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Irregular gaps between the spindles on a staircase or decking really stands out and looks pretty poor. I don't know why people use complicated formulas or trigonometry, I've done hundreds of staircases and can fill a balustrade with spindles in hardly any time using the method on this page - and it's right every time. (At time of writing spindle spacings need to be less than 100mm in the U.K. to prevent toddlers falling through them, check with your local building control if you are unsure of the maximum permitted dimension in your area).
If you haven't already done so, use the formula to calculate how many spindles are required here first.
Below is how in a nutshell I work out the spacings. It's the quickest and most accurate method I've found. And because you put them in at one end at the start, they're all to hand when you start installing them. You can easily grab them, glue them fit them etc. Scroll down for step by step installation of an angled run to get them perfect.
Whether you are installing balusters on a staircase, balcony, garden decking, veranda, porch or anywhere else - the theory is the same.
I use my DeWalt DC618KB Heavy-Duty Cordless Nail Gun for fixing spindles and railing spacers. I used to have a Paslode which is great when it works, but it was always going wrong or needing servicing. The Dewalt Nailer just seems to keep on going and going. It saves a lot of time and the hassle of having to punch the nails under the surface so they can be filled. Also, using a hammer and nails can lead to the handrail 'bowing' up when you're nailing the top of the balusters to the handrail (mainly on an angled run). A nail-gun and 600mm spirit level helps to avoid causing this problem.
If you don't have a nail-gun, use 20-40mm panel pins to fix the balusters and railing spacers. Keep an eye on the hand rail all the time as you fix them by looking down the length every so often, to make sure it's straight.
Step 1: Cut the spindles.
Cut the spindles to length and put them all in at the bottom of the run, up against the post and each other. Set up a stop like above to cut them all the same length quickly.
Step 2: Measure the remaining space
Hold a tape measure perfectly horizontal and measure the width left in between the top spindle (square part)
Take half the amount of spindles out of the bottom. Glue and nail a spindle in place so the middle of the spindle is exactly in the center and use a spirit level to make sure it is perfectly plumb. Now I'd treat this as two individual runs to make life easier.
Step 3: Divide by the number of baluster spacings (gaps).
You should have one more spacing than there is spindles. So if you have 10 spindles, take the measurement from step 2 and divide it by 11, for example. This will give you the horizontal distance apart each spindle spacing will be. In the UK, the regulations state that you cannot have a spacing of more than 100mm (horizontally) between spindles, because a child could fall through.
To be on the safe side I never go over 90mm. If the answer comes to more than 100, cut another spindle and add it to the others before re-doing step 2. Add or remove spindles until you have a gap you are happy with and that complies with your local regulations. Normally around 70-80mm looks good.
If there are several 'runs' that need spindles I will work out the spacings for each run before installing any, then add/remove spindles from some to try and get the gaps as close to each other as possible so they all match.
Step 4: Cut the spacers.
Grab a bit of the spacing strip about 300mm long, or cut a piece this long with the angle of the staircase pitch (set a bevel up where the hand rail meets the newel post). Use a tape measure or ruler to place a spindle in the grooves the correct distance away from the top newel post. If the spindle doesn't fit tightly in the hand and base rails, pin the bottom of the spindle temporarily si it stays put. Like in the picture below. Hold the spacer up to it and mark the length with a sharp pencil.
With the chopsaw still set to the staircase pitch, cut the spacer to the length marked. Use it to mark another so you have an identically matching pair. Put these in the grooves of the hand and base rail dry. Put a spindle in and check they are the right length. If they're OK and leave the correct spindle spacing calculated in step 2, move to the next step otherwise trim or cut two new ones until you are happy with the length.
Next, grab an off-cut of timber and a good woodworking clamp. If possible, lock the Chopsaw blade down low and hold the spacer up to the blade. Clamp the off-cut to the saws fence (like below). Hold the spacer up to the blade, and the timber up to the spacer before clamping. This will act as a 'stop' for making fast repeat cuts. You will need two spacers per spindle spacing, but only cut half the amount needed for now. Again, cut a pair and check they're right before cutting more.
Step 5: Glue and pin in place.
Using p.v.a. glue and some 20mm panel pins or a finish nail gun fix the first two spacers in place, one top one bottom. I normally put glue on one spacer and rub the two together before putting them in. If they're really tight use a block plane to put a leading edge on them. Punch the nail heads under the surface so they can be filled. Glue the top and bottom of a spindle and fit in place, then pin and punch. As mentioned, it's important now to keep an eye on the handrail as you fill it up to make sure it doesn't start to bow upwards when you nail the top spacer. Using a nail-gun pretty much eliminates the chances of that happening.
Step 6: Double check
When you've fitted half the spindles in place stop and re do step 2. If you are 1mm out on each spindle spacing and there are 20 spindles, by the time you get to the bottom you'll be 20mm out and it will look rubbish! Check now, and if you need to adjust the spacers up or down a mm or two do it while there's time to lose the difference gradually. Now take all the remaining spindles out of the bottom and start fitting from the bottom up. I do this because you almost always have to adjust the last two spindle spacing's at the end by 1 or 2mm and its better to have that slight variation in the middle of the run where it will blend in and be much less noticeable than at the post where it will stand out.
Step 8: Perfectly spaced spindles.. Admire!
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