Your house is prone to deterioration due to overuse and aging, and you should know the different components of the house as they will guide you during reconstruction. For instance, the beam is a structure that supports the roof or the floors above the ceiling.

You should know the type of structure incorporated into your home with the joist and girder support beams. These roof structures have different sizes, designs, and functionality.

Here is a Joist vs. beam vs. girder comparison, which will guide you during indoor repairs.

What is a Joist?

The joist is a roof structure made of steel or timber and reinforced with concrete; the joist is laid parallel to the roof or floors. They are a secondary support system over the roof and the subsequent upstairs floors.

They are laid together in more significant numbers, and the more extensive a roof is, the more the joist it will need.

They enhance the roof’s ability to distribute weight, and they have spaces that depend on the construction type. The joists act like a skeleton that supports the weight the floor would bear. Some construction experts refer to the joists as beams that span short distances and strategically support the whole structure.

The joist supports the subfloors and is attached to beams and girders, and in some constructions, the trusses are used as joists since they are attached to the girder. They are steel, dimensional lumber, LVK, and engineered lumber.

Although the joists come in different sizes, standard dimensions include the 2×8, 2×10, or 2x12s. The joists are spaced 24″ 19″ and 16″ apart, depending on the building size.

These are the types of joists you can incorporate into your building:

The Floor Joist

The joist is used on the subfloors on a live load floor structure, and they are designed to support the dead weight. The deadweight is the weight of the non-moving structures above the structure. The floor joist also supports the live weight, including people’s movement on the floor.

These joists are supported by girders that run across the floor, distribute weight, and work with the guide to reinforce the building’s weight.

The Ceiling Joist

The ceiling joists support the roof and are the horizontal support systems in your home. They form the horizontal frames which support the whole roof frame, and they transfer the roof loads from the roof to the perimeter frame in the home.

The Rim Joist

The rim joists are attached to the perpendicular ends of the floor joist and keep the floor joist from warping leading to effective weight distribution.

These joists are not direct weight supporters, but they distribute weight evenly from the rims and floors and function in tandem with other joists to keep the structure compact.

Joist Functions

Joints are load-bearers; the floor joists and beams bear the dead load on the subfloors and the building frame. The joists also bear all the live load such that as people move in a home.

They take all the weights above the frame and direct it to the girder, beams, and sidewalls, thus ensuring the house stays compact and does not crumble under uneven weight distribution.

What is a Beam?

The beam carries the whole roof’s weight in the building and is an essential component that ensures the home stays compact. They are more solid than other roof structures as they are the leading weight carriers.

They transfer the roof weight to the vertical elements; thus, they should be made of solid materials like steel or engineered wood.

The beams are in all house parts, reinforcing the basements, attics, and roof. They may or may not have specific names which distinguish one beam from another. For example, the 2×4 stud frame beam has 8′ vertical studs, topped by horizontal studs and reinforcing the wall framing.

Most top beams don’t have a name, and the construction team refers to them as roof beams.

Rafters connect to the ridge beam, and the headers scan the space above the doors and windows, and they transfer the loads above it to the vertical studs on the sides.

Regardless of the beam type, size, and position in the house frame, they effectively support and weight distribution throughout the structure.

Here are beams you would find in your house:

Steel Beam

Although the steel beams are expensive, they offer more support to the roof as they are sturdy. Large and taller buildings such as malls and apartments incorporate the steel beams into their structures to support adequate weight.

If you live in a bigger house, it is likely for it to have steel beams; thus, during reconstruction, you should check the beams and ensure you use the suitable materials which make it possible to maintain stability.

Combined Dimensional Lumber Beams

These are intense beams compared to wooden structures. The beams are well constructed and are made from several dimensional lumber joints which form longer beams. They are suitable for homes, and you can ask the construction experts to incorporate them into your house.

Flitch Beams

The flitch beams are dimensional lumber that incorporates steel sheets into the beam. The steel sheet increases the combined dimensional lumber strength making the structure stronger. The flitch beams ensure the homes withstand weight pressure so the building will stand for an extended time.

The combined dimensional lumber sandwich the steel, thus preventing roof flexing, which keeps the roof intact for an extended time.

The Engineered Beams

You can modify the beams with innovative technology to get more robust roof support and load transfer. These engineered structures have I-shape cross-sections which distribute the weight effectively.

Although most of these beams are made of plywood, you can customize ones that meet your needs based on the type of building.

LVL Beams

The LVL beams refer to the laminated veneer lumber. They are primarily found in the girders as they form intense beams that distribute weight evenly in a building. The LVL is similar to plywood, but it is very sturdy, supports more weight, and has a uniform orientation.

It is possible to fabricate longer LVL lengths, making it easy to create different beam lengths and incorporate them into the girder.

What is a Girder?

The girder is one of the supportive beams larger than other beams and offers more support; it acts as a central beam and is a horizontal structure that supports the whole building. They are load bearers and have a high capacity than any other beams to support the concentrated loads.

Since the girders are susceptible to more weight, they are made of sturdy materials which support the building effectively. Mostly, they are made from concrete or stainless steel, and they stand out from the other beams.

However, all girders are beams supporting the other beams, and not all are girders. Depending on the roof’s surface area, a house can have more than one girder.

Some girders are found at the home’s base, and they support the whole house frame as they are connected to the vertical posts leading to effective weight distribution. The home girders work similarly to constructions like bridges and industrial buildings.

These are the girder types you will find in a home:

Steel I-Beams

The steam I-beam is shaped like the capital ‘I,’ allowing for a flat lower and upper surface, leading to effective weight distribution in a building. The posts can sit effectively under these girders due to the even shape leading to more support.

The Dimensional Lumber

The dimensional lumber is cheap and is found in most homes. The strength depends on the wood species lumber dimension and project size. You might need to fasten this grinder with other roof support using 4″ or 3″ nails.

Solid Wood

Some girders are made from solid hardwood, and they are constructed from one tree, making them sturdier. They act as a support for the roof, and some homeowners would go for solid wood due to the aesthetic value.

Differences Between Joist Beam and Girder



It is made of lumber steel, LVL, and engineered lumber to support the roof and floors.

It is made of lumber, LVL, steel, and engineered lumber for better support.

It is made of lumber, steel, LVL, and engineered lumber.


They are smaller and run from one side of the roof to another.

They are the giant beam that supports the house making it compact.

The beam size varies depending on the structure. Larger buildings have larger beams, while smaller ones have small beams.


They act as secondary support to the beams, and they distribute the weight in the roof and the whole building.

It is the main roof support, and some buildings have one, and others have a few girders. They are larger beams that support the other beams and the whole building.

The beams support the horizontal load and distribute roof weight effectively in a structure.

The Point of Support

The joist is attached to the girders, which act as the main point of support.

The girders are attached it the vertical posts in a structure.

They are attached to the other beams, the girder, the vertical posts, and different framing.


The joist are many and are attached to the girder.

Only a few, and some buildings have one or two girders, and the number depends on the roof’s weight.

The number of beams depends on the structure size, but different homes have different beams.


The joist spans the entire roof.

It spans the roof’s length, and extra posts might be needed to support it.

The beams extend the width and length of the roof or floor.


What is the Important Structural Member of a Building?

The beam is the most important structure in a building as it supports the floors, roofs, and walls. The beams lead to effective weight distribution and ensure the structure is compact. It works with the joists and the girder, a type of sturdy beam.

What is a Girder Truss?

The girder truss is steady beams that run through the house’s length. They are made of sturdy materials such as LVL, steel, hardwood, and engineered lumber. The girder truss directs the weight from the roof to the floor through the vertical posts.

Why is Structural Member Between a Joist and Beam Important?

The structural member between a beam and joist is important as the joist are weight distributors attached to the roof’s main support areas. The joists are small, and by themselves, they would not distribute the weight, thus feeding the weight to the beams, which then transfer some of the weight to the girder

Are Beams and Girders Similar?

Yes, the beams and girders are similar, as all girders are beams, but not all are girders. They function; differently, girders are larger beams that direct the roof weight down the vertical posts.

On the other hand, beams collect the weight from the joist and other structures and redirect it to the main beam, the girder, and the walls.

Are Joists Load Bearing?

Yes, joists are load-bearing; the joists also bear all the live load, such as people moving in a home, raking all the weights above the frame, directing it to the girder, beams, and sidewalls. Thus, they ensure the house stays compact and does not crumble under uneven weight distribution.

Can Steel Joists Be Used as Girders?

Yes, the steel joists can be used as girders, especially if they are attached to the vertical posts

Are Trusses and Joists the Same?

The wall trusses sometimes act as joists as they distribute weight evenly in a building.

Can a Girder Be Wood?

Some hardwoods act as girders and are found in old homes, but some people would have them in their homes for aesthetic values.


You should know important parts of the house, especially if you decide to construct a home by yourself or if you decide to conduct repairs. The beams, joists, and girders are important parts of a house as they aid weight distribution and stay compact. You should know each structure’s function, dimension, and location on the roof.

Moreover, you should connect these roof structures with other supporting structures such as vertical posts and walls which aid weight distribution. Good luck learning about the structural features of a home that offer support.