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Stumped on math.

3/9/21       
Mike

I am trying to figure out the spacing between spindles and how many spindles I will need for a staircase. The opening from the Newel post at the top to the newel post at the bottom is 89 5/8. The measurement that is horizontally between the two is the 89 5/8. I need to use 1-1/2” spindles for this job. Code is spindles must be spaced apart 4 inches or less in between, so I’m trying to figure out how many spindles I need and what the spacing will be between the spindles at the dimension of 89 5/8. I’m tired of doing it the crude way and that is taking 1.5 times whatever amount of spindles I think might go in there and then reversing it back to do the math from there.

Help! (Pulling my hair out)

Thx!!

3/9/21       #2: Stumped on math. ...
Leo G Member

1÷((1+4.5)÷89.625)=16.3

So 17 spindles.

3/10/21       #3: Stumped on math. ...
Mike

Leo. Care to explain what u did and how u came up with it?

3/10/21       #4: Stumped on math. ...
Leo G Member

oops

1÷((1.5+4.5)÷89.625)=16.3

Take the spindle and add it to the spacing you require and divide it by the spacing you have. Then take the reciprocal of the number and you have you spacing.

3/10/21       #5: Stumped on math. ...
Mike

I unfortunately rely on autocad to do my math for me.

I don’t understand what the 4.5 is for.

Code is, doesn’t matter what the spacing is, as long as it doesn’t exceed 4” in between.

So if you can dummy it down when u explain the math, I’d appreciate it. The space turned out to be 88.8125. I had the dimension snapped wrong

It turns out i needed 16 spindles, with the space between being 3-13/16”

Will u pls show me how, why and the written out version of math, vs (what’s in parentheses goes first)

Thx

3/10/21       #6: Stumped on math. ...
Leo G Member

Arrrggg... This is what happens when I work 12 hour days.

1÷((1.5+4)÷89.625)=16.3

1
---------------------------------------------------------
((Spindle Width + Spacing width)/total width

3/10/21       #7: Stumped on math. ...
Leo G Member

Fine, make me do it the hard way forum..

LOL


View higher quality, full size image (1024 X 491)

3/10/21       #8: Stumped on math. ...
Sam

If it were me, I'd use a Git-R-Done approach for this one
and markup a sheet of plywood using the technique in the
image below, adjusting the divisor until the spacing between
spindles was just less then 4 inches.

Might not be as impressive as deciphering the formula
offered, but for me, it'd be far less prone to error.


View higher quality, full size image (459 X 155)

3/10/21       #9: Stumped on math. ...
Mark B Member

I use Leo's method in the shop and a modified version of Sam's method in the field with a tape measure.

You can also use something like this

https://www.blocklayer.com/baluster-spacing-eng.aspx

3/10/21       #10: Stumped on math. ...
Keith Newton

Put all your fancy software away and use your head. A typical rise run ration is 7.5" rise and 11" run. Two at 1.5" = 3"
11'' - 3 = 8", so 2 per tread ought to do it, so just count the number of treads and multiply x 2 ought to be something you can keep under your hat and figure at a glance.

Then if the stairs are open ended treads, then balusters end on the tread, so start with one 2" each side of the center measured back from the nose of the tread.

Although I think 3 per tread is a better look if it's your choice.

3/10/21       #11: Stumped on math. ...
Nicko Member

Down load the BalusterPro app on your phone. I use it all the time for spacing balusters and anything that needs equal spacing. It will even call out the measurements to you as you mark them down.

Nicko

3/10/21       #12: Stumped on math. ...
Mike

While I appreciate the different in sights, I really need someone to break the mathematics down for me without the parentheses.

I know how to divide something evenly, but I really want to know the mathematical equation behind finding out how many spindles and what the dimension in between each of them will be without gassing it on a whim and backtracking

3/11/21       #13: Stumped on math. ...
Matt Calnen

I can’t believe I’m replying to this one but...

88 13/16 divided by 5 1/2”(which is your max space and spindle width) gives you 16.something, telling you you need 16 spindles.

16 times your spindle width of 1 1/2” is 24”, or the space they will occupy.

88 13/16” minus the 24” of spindles equals 64 13/16” of space between the spindles.

Divide the 64 13/16” by 17(spaces needed) gets you 3 13/16” of space between spindles!

3/11/21       #14: Stumped on math. ...
Mike

Matt, that’s EXACTLY what I wanted to know! THANK YOU! (Ex) 88-13/16 / 5.5

That’s where I was stuck.

3/11/21       #15: Stumped on math. ...
Keith Newton

Well,,,, Yes, but you make no mention of where the top and bottom newels are on their respective treads so you may end up with some of your balusters being centered on the drop from the nose of one tread down to the next. I still would think about finding the center of the treads from nose to nose, and see how that works out at each end relative to the newels. Having a bit less than 4" on each end isn't a problem. The location of the newels at each end is a variable you have not shared with anyone, unless I missed that.

Before you start cutting and drilling, if that is part of it, I would start by measuring over level from the top of the bottom newel to a tread to see where they fall, relative to the ones lower down. Your really going to be pulling your hair out if you find those numbers have some balusters trying to be half over the nose of a tread due to creep because it doesn't match up to the tread length. Do you understand my concern?

3/11/21       #16: Stumped on math. ...
Mike

I do Keith. Even tho I did it the long way math wise, it all worked out in the drawing acad. i just wanted to know a faster way of determining how many spindles, based upon the required spacing and Matt’s formula was exactly what I needed

3/11/21       #17: Stumped on math. ...
David R Sochar Member

Not so fast........ If you are using something like a 5015, with 1-1/4" square base and a 7/8" taper turned top, then the calculations need to be made at the top of the spindles, using 7/8" and 4". Not 1-1/4". That 3/8" can cause a lot of misery.

And, sidebar here. The spindles do not locate off the center of a tread. The front face of the spindle needs to be aligned with the face of the riser and the outside face of the spindle base needs to be aligned with the skirt board or bracket.

The photo is a good example of what NOT to do. That first spindle is pushed out to the start of the bullnose, and looks like it is out in space instead of bearing the load directly.


View higher quality, full size image (720 X 491)

3/11/21       #18: Stumped on math. ...
Mike

David, the treads have nothing to do with this situation, as there is a board that rides on top of the stringer that the spindles nail to on the angle.

3/11/21       #19: Stumped on math. ...
Keith Newton

David has got that right. However it would have been nice to know you were using a closed end treads when you ask the question.

3/11/21       #20: Stumped on math. ...
Mike

Sorry Keith. I tried to post a picture from my iPhone, but it says it is too big and I don’t know how to resize it to allow it to let me post it. I don’t know why they have a file restrictions set to such a small file size

3/11/21       #21: Stumped on math. ...
Scott

I could be wrong , but I think David was saying if at some point the spindle is 7/8 diameter, you need 4 inches between the narrowest part of the spindle. So if you have 4 inches at the bottom you would have an opening of 4 3/4 at the narrowest part of the spindle, which would not meet code. A child could maybe fit their head through the wider spot?

3/15/21       #22: Stumped on math. ...
David R Sochar Member

You are correct Scott. That difference is real, and can cause real problems. Given the freedom of a curbed stair, spindle placement is more critical.

I mentioned the placement since it was incorrectly stated by an earlier responder.

3/18/21       #23: Stumped on math. ...
Steve Member

Ok, as pointed out, the placement of the spindles is critical if your stairs are like the pic posted, e.g. placement on each step. But the OP does not state, what if the construction is similar to this pic (sorry my nomenclature on stair parts is deficient) in which they are evenly spaced regardless of each step?

3/18/21       #24: Stumped on math. ...
Steve Member

Mike, in order to change the file size of your iPhone pic, google "how to change file size of iPhone pictures". That will list numerous articles and sites to show you.

Or, simply forward it to yourself (email) and edit it likety split in Preview/change size.

8/19/21       #25: Stumped on math. ...
ian

Simple way to wrap your head around it, extend the length so it becomes a straight run, remove the angle of the stairs from the picture.


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