There are several methods to reinforce floor joists for plumbing. Choose a reinforcement solution, including the material type, that fits your joists and their current condition. Also, you must adhere to the International Building Code and the applicable state or local regulations.

In this article, we’ll take a look at six effective ways to reinforce floor joists for plumbing. Each of these methods is not equally effective for joists of different sizes and spans. Also, choose a solution based on your needs, such as whether you have to reinforce joists with holes or members that are cut, notched, or damaged. This guide illustrates every practical solution. 

1. Scabbing

Scabbing is a simple and affordable yet effective joist reinforcement method. You take a piece of lumber or plywood of appropriate size and affix it permanently to a joist at the precise site of concern. You can scab one or both sides of a joist, based on the reinforcement requirement.

You will need sufficient lumber or plywood, construction glue or adhesive, and nails or screws. Using the same lumber that your joist is made of is ideal. Plywood is handy if you have to bend or tweak the scabbing a little.

Ordinary scabbing for a relatively small part of a joist is sustainable in many cases. Extensive damage, such as notches, cracks, and unsteady joists, requires the scab to be bolted.

Watch this video to see how to bolt a new partial joist using the scabbing method:

Don’t use these methods for engineered wood. Refer to the manufacturer’s recommendations.

2. Sistering

Sistering is similar to scabbing. You have to permanently affix a piece of lumber onto the sides of the existing joist. However, there are a few significant differences.

Sistering is usually done on both sides, whereas scabbing is sometimes limited to either surface of the member. Also, sistering spans much longer than scabbed parts of a joist.

Furthermore, you should ideally use the same type (or grade) and size of lumber instead of plywood for strong and durable reinforcement.

Check this brief video to see how you may sister floor joists:

Sistering is essentially reinforcing an existing joist with an identical one, effectively doubling up the structure. Thus, use the same lumber grade, size, and spacing specifications as the original joist.

3. Blocking

Scabbing and sistering are parallel reinforcements affixed to the surface or sides of an existing joist. The reinforced span may vary depending on the requirements. Blocking is a perpendicular reinforcement.

You may use the same lumber grade or plywood. Choose the same depth and thickness of the piece of wood as of the original joist. The length or span of the blocking material depends on the distance between two adjacent joists.

Blocking is essentially installing a piece of wood between two joists at a 90° angle to provide structural support if one or both existing members need reinforcement.

This video shows how you can install blocks on joists for a new floor:

Watch this video for blocking joists on an existing floor:

Blocks work well when you opt for the entire perpendicular cross-section of the joists in a given area. As shown in the videos, all the blocking pieces should form a neat, straight line. 

4. Bridging

Bridging is similar to blocking but with one significant difference.

The bridges are X-shaped grid-like reinforcement instead of perpendicular blocks. You may use the same lumber grade or other materials like metal or steel.

Here’s a video for galvanized steel bridging:

The following video shows the X type bridging or herringbone strutting method:

A few building codes apply to lateral supports or diagonal bridging for joists. You can find the relevant information here.

5. Plating

Joist reinforcers, such as steel or flitch plates, are an effective and convenient solution for plumbing. The other methods, whether scabbing and sistering or blocking and bridging, are relevant for various types of damage and reinforcement requirements.  

Specially designed plating can flawlessly suit holes of different sizes required for plumbing fixtures, such as drain or waste pipes to run through floor joists.

Plumbing installations and remodeling projects often require more than just drilled holes. You may have to cut or notch a few parts of several joists. Plating is an appropriate reinforcement method for all such structural changes in joists.

Here’s a video with an apt pictorial representation of how plating works:

Steel plates have one drawback. You will not be able to drill any holes or make other structural changes to the parts of the joists reinforced with the flitch plates.

Sistering or scabbing does not have such a limitation as they are lumber or plywood, and you can drill through the reinforced parts of the joists for a project in the future.

6. Sheeting

You may use a plywood sheet to provide additional support to a wobbly, bouncy, unsteady, or creaking joist. This solution is not specifically for reinforcing floor joists for plumbing, but it is practical and effective in improving the members’ overall stability and durability.

Can Plumbing Go Through Floor Joists?

Plumbing can only go through floor joists if the drilling, notching, and cutting do not violate any building codes and local regulations. Furthermore, the pipes or other fixtures should be appropriately sloped depending on the bore size and floor span.

Generally, you cannot notch or cut any part of the middle 1/3rd section of a joist bearing a load. The notches towards the sides must be less than 1/4th of the depth of the joist.

Hence, if you have a 2 inch x 12 inch joist, the notch can be up to 3 inches into the lumber.

Any hole drilled through a load-bearing joist must not be within 2 inches from the top or bottom of a joist. The diameter of such a hole must be less than or a maximum of 1/3rd of the joist depth.

Thus, for a 2 inch x 12 inch joist, the maximum diameter is 4 inches . However, there is a difference between nominal and actual sizes.

A 2 inch x 12 inch joist has an actual size of 1.5 inches x 11.25 inches. Hence, you cannot have a hole with a 4-inch diameter in such a joist.

Refer to these codes for cutting, notching, and boring wood members for plumbing fixtures. Your state or local regulations may be more stringent than these codes. Check the local codes before you finalize the project specifics, or you may have trouble if a building inspector shows up.

The Diameter and Slope Conundrum

The restrictions for the hole diameter and its position per the joist depth are not the only problem. You will have to solve the diameter and slope conundrum, especially if you have a large floor span.

A regular drain pipe requires 0.25 inches sloping for 1 linear foot. The position of every drilled hole should be calculated to create and facilitate this slope. If you have a large floor span with several joists, you may run into possible violation of the 2 inches exemption at the top and bottom of every member or stud.

You can include notches and cuts in your design or layout to avoid drilling holes closer to the top and bottom. However, you cannot notch or cut several parts of the joists per the building codes and local regulations cited and linked in this guide.

Can You Run Waste Pipe Through Joists?

You can run a 3-inch but not a 4-inch waste pipe through floor joists if the lumber nominal size is 2 x 12 inches or smaller. Other pipes with a smaller bore size or suitable per joist depth can run through the members.

The holes necessary for plumbing pipes to go through floor joists have to be slightly larger in diameter than the available bore sizes. You must factor in the specific installation process, too. These intricate details can complicate the process and could make your project illegal.

You may attend to the space around the pipes and use fillers or inserts. However, the building codes and local laws may prevent you from drilling the holes in the first place. Some states and areas have laws against drilling holes any larger than three inches in diameter in floor joists.

How Do You Reinforce Joists With Holes?

Reinforce floor joists with holes by scabbing, sistering, blocking, bridging, or plating. Joist reinforcers such as flitch plates can be handy. You may use plywood for reliable reinforcement. Choose lumber of the joist’s grade and size for stronger support.

How Do You Reinforce Cut Floor Joists?

Notched and cut floor joists need a bespoke reinforcement method. The severity of damage may call for a combination of reinforcements, such as sistering in conjunction with blocking or bridging. You may need flitch plates if a joist also has a hole or two.

Final Thoughts

Reinforcing floor joists for plumbing is a complicated project. Structural engineers strongly recommend against violating the relevant codes. Consult a pro if you are unfamiliar with the codes, suitable materials, and reinforcement methods for floor joists.

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