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Sistering Floor Joists to Repair Sagging Joists: Guide



V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com


Sistering Floor Joists

Sistering of joists can be a great way to protect your floor joists from sagging.

Before sistering any sagging joists, it is important to understand how sistering works and how to do it correctly.

What is Sistering of Floor Joists? Explained

Sistering is the process of joining two or more members together to act as one continuous member. When sistering floor joists, the new joists are placed next to the existing joists and connected with screws or nails at each end and along the length of the new joist.

The new joists must be of the same thickness and depth as the existing joists.

Sistering is done to add strength, stiffness, and support to a structure.

It is frequently used to restore weak or damaged members, but it can also be employed to stop further harm.

Sistering can be used on both load-bearing and non-load-bearing members.

It is important to remember that joist sistering and blocking are not the same.

While sistering can improve the strength of your floor joist, for large spans and loads, a floor truss might be a better option.

Why is a Floor Joist Sistered? Reasons

There are several reasons why you might need to sister your floor joists.

If your house has settling issues, the sisters will help support the weight of the house and distribute the load more evenly.

This can help prevent further settling and possible foundation damage.

Additionally, if your floor is bouncy or saggy, sistering the floor joists can help stiffen it up.

This can make your floors more comfortable to walk on and reduce creaking noises.

Sistering floor joists can also help to level out an uneven floor.

Furthermore, if your house has been damaged by fire, insects, or water, sistering can help repair damaged floor joists while also protecting against future damage.

Building Code for Sistering Joists: Understand

In most building codes, minimum requirements are established for the building and designing of floor systems, but not for the upkeep or reinforcement of these systems.

Consequently,there is no sistering joists requirement in the building code because this action is seen as strengthening a framing system.

You just need to ensure that the new joists are of the same size or larger than the existing joists and that they are properly attached.

Typically,homes are intended to be erected on single joists at established spacing.

The new joist must support that weight even if the old joist cannot.

Licensed contractors aim to fix these floor joists at a distance of 12 to 24 inches, with 16 inches in the center being the ideal spacing.

Moreover, perpendicular headers over 4 feet must be sistered.

The header is then fastened to the side joists.

Furthemore,it is mandatory for these joists to be sistered and supported at both ends by a beam or wall.

What Length Should a Sister Joist be?

When it comes to deciding how long a sister joist should be, there are a few factors to consider.

The first factor is the width of the joists. Generally, the wider the joists, the longer the sister joist should be.

Second, the span length must be taken into account. The longer the span, the longer the sister joist must be.

And finally, the third factor is the weight of the flooring material. The heavier the flooring material, the longer the sister joist should be.

Moreover, sistered joists always do not need to match the original joist length.

A sistered joist that is 2/3 the length of the original is sufficient if you can’t run the joist the entire length.

The sister joist, however, must be at least 2 x 6 in size.

Furthemore,if a joist is sagging or broken and you need to repair it, but you don’t have much room, you can partially sister a joist as long as you are at least 3′ away from the damaged area on both ends.

However,position the sistered joist as far away from the deflection point as you can.

How to Sister Joists Correctly: Step-by-Step

If your floor is sagging or starting to show signs of wear and tear, sistering the joists is a great way to reinforce the structure.

Sistering joists is a pretty straightforward task, but it can take a while if you have to remove a lot of utilities or if the joists have shrunk and do not fit together properly.

In such situations, it is usually preferred to use a shorter and wider joist over a longer and narrower one.

Sistering Floor Joists
Sistering Floor Joists

Here’s a step-by-step guide on how to do it.

Step 1: Consult a Structural Engineer

As the first step, consult a professional if you have concerns about the structural integrity of the building.

The professional will examine the floor and recommend strengthening measures to eliminate droop and bring the frame into compliance.

Any structural problems, such as too-small joists, will be revealed by the inspection and require the services of a professional.

Step 2: Eliminate Wiring and Additional Blockages

We will spend the most time on this step.

The wiring, plumbing, HVAC, and other obstructions must all be taken out.

Typically, this entails ductwork removal, pipe detachment from valves, and wire removal from the breaker box.

Many times, taking out so many ducts, pipes, and wires causes a joist to collapse partially.

We recommend running a sistered joist down the whole thing for the best support of your subfloor.

Step 3: Size and Cut the New Joist

Remember to maintain your new joist’s dimensions relative to the old joist if you want everything to line up.

If at all possible, apply the 2/3 rule when cutting a partial joist.

This indicates that if your joist is 12’, a sistered joist of at least 8’ is sufficient.

Step 4: Fit the Sistered Joists

To make sure the sistered joist fits tightly against the old joist, shim either end.

A full-sized sister joist might not fit snugly against the old joist at either of the bearing ends.

If the sister joist doesn’t fit snugly against the old joist, some easy adjustments can be made without damaging the walls above.

To ensure that the new joist is aligned with the old one, put a piece of wood on the floor where the joist bears on it or against a wall, and then use a jack or jack post to lift up on it.

Once everything is in place, install the bolts and make sure everything is snug and plumb before moving on to the next step.

Step 5: Fasten the Sistered Joists

Nails are better than screws or any other kind of fastener when it comes to attaching two pieces of wood together.

This is because you need a greater shear-holding power to deal with the vertical stresses on joists.

To adhere two 1.5″ thick joists together, you’ll want a 3.5” long 16d nail.

Nail the ends off, so they stick out through the other side and bend them back into the board.

The sistered joist will appear to have a zigzag pattern created by these nails.

Moreover,nail three nails a few inches apart at either end of each partially sistered joist, in the top, middle, and bottom.

Remember to use a huge amount of construction adhesive when nailing, bolting, or doing both.

While,the most important part of this project is the fasteners, glue holds the fasteners in place and strengthens the connection between the joists.

Step 6: Final Evaluation

Make sure the new boards are level and not too flexible.

Before reinstalling the utilities, any issues should also be addressed.

Step 7: Reconnect the Utilities

It’s time to reconnect your utilities after fastening and installation.

Drill joists, if necessary, to install service lines.

Again ensure your new joist is level before installing anything else, and test the floor from any other floorboards you can access to make sure there are no creaks.

Before moving, also replace any damaged items.

Sistering Engineered Floor Joists

In framing projects and, consequently, in sistering floor joist endeavors, engineered wood joists are gradually taking the place of 2 x 10 dimensional timber.

Despite being more expensive than 2×10 dimensional lumber, they are simple to use and relatively light.

Sistering Engineered Wood I-Joists


Sistering engineered wood I joists is a great way to increase the strength and stability of your joists.

When sistering, like traditional joists ,you’ll need to use two pieces of lumber that are the same thickness and width as your original joist.

You’ll also need to use a strong adhesive to bond the two pieces of lumber together.

Although dimensional timber is frequently used as sistering material, engineered lumber products actually add additional rigidity.

Because they are lighter, they also have the advantage of being quickly handled on the job site without the need for pricey handling equipment.

However, they can be vulnerable to damage if they’re not properly supported.

Moreover,it may be challenging to link an engineered I-beam to a traditional plank due to the diameter differences between the two materials.

A common practice is to sister the joist with plywood or OSB to match with dimensioned wood.

For optimal results, span the sistered joist as long as possible.

Furthermore, when sistering with engineered wood,8d nails will be used because the thickness will be smaller than if you were sistering two conventional pieces of dimensional lumber.

Additionally,engineering joists are frequently provided in bespoke lengths.

The “web” or vertical part and the outside border of the flange are typically separated by one inch in these I joists or engineered wood planks.

That will need to be filled in with two layers of 1/2″ OSB or plywood.

To prevent gaps from overlapping, space the pieces out as you go.

Use plywood as the sister joist once the web has been completely filled in, ensuring that your plywood is stacked staggered on top of one another.

Sistering Laminated Veneer Lumber(LVL) Floor Joists

LVL Joists
LVL Joists

Laminated Veneer Lumber (LVL), an engineered wood product made of several sheets of thin wood adhered together, is used for sistering floor joist installations.

You can use LVL floor joists, which are uncommon because of their high cost and scarcity, in the same way that you would use dimensional wood.

Try to choose wood whose breadth corresponds to that of your LVL joists.

When sistering laminated veneer lumber (LVL) joists, it is also important to use the same species of LVL.

It is also important to stagger the joints so that they do not line up directly over one another.

This will help to distribute the load more evenly and prevent any single point from bearing too much weight.

For added stability, you can also glue the new piece of LVL to the existing joist.

These LVLs are typically used for longer spans that dimensional lumber cannot reach.

If you need an installation that requires LVL floor joists, then we recommend first tapping into what is available from your local supply.

In the United States, one of the top producers of LVL joists is Forests Product Supply.

However, it may be cost-effective to try some of the other options for sistering the joist installation, which are currently en vogue such as engineered wood and CDX plywood.

Floor Joist Sistering Cost Guide

You should expect to pay between $100 and $300 per joist on average for sistering.

For a larger area, the cost could be closer to $1,000 or more.

The cost of sistering floor joists varies depending on the size, the number and type of joists involved, and several other factors.

For instance, if your floor is difficult for a professional to access, fixing it will take more time and effort, which raises prices.

Moreover, if your contractor needs to cut a floor or ceiling open to get to the joists, the price will go up significantly.

Your expenditures will also increase if the damage to your floor joists is severe.

Before repairing anything, you’ll need to pay to get rid of the insects or fix the moisture issue because termites or prolonged water exposure can destroy joists.

You can dry these joists using low-pressure fans and dehumidifiers.

These charges can differ greatly depending on the type of issue.

If your sagging floor is caused by a major structural problem, such as a cracked foundation, you will need to address that issue first before repairing the floor joists,which can increase the cost.

Frequently Asked Questions(FAQ)

Can I use 2×4 for Floor Joists?

Although 2x6s and 2x8s are more frequently used for floor joists, 2x4s can also be used in some circumstances.

However, it is recommended to limit the usage of 2x4s to smaller floors with close spacing.

What Size Bolts for Sistering Floor Joists?

When sistering floor joists, you will need to use bolts that are at least as long as the thickness of the lumber being used.

This application typically uses 3/8″ diameter bolts arranged in a “W” pattern, spaced about 8″ apart.

All 3 feet of wood on both sides of the damaged section should be penetrated by this.

How Much Do Sistering Joists Overlap?

The short answer is that joists should overlap by at least 3 inches, but ideally 6 inches.

This will make it more likely that your floor will be robust and resist sagging in the long run.

If you are unsure of how much to overlap your sistering joists, it is always best to err on the side of caution and go with a larger overlap.

V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com