Stairs for Sleeping Loft - 2024 IRC 315

[BN4537]

Given:​

R 315.5.2.2
Risers for stairs providing egress from a sleeping loft shall be not less than 7 inches (178 mm) and not more than 12 inches (305 mm) in height. Tread depth and riser height shall be calculated in accordance with one of the following formulas:

1. The tread depth shall be 20 inches (508 mm) minus four-thirds of the riser height.
2. The riser height shall be 15 inches (381 mm) minus three-fourths of the tread depth.

Let's assume you are on inspection and you encounter a sleeping loft served by a stair with 7" risers. You do calculation 1 and come up with a tread depth of 10.69". How does one enforce tread depth? Is 10.69 that an absolute number, so I will need to carry a calculator and an extra-large set of digital calipers on final inspections? Is 10.69" a maximum? A minimum? Is it an absolute number with a tolerance? If so, what is the tolerance intended to be?

My understanding is that we are limiting the size of sleeping lofts and requiring a steeper, more uncomfortable stairs (or ladders) to serve the sleeping lofts, to allow a narrower egress width without encouraging the use of that space for normal purposes as we would a normal "habitable" room with a full-sized means of egress. I'm guessing for that reason we do not want the treads getting bigger than allowed by the ratio? Smaller doesn't really make sense either... Any help would be appreciated.

 

[Paul Sweet]

"R 315.5.2 says Stairways providing egress from sleeping lofts shall be permitted to comply with sections R315.2.1 through R315.2.3." (my emphasis added)

I don't see why stairs that comply with the normal riser & tread sizes shouldn't be permitted. I think that normal construction tolerances would let you round to the nearest 1/2" or 1". I don't think somebody is going to trip if it's a couple hundredths of an inch off.

 

[BN4537]

be permitted to

I missed that part, thanks.

 

[Yankee Chronicler]

Given:​

R 315.5.2.2
Risers for stairs providing egress from a sleeping loft shall be not less than 7 inches (178 mm) and not more than 12 inches (305 mm) in height. Tread depth and riser height shall be calculated in accordance with one of the following formulas:

1. The tread depth shall be 20 inches (508 mm) minus four-thirds of the riser height.
2. The riser height shall be 15 inches (381 mm) minus three-fourths of the tread depth.

Those are two very weird formulas. Neither of them is anything like the stair formulas that I remember from Architectural Graphic Standards from many years ago.

Let's assume you are on inspection and you encounter a sleeping loft served by a stair with 7" risers. You do calculation 1 and come up with a tread depth of 10.69". How does one enforce tread depth? Is 10.69 that an absolute number, so I will need to carry a calculator and an extra-large set of digital calipers on final inspections? Is 10.69" a maximum? A minimum? Is it an absolute number with a tolerance? If so, what is the tolerance intended to be?

My understanding is that we are limiting the size of sleeping lofts and requiring a steeper, more uncomfortable stairs (or ladders) to serve the sleeping lofts, to allow a narrower egress width without encouraging the use of that space for normal purposes as we would a normal "habitable" room with a full-sized means of egress. I'm guessing for that reason we do not want the treads getting bigger than allowed by the ratio? Smaller doesn't really make sense either... Any help would be appreciated.

10.69 isn't a number that any carpenter's tape rule I've ever seen can measure, and I doubt stair makers have rules calibrated in hundredths of an inch. So I think we need to convert to fractional values to get a usable nominal value, and then apply the tolerances established in the code for stairs. .69" is 11.04/16, and most tape rules are marked in 16ths, so I would go with 10-11/16" as the nominal number. Then the standard 3/8" difference between the largest and smallest from R311.7.5 would apply.

 

[Yankee Chronicler]

"R 315.5.2 says Stairways providing egress from sleeping lofts shall be permitted to comply with sections R315.2.1 through R315.2.3." (my emphasis added)

I don't see why stairs that comply with the normal riser & tread sizes shouldn't be permitted. I think that normal construction tolerances would let you round to the nearest 1/2" or 1". I don't think somebody is going to trip if it's a couple hundredths of an inch off.

I agree that "normal" stairs are allowed, but I don't agree that the nominal tread dimension can rounded either way by 1/2" or 1" from what the tread dimension should be per code for a normal stair. R311.7.5.1 establishes the maximum riser height as 7-3/4" and R311.7.5.2 establishes the minimum tread depth as 10". This means that no tread can be less than 10", but some treads can be as wide as 10-3/8". It's not 10" plus-or-minus 3/8".

 

[BN4537]

On the other side of the spectrum, that section of code would allow 12" risers with a 4" tread. 4" is already pretty small, so it would make sense to treat the number as a minimum with some kind of tolerance to make the stair treads a little bit bigger.

So, what if someone wants to make 12" risers with 11" treads? Isn't that safer? It seems like this code section would not allow this.

 

[Paul Sweet]

R 315.5.2.2 would make a lot more sense if it said tread & riser dimensions shall between the values calculated by formulas 1 & 2. My comment about construction tolerances referred to these formulas, and not to allow going outside the envelope of R311.7.5.

I think the R315.5.2.2 formulas might be based on OSHA regulations for ladders to be between 70 & 90 degrees from the vertical.