# New, free product: Stairway Inspection Spheres.

It is mathematically impossible to do the 6" test with a tape measure alone. Can’t be done without additional mathematics.

Even if you disregarded the nose of the higher tread (which makes the formula much easier) and made it a simple right triangle formed on 3 sides by the spandrel, the riser, and the tread… to determine if a 6" circle (called an “incircle”) would fit within that triangle using only a tape measure would require the following mathematical calculation:

r = (a+b-c)/2.

Thanks Nick

InterNACHI’s Resident Mathematician.

Oh, and since a tape measure won’t work… if you don’t like mathematical formulas, there is another way…

I inspect without regard to the age of the house or the code to which it was built.

And yet you have not answered my question.

What’s the question? I’ll take a crack at it.

Thank you much, I believe they will be very handy. Also a nice visual aid, I’m sure when using someone will ask why, opens door to knowledgeable conversation with client. Thanks again:)

If a circle can measure distance (ruler/tape), then it has to hold true that distance (ruler/tape) can measure a circle.

It can. But a ruler or tape measure can’t determine the size of an incircle, which is why you can’t perform the 6" test with merely a tape measure. Impossible.

See post #16 with picture.

My comment would be that “a 4” sphere can pass through the balustrade’s components which is a child and pet safety concern."

You are whacked!
The flippin’ Egyptians and the Mayans have been doing this for thousands of years.
Would you please tell me the inner circle dimensions of whatever you are mumbling about?
If I know one factor I can tell you the others.

As inspectors we “usually” look for what is not correct .Not what is incorrect…

You are whacked!
The flippin’ Egyptians and the Mayans have been doing this for thousands of years.
Would you please tell me the inner circle dimensions of whatever you are mumbling about?
If I know one factor I can tell you the others.

Well, that’s the rub, Roy. He can’t. He can only tell you the “outer” circle dimensions of whatever he’s blubbering about! :shock:

Taking a break from studying ministry for a couple of hours and decided I would return home to InterNACHI. As soon as I walked in the door, I read the paper (Home Energy Newsletter) and ate well. As I was turning the channels, I see an infomercial from the award winning Nick Gromicko. It was a really cool show with Nick showing these 2 colored circles. I really enjoyed learning this math. I too will share an equation below:
E + R = O

Solve:
The Event plus your Reaction equals the Outcome.

Nope. Mayans died out before the formula for figuring out what size sphere can fit through a right triangle (in this case, formed by the spandrel, the riser, and the tread) was invented in about 500 BC. And that’s basically the formula you need if you want to try to do the 6" staircase code check with nothing but a tape measure.

It’s impossible for a home inspector to do the 6" staircase code check with with a tape measure alone. A tape measure measures the distance between two points. Describe what two points you would measure? The inside corner of the tread and the riser to the closest point on the spandrel? If that’s what you are doing, you are wrong every time. How about the middle of the tread to the middle of the riser? If that’s what you are doing, you are wrong every time again.

Anyway, you can’t do it without a bit of math.

The way you can tell if s stairwell passes the 6" sphere test is to take 3 particular measurements, add two of them, and subtract the third. If that number is greater than 6", the stairwell fails and is considered a code violation.

I’ll write an article about it with a diagram for you.

However, if you don’t like doing basic math, there is an even easier way…

1 of the pluses about being an InterNACHI member is that there are days that you will laugh and laugh. Also, you can’t beat the education.

And exactly what recommendation would you be making based on your perceived safety issue that meets the acceptable building codes.