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Exterior stairs constructed by D Friedman & Art Cady (C) Daniel Friedman Stair Stringer & Stair Tread Support Hazards

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Stair stringer & stair tread support defects & safety hazards:

This document provides specifications, sketches, photographs, and examples of improperly constructed or otherwise damaged stair and stair tread supports. Our photo at page top shows a wood exterior stair during construction by the author D. Friedman and associate Art Cady.

Because of the tricky uneven ground and the difficulty of accurately measuring rise off of a rough slope, the builders set the downhill stringer in place first, and determined remaining stair construction measurements based on the location of that stringer. Railings and balusters are incomplete in the photo.



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Stair Stringer Defects: structural & safety hazards in stair & stair tread supports

Stair Collapse during renovation (C) Daniel Friedman Stair Collapse during renovation (C) Daniel Friedman

[Click to enlarge any image]

Article contents

Definition of stairway stringer

What is a stair stringer? the stair stringer is the diagonal supporting structure that carries the weight of the stair assembly as well as people using the stairs. In our page top photo you can see the stair stringer that used 2x lumber. Stringers are typically notched to accept stair treads, or treads may be mounded on cleats secured to the stringer interior face.

You can see a stair stringer along the upper-edge of the set of stairs lying on their edge in our photo at above-right, after the stairs fell from the stair opening (our photo at above left) during a building renovation project. The stair stringer is secured to the structure at stair top, stair bottom, stair sides, depending on where there are building surfaces present.

Stair Tread Support Choices & Relation to Stair Stringer Notching

Stairway Lighting requirements (C) Carson Dunlop Associates

  1. Stair treads can be placed directly on top of the horizontal surface of cutouts in the notched stair stringer. 

    This approach is very secure provided that the stringer depth is great enough and the cutout depth shallow enough that the remaining stringer material is strong enough to carry the weight of the stairway and its users (1-? arrow in sketch at left).

    More about too-deep stringer notching is below
    at Deck Stair Stringer Notching Too Deep. [Click to enlarge any photo or sketch]

    Of course if the stairway is supported by securing the stringer to walls on both sides, this cutout depth weakness problem is mitigated or eliminated.
  2. Stair treads can be placed between the stringers and supported by cleats or supported by grooves routed inside the un-notched stringer.

    If the stair treads are set into grooves routed into the stringer, in a 2x stringer the groove depth should be 1/2 the thickness of the stringer (typically 3/4") and additional support by cleats or by gluing and end-screwing through the stringer sides into the treads is good practice.
  3. Stair treads butted against an un-grooved stringer inner face and then nailed or screwed through the stringer into the stair tread.

    Watch out: We do not recommend this stair tread approach as too often we find the treads break away from the stringer causing catastrophic stair tread collapse and perhaps serious injuries. If you encounter a stair built this way you should add glued & screwed cleats to support the treads.
    Also see Unsafe Stair Tread-to-Stringer Connections later in this article.

Common Stair Stringer Defect Photographs

Rotted stair stringer (C) Carson Dunlop Associates

You can see a stair stringer along the upper-edge of the set of stairs lying on their edge in our photo at above-right, after the stairs fell from the stair opening (our photo at above left) during a building renovation project.

The stair stringer is secured to the structure at stair top, stair bottom, stair sides, depending on where there are building surfaces present.

Our photo of a severely rotted stair stringer (above-left) is provided courtesy of Carson Dunlop Associates as is the drawing at below right. Used with permission.

Rotted basement stair (C) Daniel Friedman Stairway Lighting requirements (C) Carson Dunlop Associates

Rotted stair treads or stringers

support added to exterior deck stairs (C) Daniel Friedman

Watch for rot in wooden stairs, especially at exterior entrances and decks. Where the stair stringer is mounted against a building wall water is often trapped, leading to hidden rot and sudden collapse.

Our photo shows rotting basement stair treads; from the stair top we could see that the entire stairway was twisting and in danger of sudden collapse. The sketch, courtesy Carson Dunlop, predicts stair rot exactly where it's found in our photograph.

This exterior stair ascending to a deck used a stringer pair that was so bouncy the builder decided to retrofit additional support at mid-span of the stair stringer.

A post secured to a pier and to the stringer side on each side of the stairway may have been a stronger solution.

We like to set posts at mid span as well as at the bottom landing quite securely to prevent unsettling movement when descending or ascending a tall exterior wood-framed stair such as this one.

Notice also the obsolete side railings which are not child-safe.

Stair Stringer Movement Hazards

Stairway Lighting requirements (C) Carson Dunlop Associates

Stair stringer movement, as we cited just above, can lead to separation of the connection between treads and the stringer, leading to stair collapse.

Examine the stringers for bowing and look for gaps at the ends of treads - but beware: separation and gaps may be hidden if the stair treads were set into groove routed into the stringer.

Sketch courtesy Carson Dunlop Associates.

Unsafe Stair Tread-to-Stringer Connections

Stairway Lighting requirements (C) Carson Dunlop Associates

Unsafe connections between stair treads and stringers  is also often hidden: check for:

Sketch courtesy Carson Dunlop Associates.

Also see Stair Tread Support Choices & Relation to Stair Stringer Notching at the top of this article.

Deck Stair Stringer Notching Too Deep

Unsafe exterior stair stringer and riser height (C) Daniel Friedman

The stair stringer is too-deeply notched and could split by the simple effect of weight of a user walking on these steps. Other defects in this horrible deck step construction include:

Stair Tread Support Choices

This subtopic has been expanded and moved to STAIR STRINGER SPECIFICATIONS

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