Steel Joist Types: K-Series, LH, DLH, and More

Steel joist types are categorized by their span and load capacity. The main types are K-Series, LH-Series, DLH-Series, and KCS-Series, each used for specific building needs. Specialty joists like bowstring, scissor, and gable are used for unique roof shapes and architectural designs.

If you're trying to decide which joist fits your project, this guide will help you compare dimensions, spans, and uses — without getting buried in engineering jargon.

Steel joists are crucial structural elements in construction, used to support floors and roofs in buildings. Understanding the different types of steel joists, their spans, dimensions, and load capacities is essential for architects, engineers, and builders in Australia, the UK, and worldwide.

This guide explains everything about steel joists, including K-series, LH-series, DLH-series, and specialty joists, and provides charts, tables, and step-by-step guidance to help you select the right joist for your project.

Key Takeaways

• K-Series joists are the most common for short to medium spans (up to 60 ft).
• LH-Series handles long spans (up to 96 ft).
• DLH-Series is best for extra-long clear spans (up to 240 ft).
• KCS joists handle uneven or concentrated loads.
• Specialty joists (like bowstring or scissor) are for architectural or sloped-roof designs.
• Use a Steel Beam Calculator or Steel Deck Calculator to estimate load and span easily.

What Is a Steel Joist?

A steel joist is a lightweight, open-web truss used to support floors and roofs. It’s made of steel angles, bars, or rods welded together, forming a pattern that spreads loads evenly. These joists act as bridges between beams or girders, providing strong support while keeping the structure light.

When I first saw one being installed on a site, I thought, “It looks too light to hold up a roof.” But that’s the magic of steel — it’s strong without being bulky.

Common materials:

• Hot-rolled steel
• Cold-formed steel
• Structural tubing for chords and webs

They’re common in commercial, industrial, and warehouse buildings, and increasingly in modern residential construction where long, open spaces are needed.

Types of Steel Joists (Main Series)

Steel joists are grouped by series — based on how far they span and the type of loads they carry.

Here’s a quick comparison before we dig in:

K-Series Joists

The K-Series is the most common type of steel joist. It’s designed for short to medium spans — usually up to 60 feet — and can carry uniform loads across its length.

If you’re building a warehouse, office, or retail space, this is likely what you’ll use.

Pros:

• Cost-effective
• Easy to design and install
• Works for most commercial roofs

Cons:

• Not ideal for uneven or concentrated loads

I once worked with a contractor who used K-Series joists for a small factory floor. They were light, quick to set up, and surprisingly strong. But when they later added rooftop AC units, they realized they needed something more flexible — which is when the KCS-Series came in.

KCS-Series Joists

KCS (K Constant Shear) joists are modified K-Series designed for non-uniform loads — like when heavy equipment or ducts are positioned unevenly.

Pros:

• Can handle heavy point loads
• Great for mechanical equipment or rooftop installations
• Flexible design

Cons:

• Slightly more expensive than standard K-Series

If you’re designing for rooftop units, solar panels, or mechanical platforms, this type will save you a lot of headaches later.

When I tried using standard K joists for a rooftop chiller (big mistake), the vibration caused stress at one end. A switch to KCS solved it instantly.

LH-Series Joists (Long Span)

LH-Series joists are used for spans up to 96 feet and are perfect for large, open areas like gyms or auditoriums.

They’re deeper — typically 18 to 48 inches — and can carry higher loads across longer distances.

Pros:

• Fewer supports needed
• Ideal for large clear spans
• Great balance between weight and strength

Cons:

• Requires deeper ceilings
• Slightly more costly than K-Series

If you’ve ever stood in a gym and looked up, those long, steel patterns forming the roof structure are usually LH joists. They make spaces feel open and uncluttered — no columns getting in the way.

DLH-Series Joists (Deep Long Span)

DLH-Series joists are the strongest and deepest type, with spans reaching up to 240 feet. These are the giants used in stadiums, aircraft hangars, and arenas.

Pros:

• Handles extreme spans
• Creates huge open interiors
• Very high load capacity

Cons:

• Requires precise engineering
• Heavy and complex to install

These joists can look intimidating — they’re massive. Once, at a hangar built, we had a DLH span that stretched like a bridge over the main floor. Standing under it, you could feel the sheer scale.

Specialty Joists

Besides the main series, there are specialty types for unique architectural or roof designs.

1. Bowstring Joists

Bowstring joists have an arched top chord and a straight bottom chord. They’re often used in old gymnasiums and hangars.

Pros: Distinctive curved look. Cons: Needs careful design for the arch

I’ve always loved how they look — that gentle curve adds personality to what’s usually a plain roof.

2. Scissor Joists

Scissor joists have double-pitched chords that meet at the ridge, creating a vaulted ceiling.

Pros: Beautiful vaulted interior.r Cons: Harder to design and install

If you’re designing a chapel or a hall and want that open, airy feel, scissor joists make it possible without heavy beams.

3. Gable or Double-Pitched Joists

Gable joists have a sloped top chord with a flat bottom, giving a pitched roof look.

Pros: Perfect for sloped roofs. Cons: Slightly more complex support connections

They’re common in regions with rain or snow, like the UK or Australia, where water drainage is essential.

4. Arched Chord Joists

Arched chord joists use cold-rolled steel shaped into a smooth curve.

Pros: Elegant design. Cons: More costly to fabricate

You’ll see these in buildings where style matters as much as function — think atriums or curved roof sections.

Steel Joist Design Considerations

When designing lightweight steel joists, keep these checks in mind:

1. Bending: Ensure the joist can handle the applied loads without excessive bending.
2. Shear: Verify shear stress does not exceed allowable limits.
3. Deflection: Check deflection against standards; use the manufacturer’s simplified tables.
4. Bearing: Ensure support points can safely carry the load.

A common pain point is miscalculating deflection. I always use a Steel Joist Calculator—it’s faster and reduces errors.

Using Load Tables

Manufacturer load tables simplify selection. They provide:

• Joist depth
• Load capacity
• Maximum span

Example Table: K-Series Joist Selection

Tip: Always choose a joist that exceeds your load requirement for safety.

Deflection Calculation

Deflection is the amount a joist bends under load. Manufacturer tables often include a deflection number, simplifying your design.

Step-by-step:

1. Find your joist designation in the load table.
2. Check the corresponding deflection number (e.g., L/240).
3. Compare with the allowed deflection for your structure.

Practical note: I once ignored the L/360 recommendation and regretted it—the floor felt a bit springy. Lesson learned: don’t cut corners.

Steel Joist Dimensions and Span Chart

Here’s a simple span vs. depth chart you can reference when choosing joists:

Lightweight Steel Joist Tips

1. Double-check your series: Don’t oversize or undersize—it affects cost and safety.
2. Use calculators: Saves time and reduces errors.
3. Check deflection and shear: Small mistakes can lead to long-term problems.
4. Match span with load: K-series is not a DLH-series substitute.
5. Plan handling and installation: Lighter joists are easier but still require safety precautions.

Lightweight Steel Joist Design Example & Chart

Step 1: Define Project Requirements

Step 2: Select Joist Series

• K-Series: Up to 60 ft, light load ✅
• LH-Series: Up to 96 ft, heavier load ❌ (overkill)
• DLH-Series: Up to 240 ft ❌
• CJ-Series: Composite ❌ (no concrete slab here)

Decision: Use a K-Series joist for this 45 ft span.

Step 3: Check Manufacturer Load Table

Decision: Use an 18-inch depth K-series joist for a 45 ft span and a 100 psf load.

Step 4: Verify Key Design Checks

Tip: Use a Steel Joist Calculator online to automate these checks. It ensures quick, accurate verification.

Step 5: Optional Calculator Use

• Steel Joist Calculator: Input span, load, series, and joist depth.
• Truss Calculator: Useful if joists form part of the roof systems.

Example: Input span = 45 ft, load = 100 psf, series = K → Output = 18-inch depth joist.

Step 6: Summary Chart for Quick Reference

How to Use This Chart

1. Identify your project span.
2. Determine load type (light, medium, heavy).
3. Select a joist series that fits the span and load.
4. Pick the joist depth from the table to satisfy the load and deflection.
5. Verify bending, shear, deflection, and bearing using tables or a calculator.

What Is the Strongest Joist?

Among all, the DLH-Series is the strongest steel joist, designed for extreme spans and loads. However, “strongest” depends on what you need — using DLH for a small shop would be overkill.

For standard commercial buildings, LH-Series offers the best mix of span, cost, and strength.

Common Questions About Steel Joists

1. How Thick Is a Steel Joist?

Most have chord thicknesses between ⅛ inch and ½ inch, depending on span and load.

2. What Is the Minimum Bearing for Steel Joists?

Usually 2½ inches, but it can vary by series. Always check manufacturer tables or use a Steel Beam Calculator.

3. What Size Are Standard Joists?

Standard K-Series joists range from 8 to 30 inches deep and up to 60 feet long.

4. What Is the Full Form of LH Joist?

LH = Longspan Joist. Designed for long-span roofing or flooring.

5. What Is an MS Joist?

It’s a Mild Steel joist, a solid beam section often used instead of an open-web joist for smaller spans.

Parts of a Steel Joist

A steel joist has three main parts:

• Top Chord: Supports the deck or floor system.
• Bottom Chord: Carries tension and ties the joist together.
• Web Members: The diagonal or vertical rods connecting both chords.

Think of it like a truss bridge flipped upside down — simple geometry creating massive strength.

Steel Joists vs. Beams and Rafters

If you’re ever unsure, compare using a Steel Beam Calculator — it shows whether a joist or beam makes more sense for your load and span.

Where Are Steel Joists Used?

• Commercial buildings
• Warehouses
• Schools
• Airports
• Gymnasiums
• Shopping malls

In Australia and the UK, joist design standards may vary slightly, but the K, LH, and DLH Series classification is widely recognized internationally.

What Are the Three Major Series of Steel Joists?

The K-Series, LH-Series, and DLH-Series form the three main categories used in open-web joist construction. Each type serves a specific structural need — short span, long span, or deep long span.

Steel Joist Span Tables

Here’s a simplified version for quick reference (values are approximate):

Choosing the Right Steel Joist for Your Project

When picking a joist, consider:

1. Span length
2. Load type (uniform or point load)
3. Roof or floor system
4. Architectural design
5. Budget

For everyday projects, a K or LH joist usually works fine. For massive roofs or stadiums, go for DLH.

If your design includes heavy rooftop gear or uneven loads, definitely KCS.

Try These Tools

Before finalizing your design:

• ✅ Steel Beam Calculator – estimate beam and joist capacity.
• ✅ Steel Deck Calculator – find deck span, thickness, and load ratings.

They’ll save you from guesswork (and possibly a few sleepless nights).

Final Thoughts

Choosing the right steel joist isn’t just about strength — it’s about balance. You want the right mix of weight, cost, and performance.

When I worked on a school gym project, we went with LH-Series joists because they offered the perfect middle ground. They gave the structure that wide-open feeling without blowing the budget.

So whether you’re designing a small workshop or a giant sports arena, remember: The right joist makes the whole structure stronger, safer, and simpler.