It’s really simple: when the tread is less than 11 inches deep, you have to add a nosing to extend it slightly past the front face of the riser below it, which allows enough additional surface to solidly plant most of your foot on the stair. If the tread is too short and there is no nosing, then you will be walking up the steps with your heel—which is where most of your weight is normally transferred to the floor—sticking over the back of the tread, like in the photo below. This is dangerous and makes it easier to lose your balance.

    Here’s how the International Residential Code (IRC) and Florida Building Code (FBC - R311.7.5.3) defines the requirement:

A nosing not less than 3/4 inch (19 mm) but not more than 1-1/4 inches (32 mm) shall be provided on stairways with solid risers. The greatest nosing projection shall not exceed the smallest nosing projection by more than 3/8 inch (9.5 mm) between two stories, including the nosing at the level of floors and landings. Beveling of nosings shall not exceed 1/2  inch (12.7 mm).

Exception: A nosing is not required where the tread depth is a minimum of 11 inches (279 mm). 

So, since the minimum tread size is 10”, a nosing will extend the tread depth to between 10-3/4” and 11-1/4”. However, extending the nosing too far, which was sometimes done on excessively steep stairs of old houses, creates a protrusion that is a trip hazard when ascending or descending the stairs. The stairs in the photo below had a 2-1/2” nosing and were truly awkward–and also dangerous–to use, especially when descending the stairs. 
 

   A senior homebuyer changed his mind about purchasing the house after walking up and down these stairs several times. We inspected the house again for a younger couple that decided they could tolerate it. But you also have to consider that, while you may become accustomed to climbing poorly designed stairs like this on a daily basis, a houseguest encountering them for the first time in your home may not be so lucky.

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Here’s links to a collection of our other blog posts about STAIRS:

• What is the minimum headroom clearance for stairs?

• Is a landing always required at the top and bottom of stairs? 

• When is a railing required at stairs?

• What is the building code for the minimum height of stair steps (risers)? 

• What is the building code requirement for receptacle outlets at stairs and stair landings?  

• Are open stair risers acceptable?

• What is the steepest residential stair allowed?

• Why is a single step dangerous in a house?

• Do I need stairs at all exit doors from a mobile home? 

• The stairs feel too steep. What's the building code? 

• What do home inspectors check when inspecting stairs?

• What is the longest stair run allowed? 

• What is the lighting requirement for stairs?

• A light is required over a stair after how many steps/risers? 

• When is safety glass required for windows at stairs and stair landings?

    Visit our STAIRS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.