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Modern stair tread code interpretation

S

SMA

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May 11, 2018
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4
Location
NY
Hi all,

We're having a code disagreement in our office I was hoping you could weigh in on. Does the stair design in the image attached meet the IRC 2021 (Connecticut)? The intention is for the stair to have a modern design, with no squared off nosing projection, with a tread depth less than 11".

The debate centers around whether the radius of the tread (R311.7.5.3 Nosings) and/or the slope of the riser (R311.7.5.1 Risers) negate the need for a nosing projection?
  • One person says "If you have a nosing, you meet code. Why is there an exception for a slanted riser if you are required to have a nosing in all cases. The slanted riser becomes the nosing."
  • The other person says "the nosing projection requirement is completely separate from the riser requirements and a nosing is not achieved by a slope or radius. It’s just a slope or radius, not a projection. Adding a slope doesn’t negate the need for the projection. Making the tread 11” is the only exception that absolves you from needing the nosing projection. The code also states that the riser is from the underside of the nosing, meaning a nosing would be present."

Thanks in advance for your help.


Stair code question.jpg
 

2022 CT Res. Building Code (Amended 2021 IRC)​

R311.7.5.1 Risers​

The riser height shall be not more than 8 1/4 inches (209.5 mm). The riser shall be measured vertically between leading edges of adjacent treads. The greatest riser height within any flight of stairs shall not exceed the smallest by more than 3/8 inch (9.5 mm). Risers shall be vertical or sloped from the underside of the nosing of the tread above at an angle not more than 30 degrees (0.51 rad) from the vertical. Open risers are permitted provided that the opening between treads does not permit the passage of a 4-inch-diameter(102 mm) sphere.
Exceptions:
  1. The maximum riser height of existing stairs serving existing unfinished attics or existing unfinished basements being converted to habitable space or replacement stairs where the pitch or slope cannot be reduced because of existing construction shall be 9 inches (229 mm), measured in accordance with Section R311.7.5.1.
  2. The opening between adjacent treads is not limited on spiral stairways.
  3. The riser height of spiral stairways shall be in accordance with Section R311.7.10.1.

R311.7.5.2 Treads​

The minimum tread depth shall be 9 inches (229 mm). The tread depth shall be measured horizontally between the vertical planes of the foremost projection of adjacent treads and at a right angle to the tread's leading edge. The greatest tread depth within any flight of stairs shall not exceed the smallest by more than 3/8 inch (9.5 mm).
Exception: The minimum tread depth of existing stairs serving existing unfinished attics or existing unfinished basements being converted to habitable space or replacement stairs within existing dwellings shall be 8 inches (203 mm), measured in accordance with Section R311.7.5.2.​

R311.7.5.3 Nosings​

Nosings at treads, landings and floors of stairways shall have a radius of curvature at the nosing not greater than 9/16 inch (14 mm) or a bevel not greater than 1/2 inch (12.7 mm). A nosing projection not less than 3/4 inch (19 mm) and not more than 11/4 inches (32 mm) shall be provided on stairways. The greatest nosing projection shall not exceed the smallest nosing projection by more than 3/8 inch (9.5 mm) within a stairway.
Exception: A nosing projection is not required where the tread depth is not less than 11 inches (279 mm).​
 
classicT said:
A nosing projection not less than 3/4 inch (19 mm) and not more than 11/4 inches (32 mm) shall be provided on stairways.
Click to expand...
The difficulty in the OP arises because R311.7.5.3 does not define how to measure the "nosing projection." Note that "nosing" is defined in Chapter 1 as "The leading edge of treads of stairs and of landings at the top of stairway flights," so one of the two points to use in the measurement is specified, but the other point is not.

This is in contrast to R311.7.5.1 and R311.7.5.2, which clearly define how to measure riser height and tread depth. Which are 7-1/2" and 9", respectively, in the diagram in the OP.

It is clear that R311.7.5.3 does require a "nosing projection" between 3/4" and 1-1/4", as the tread depth is less than 11". So the only hope for considering the profile in the OP to be compliant is to say that as the nosing projection measurement is not specified, you have the leeway to choose where to measure it from. If you choose to measure it at the point on the riser that is 2-1/2" below the surface of the tread above, then you'll get a nosing projection of about 0.96", which would be compliant.

Doing that is arbitrary for the profile in the OP. You could, of course, alter the profile slightly to introduce some additional curve in the correct location and use that change in shape as a less arbitrary justification for where to measure the nosing projection to.

Cheers, Wayne
 
So Question?

If I overlay the maximum nosing projection in R311.7.5.3 Nosing of 1-1/4" I fall under the position that the required nosing for a full slant does not comply.

The fact of the matter is the restricted depth is to stop foot draggers who ascend from getting their foot caught on closed risers. However, the IRC allows open risers with a 4" sphere rule, so the foot could slide forward, just no place to rest past the maximum 1.25". So the foot dragger loses the stop option.

Contrary to what most people think the nosing is not there for ascending it is there for descending on shorter treads, under 11-inches, so the heal of the shoe can rotate under further. Thus, the reason the requirement is removed once you reach a minimum tread depth of 11-inches.
1702945357646.png
 
If you create a little more of a nosing but still slant back … would that be a workable compromise? (That’s supposed to be a continuous radius).
 

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e hilton said:
If you create a little more of a nosing but still slant back … would that be a workable compromise? (That’s supposed to be a continuous radius).
Click to expand...

Why is a "workable compromise" even necessary when the stair shown in the original; post isn't a code violation?
 
YC,

So here is the question that you are saying does not exist and note as compliant:

Definition of a nosing CT IRC: Chapter #2: "NOSING. The leading edge of treads of stairs and of landings at the top of stairway flights."

So, per the definition the forward most point within the 9/16" radius is the leading edge and the same goes for the 1/2" bevel.
  • A nosing is required for a tread with a depth of less than 11-inches.
    • The OP's post is 9-inches so a nosing projection is required.
    • The nosing is required to be a minimum of 3/4" to a maximum of 1-1/4"
    • In the diagram in your opinion where is the 3/4" - 1-1/4" range defined with start and stop points?
  • With the radius of the nosing being 9/16" the transition point from the radius to the slant begins prior to the 3/4" minimum being met and continues past the maximum 1-1/4" point when measured from the leading edge.
    • So for a tread under 11-inches please explain your defined start and stop points for the required nosing measurements to be taken at?
 
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All I can tell you is that the nosing profile shown in the opening post is specifically shown in the 2021 IBC Commentary. The only part about the stair shown in the OP is that the nosing overhang appears to exceed 1-1/2".

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I believe I agree with Tom in that the nosing can be the first 3/4" and then it is a sloped riser face beyond that.....Remember, the nosing and the sloped risers are separate code sections even though they are the same feature at times....The language probably needs to be clarified and maybe homogenized between IBC and IRC to help with the confusion....

1702998439015.png
 
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Yankee Chronicler said:
All I can tell you is that the nosing profile shown in the opening post is specifically shown in the 2021 IBC Commentary.
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In the diagram you posted, the nosing projection is measured in both cases to the rear-most point of the riser face. [While this is a natural choice, it is not actually specified in the IRC.] Applying that methodology to the profile in the OP gives a riser projection of about 3-3/4". So I don't think that diagram helps support the idea the OP's profile is compliant.

Cheers, Wayne
 
Yankee Chronicler said:
All I can tell you is that the nosing profile shown in the opening post is specifically shown in the 2021 IBC Commentary. The only part about the stair shown in the OP is that the nosing overhang appears to exceed 1-1/2".

View attachment 12331
Click to expand...
Yankee,

Look at what you posted it is not the same.

What you posted notes the 1.25" maximum, and notes 30Deg is the max slope. But if you actual measured that diagram you would see that its not 30 degrees.

it is only 10.29 degrees aprox...

1703007367811.png
 
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