overview of the topic.


Thank You for reading our blog post on Sistering Joists in Townhouse Renovations.

I hope this was helpful. If you want to discuss a specific project with an architect, please get in touch with us at Fontan Architecture directly.


Contact Fontan Architecture

Jorge Fontan

Jorge Fontan

This post was written by Jorge Fontan AIA a Registered Architect and owner of New York City architecture firm Fontan Architecture. Jorge Fontan has earned 3 degrees in the study of architecture including two degrees from the City University of New York and a Masters Degree in Advanced Architectural Design from Columbia University. Jorge has a background in construction and has been practicing architecture for 20 years where he has designed renovations and new developments of various building types.