What Size Beam Do I Need To Span 20 Feet?

Nathan Clark | 🗓️Modified: March 3, 2024 | ⏳Time to read:12 min

Choosing the right beam size for your construction project is a crucial step in ensuring the structural integrity of your building or structure. Factors such as load-bearing capacity, material type, and design considerations all play a role in determining the appropriate beam size. In this comprehensive guide, we’ll explore the key aspects of selecting the right beam size for your project, with a particular focus on residential construction.

The role of beam

A beam is a strong part of a building that holds up the roof. It helps spread out the weight of things on top of the building, like people or furniture. Beams are like long sticks that go across the top of the building.

Beams also help move the weight down to the ground. They do this by connecting to big columns in the building. The size of the beam depends on how far apart the supports are.

It’s important to pick the right size beam so it can handle all the weight properly. There are different types of beams, like ones that stick out on their own (cantilever), ones that have supports at both ends (simply supported), and others.

When you’re making changes to a building, it’s really important to make sure the beams are strong enough. For example, if you want to make a room bigger, you need to add more support to the beams.

For a 20-foot space, you need a beam that’s at least 18 inches deep if it’s made of wood. Different types of beams have different jobs. Some are really strong and don’t move much (like cantilever beams), while others move a bit (like simply supported beams).

Making sure you have the right size beams for the job is super important for keeping the building safe and strong!

Size of Beam for 20-foot Span

When we build roofs, we need to make sure the beams, which are like strong sticks, are just the right size. Beams are really important because they help hold up the columns and other parts of the building.

They also help protect the building from bad weather like storms or strong winds. Beams have different parts: the top part deals with pressure, while the bottom part deals with pulling.

RCC Concrete Beam

For a certain type of beam called RCC Concrete Beam, we usually use a beam that’s about 9 inches wide and 9 inches deep. The size of the beam might change depending on how long the span is or how much weight it has to hold.

We follow some rules to figure out the size of the beam. One rule says that for a 20-foot span, the depth of the beam should be about 12 inches. Then we need to add some extra for the bars inside the beam and some cover on the outside.

Continuous Beam

For another type of beam called Continuous Beam, which is even stronger, we might use a beam that’s 9 inches wide and 12 inches deep for a 20-foot span. Again, we follow some rules to figure out the size.

These rules help us make sure the beams are just right for keeping buildings safe and strong.

Thumb Rule Method for Size of Beam Need for Span 20 Feet:

When determining the appropriate size of a beam for a 20-foot span using the Simply Supported beam method, we apply the following calculation:


  • Span/Effective Depth ratio for Simply Supported Beam = 20
  • Span = 20 feet
  • Diameter of Bar = 16mm
  • Clear cover = 1 inch

We can express the calculation as follows:

  1. Effective Depth (D_eff) = (Span × 12 inches) / 20 = 12 inches
  2. Total Overall Depth (D_total) = Effective Depth (D_eff) + Clear cover + (1/2 × Diameter of Bar) D_total = 12 inches + 1 inch + (0.3 inches) = 13.3 inches

Rounding up to the nearest whole number, the beam’s depth would be 15 inches.

  1. Beam Width (W) = D_total / 1.5 W = 15 inches / 1.5 = 10 inches

Thus, the final result indicates a beam depth of 12 inches.

Continuous Beam

When determining the size of a Continuous Beam for a 20-foot span, we use a specific ratio and follow calculations to ensure stability. Here’s how it’s done:


  • Continuous Beam Span to Depth Ratio = 26
  • Span = 20 feet
  • Diameter of Bar = 16mm
  • Clear cover = 1 inch

We can express the calculation as follows:

  1. Effective Depth (D_eff) = (Span × 12 inches) / 26 = 9.23 inches
  2. Total Depth (D_total) = Effective Depth (D_eff) + Clear cover + (1/2 × Diameter of Bar) D_total = 9.23 inches + 1 inch + (0.3 inches) = 10.53 inches

Rounding up to the nearest whole number, the beam’s depth would be 12 inches.

  1. Beam Width (W) = D_total / 1.5 W = 12 inches / 1.5 = 8 inches

Thus, the beam width is considered as 9 inches.

For a continuous beam in a 2 to 3 storey residential building, with a 20-foot span, a beam of size 9 inches by 12 inches is typically used. This allows for easier placement of reinforcement bars, such as 12mm bars at the top and 16mm bars at the bottom.

Sizing of Beam & Wood Strength

No matter what type of material is used, it’s important that it’s strong enough to handle pressure, resistance to being cut, and overall strength. There are different factors that help decide how long and how big a beam should be.

These factors depend on how well engineered and sturdy the wood beams are.

The calculations use many different things like the type of wood, its quality, size, the weight it needs to support, how much it can bend without breaking, and more.

Supporting Weight – According to the International Residential Codes, beams or headers must have at least 1-1/2 inches of support underneath. If it’s longer than 5 feet, it needs to have double support.

Space for Support – Sometimes, more space is needed for support, which makes the structure stronger and safer.

Securing Together – Beams with more than one layer need to be securely fastened with nails or bolts. The International Residential Codes say they should be nailed at least every 32 inches apart in a staggered pattern, using nails that are at least 3 inches long and 0.120 inches thick. Using thicker nails like 3-1/4 inches long and 0.131 inches thick across a laminate of 4 feet provides even better results.

For Beam Required to Span 12 Feet:

When figuring out the right beam size for a 12-foot span, there are several methods to consider. If the span is exactly 12 feet without any overhang, a double-ply beam is often effective, spanning the equivalent of its depth in inches.

For instance, if a 2×12 beam is used, it can span 12 feet, while a 2×10 beam can span up to 10 feet. Consulting experts for this analysis is crucial, ensuring all aspects are considered and saving considerable time in the process.

It’s also important to calculate the total load per foot that the beams will need to support.

Lumber Size of the Beam for a 20-Foot Span: Max. Live Load: 60 lbs/ft² (2873 N/m²)

Maximum Span (ft – in)Nominal Size (inches)Joist Spacing Center to Center (inches)Lumber Grade2 x 122413′ – 2″2 x 141220′ – 10″1618′ – 0″The formula for the section modulus of the beam is based on the width multiplied by the square of the beam depth, divided by 6. Typically, two standard 2-by-6 beams have actual dimensions of 1.5 by 5.5 inches. A double 2×12 beam can span 12 feet, while two 2×10 beams could span 10 feet.

Determining Wood Beam Size for a 20-Foot Span

When you’re faced with a 20-foot span, the size of the wood beam becomes a critical factor. For spans exceeding 20 feet, it’s essential to use a beam that is at least 18 inches deep for optimal structural support. There are online calculators available to help you determine the required beam size for specific spans.

As a general guideline, for a 20-foot span, you should opt for a 4-2×16 or an 8×16 wood beam. In other words, a 2×16 wooden beam with four nails or a 4×16 wooden beam with eight nails can effectively span 20 feet. These beams should have a minimum depth of 16 inches and a width of 8 inches, making them suitable for residential construction projects.

If you choose to use multiple-ply wood for your beams, ensure that they are installed plumb and tightly together. This ensures they can safely bear the intended load.

Load-Bearing Beams vs. Decorative Beams

Traditionally, beams served a load-bearing role in supporting the structure of a building. However, over time, beams have also become a design element in architectural projects. Some beams today are purely decorative and don’t serve a structural load-bearing purpose. These decorative beams are often thinner and more affordable than their load-bearing counterparts.

For load-bearing purposes, beams are typically concealed within the structure, hidden from view. The structural integrity of these load-bearing beams is essential for the safety and stability of the building. On the other hand, decorative beams, while adding aesthetic value, are not intended to carry structural loads.

Faux beams made from 1-inch-thick boards can mimic the appearance of load-bearing beams when assembled together. They are easy to design and install to achieve various ceiling beam layouts.

Understanding Different Loads on Beams

In the world of construction, it’s essential to understand the different types of loads that beams may encounter. Two primary load categories are interior loads and exterior loads, and the placement of the beam within the structure determines the load it will bear.

Interior Beams: Interior beams commonly support the load of the roof. In most cases, the roof load is included in the calculation of interior beams. For guidance on specific load calculations, it’s advisable to consult an engineer or supplier. Generally, most applications adhere to the guidelines outlined in the International Residential Code (IRC).

Exterior Beams: Beams located on exterior walls, especially those supporting clear span trusses, carry a different load compared to interior beams. For example, in a 24′ x 24′ building with trusses and a roof load of 30 lbs. of snow, the load on the exterior wall will be roughly half that of an interior wall due to the absence of storage in the ceiling. It’s crucial to select the load calculations that apply to your particular construction scenario, as most calculators provide options to account for these variations.

Using a Beam Calculator

To calculate the number and size of beams required for a specific span, you can rely on a beam calculator. However, accurate calculations hinge on knowing the material to be used, such as LVL (Laminated Veneer Lumber) or No. 2 pine. It’s important to cross-check your calculations with span charts and remember that calculated sizes are based on even distribution of loads.

The beam calculator typically prompts you to provide information about whether the beam is located within or outside a wall, the presence of trusses, and other relevant details such as snow load, storage in ceilings, and second-floor considerations. By combining this information with the span and width of the loaded area, you can accurately determine the size of the beam required for your project.

Columns and Decking for Flooring

In addition to beams, columns and decking are essential components in the construction of flooring systems. The size of columns is determined by the floor area they support. You can calculate column size by dividing the area of the flooring by the distance between adjacent columns and the main columns.

For instance, an 8×8 column can support approximately 3,000 square feet of space, while a 12×12 column can support 6,000 square feet. Adding two inches to the column size provides an additional 1,500 square feet of column support.

When it comes to decking and joists, they may be used in place of beams and girders. Sheer studs, often used in metal decking, have a 2-inch depth for spans up to 15 feet and a 3-inch depth for spans up to 10 feet. Increasing the depth by an inch can extend the span by approximately 5 feet. To enhance stability, several inches of concrete may need to be poured on top of the floor.

Practical Tips for Beam Selection and Installation

Ensuring the correct selection and installation of beams is crucial for the structural integrity of your building. Here are some practical tips to consider:

  1. Minimum Beam Size: Professionals typically recommend using a beam no smaller than a two-ply 2×8. Meeting this minimum size provides necessary strength for various applications, including openings around interior doors.
  2. Consult Professionals: If you lack experience in beam selection and installation, it’s advisable to seek the expertise of professionals. Incorrect calculations or installation can have serious consequences for the structural integrity of your building.
  3. Fastening: When using two- or three-ply beams, ensure they are securely fastened together with bolts or nails. The International Residential Codes recommend a minimum of 32″ on-center staggered pattern with a minimum 3″ x .120″ nail. Alternatively, a 3-1/4″ x .131″ groove shank nail every four feet across the laminate is a reliable choice.
  4. Bearing Requirements: Follow the International Residential Code’s guidelines, which specify that any joist, beam, or header must have a bearing of at least 1-1/2″. For individuals 5′ or taller, it’s advisable to double this amount. In cases of longer spans, additional bearing space may be necessary, and online bearing tables can serve as a reference.

Estimating Beam Span for 12 Feet

To estimate the beam span for joists spanning 12 feet without overhang, a straightforward rule can be applied. The joist span, in feet, can be determined by 1.5 times the depth in inches. For example, a 2×8 beam can span 12 feet without requiring additional support. A 2×10 beam can span 15 feet, a 2×12 can span up to 18 feet, and so forth. Keep in mind that larger decks typically necessitate larger joists.

Final Thoughts

Selecting the right beam size for your construction project is a critical aspect of ensuring structural stability and safety. If you have any doubts or lack experience in this area, it’s always wise to consult with a professional engineer or contractor. Their expertise, knowledge, and experience can ensure that the job is carried out correctly and safely.

About Nathan Clark

Nathan Clark is a seasoned professional framing expert boasting an extensive career spanning over two decades. With over 20 years of hands-on experience in the field, Nathan has established himself as a trusted authority in the realm of framing and carpentry. His unwavering commitment to craftsmanship and attention to detail have earned him widespread acclaim among clients and peers alike. Besides work, he loves exploring places, traveling, and fishing.

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