Structural Steel Plans: Penthouse Framing & Joist Designations

Once foundation and footing work is complete, the next step in structural blueprint reading is understanding how the building transitions upward into its elevated framing. A roof and penthouse framing plan shows how structural elements at a higher level tie into the structure below and support the structure above. It also introduces key steel member conventions that appear throughout steel-framed buildings.

What the Low Roof and Penthouse Framing Plan Shows

A plan labeled as a low roof and penthouse floor framing plan focuses on the structural layout at that level of the building. It helps answer a few essential questions:

• What steel members exist at this level, and how are they laid out?
• How does the framing connect back to foundation walls and exterior structural systems?
• Where do beams, joists, and other members transfer loads into the structure below?
• Which areas are framed, and which areas are intentionally left open?

Compared to foundation plans, framing plans usually look much denser because they include many parallel lines. Those lines represent structural members, often a mix of steel joists and wide flange beams.

Recognizing What You Are Looking at

At first glance, roof framing plans typically show:

• Joists, usually drawn as repeating parallel members spanning between beams
• Wide flange beams, typically larger members who joists frame into
• Exterior structural elements such as foundation walls continuing upward as perimeter structure
• Additional member types, including items like HSS members depending on the design

These plans often include supporting schedules and general notes off to the side, but the clearest starting point is usually learning to interpret the member tags on the main framing field.

Understanding “Open” Areas on a Framing Plan

Not every portion of a floor plate will have structure at every level. On framing plans, areas labeled OPEN typically indicate that there is no framing at that particular level in that zone.

This usually happens when a space spans multiple floors vertically such as:

• An atrium that rises two or three stories
• A gymnasium or other large-volume space
• Any area designed as a multi-level open volume

The key takeaway is that “OPEN” does not mean the area has no structure anywhere. It means there is no structure at that specific level. Framing will typically appear at higher levels where the open volume ends and the building resumes floor or roof structure.

Common Steel Member Designations

Two of the most common labels you will see on steel framing plans are K-series joists and W-shape wide flange beams. These designations communicate member type, depth, and structural capacity in standardized shorthand.

K-Series Joists: Example 24K8

When you see a designation like 24K8, you are looking at a steel joist.

What a Steel Joist is

A steel joist is not a solid beam. It is a truss-like member with:

• A top chord
• A bottom chord
• Angled or web members between them

This open web configuration makes joists useful for running systems through the framing zone because there are natural openings between the chords and web members.

How to Read 24K8

• 24 indicates the nominal depth of the joist in inches, measured from the top chord to the bottom chord.
• K indicates the joist series, a standardized category of joists.
• 8 relates to the joist’s chord size and load capacity designation.

These joist designations correspond to standardized tables published by the Steel Joist Institute. You do not need to memorize the tables to recognize what the tag means. The goal is to understand that this is a joist, identify its depth, and communicate the correct joist series and designation to fabricators and installers.

Wide Flange Beams: Example W16x31

When you see a designation like W16x31, you are looking at a wide flange steel beam, often described as the classic I-beam shape.

What a Wide Flange Beam is

A wide flange beam is a solid steel member with:

• A top flange
• A vertical web
• A bottom flange

Unlike a joist, a wide flange beam is solid through the web area. Penetrations can be created in some cases, but the member is not inherently designed with large openings the way a joist is.

How to Read W16x31

• W indicates a wide flange shape.
• 16 indicates the nominal depth in inches, measured from the top flange to the bottom flange.
• 31 indicates the weight in pounds per foot.

So a W16x31 member is approximately 16 inches deep and weighs 31 pounds per foot of length.

Why the Weight Matters

Steel is often purchased, estimated, and tracked by weight, commonly in tons. Understanding the pounds-per-foot portion of the designation allows teams to estimate total steel tonnage by multiplying member length by weight per foot, then summing across the project. This is part of the language used for takeoffs, procurement, and coordination with steel subcontractors.

How Joists and Beams Work Together on the Plan

On a typical roof framing layout, you will often see:

• Multiple joists spanning across the plan in repeating lines
• A larger wide flange beam running perpendicular or centrally, acting as the main support line that the joists frame into

Reading the plan becomes much easier once you recognize the pattern: joists are the repetitive framing members, and wide flange beams are the primary supports that collect those joist loads and transfer them into columns or walls.

With these member conventions in place, you can start interpreting framing plans more confidently and communicate clearly with fabricators and installers using the same standardized terminology found directly on the drawings.