Steel Stair Stringers: Material Thickness Guide

What Controls Stringer Thickness

The thickness of a steel stair stringer is not chosen from a single rule of thumb. It follows from the clear span between supports, the number of steps the stringer carries, the total load including people and furniture, and the deflection the design allows. A short residential staircase and a long public stair with the same width can need very different sections.

Profile shape matters as much as steel thickness. A deeper channel or a closed box section resists bending far better than a thick flat plate of the same weight, so fabricators often compare several options before fixing a value.

Common Profiles and Their Thickness Range

Stair stringers are usually built from channels, angles, flat plates or hollow sections. Each has its own practical thickness range for the sizes typically fabricated in the workshop.

• Channel stringers: web and flange thickness selected from standard section tables.
• Flat plate stringers: thicker plate with stiffening, often cut to a sawtooth or notched profile.
• Box sections: good stiffness for longer spans and exposed visible stringers.

The chosen profile sets the starting point, and the engineer then verifies bending and deflection for the actual geometry.

How Span and Spacing Influence the Choice

Span is the most important variable. As the distance between top and bottom supports grows, bending moment rises quickly, so the required section modulus increases faster than the span itself. Stringer spacing also matters: closer stringers share the load and allow thinner members, while widely spaced stringers each carry more.

When sizing a stair from drawings, check the landing supports and the wall or beam connections, since the assumed support conditions change the calculated bending.

Finishing and Galvanizing Effects

Finishing affects the design more than many people expect. Hot-dip galvanizing adds a coating layer and can slightly change dimensions, and the process requires vent and drain holes to be planned in the steel. Powder coating adds a thin film that does not change structural size but must be applied to clean, properly prepared surfaces.

Thickness should therefore be specified together with the finish, so the fabricator can plan cutting, welding and handling around the coating process.

Practical Sizing Workflow

A reliable workflow starts with a written specification: stair width, rise, tread, total height, support type and intended use. Measure on site where the stair meets the structure, then have the engineer calculate the stringer section for the design load. Fabricators in our workshop at 37 Rival Street, Tel Aviv, work from that approved specification rather than estimating thickness from appearance.

• Record span, rise and support details.
• Confirm the design load with the engineer.
• Approve the section, profile and finish in writing before production.

Checking Against Israeli Railing and Stair Requirements

Stairs and their guarding in Israel are commonly reviewed against the national standard SI 1142, which covers railings and related dimensions. This page does not list the exact values, because they depend on the building type and must be confirmed with the engineer and the local approval process.

Keep the stair geometry and the guard details in the same drawing set so the stringer design, the handrail height and the infill spacing are checked together.

Frequently asked questions

Is there one standard thickness for steel stair stringers?

No. Thickness depends on span, load, profile and spacing, so each staircase needs its own calculation or engineer confirmation.

Does galvanizing change the stringer thickness?

It adds a coating layer that slightly alters dimensions, so the fabricator should account for it, but the structural section is still set by the design calculation.

Is a thicker plate always better?

Not necessarily. A deeper channel or box section can give more stiffness for less weight than a thick flat plate, so profile matters as much as thickness.

Who decides the final stringer size?

The structural engineer confirms the section, and the fabricator builds to that approved specification after site measurement.

Common mistakes to avoid

Choosing thickness by appearance alone, or ignoring the finish and span, often leads to stairs that feel springy or fail inspection. Always base the size on an engineered calculation.

Our recommendation

Send the engineer the span, rise, width, support details and intended load before choosing a profile, and approve the finish at the same time so cutting and galvanizing are planned correctly.

Summary

Steel stair stringer thickness is set by span, load, profile and spacing, then confirmed by an engineer and matched to the chosen finish.

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