How To Build a Spiral Staircase Around a Tree

Before this project, I had never built/designed a staircase, let alone one that spirals.  My early research consisted of measuring different staircases around my house looking into rise, run, stair length, height, railings, etc.  All of this helped me get my bearings in terms of how the steps would have to line up, so I made a list of...

Factors To Consider

• height of each stair
• staircase width
• stair size and shape
• number of stairs per revolution
• how to connect to the tree
• fitting the spiral between the big and small tree trunks
• railings
• how to support the steps and leave room to walk up without hitting your head???

This set of problems was pretty daunting, and impossible to solve independently since they all affect each other.  So I went to the drawing board of my driveway for better visualization.

Early chalk sketches

Besides being functional, I wanted the steps to be comfortable to walk up- not too steep, not too narrow. I used piles of scrap wood to model the height of the steps and my chalk drawings to approximate the width.  Of course, the step width was limited by the space between the main tree and it's second trunk.  I measured the distance between them at about 6 feet off the ground and got 18".  This estimate was conservative, the trees seemed to grow slightly apart going upwards and are closer together at the bottom. (This would cause some problems later on with the first steps but that's another post).  Most of the spirals I came across while researching had 12 steps per revolution so I used that number too.

Going Up

Between my chalk drawings and trying to fit through a sliding door opened 18", I decided this width was too narrow.  The steps would have to squeeze down to 18" between the trees but could be wider elsewhere.  So now I was looking at some sort of variable width spiral? This seemed impossible until inspiration struck:

A spiral squarecase

Spiral steps inscribed inside a square!  My hard work googling pictures of spiral steps had paid off.  I could make the staircase square shaped and have the a middle side of the square between the two trunks.

This gave me the basis for my outer support system.  The sides of the square would support the outside of the stairs as in picture above, except I didn't have a wall to anchor to.  Connecting to the tree would be done from the square corners with some sort of knee brace.  The square sides would have to be 'stringers' and I'd need another way support the insides of the steps at the tree.  These problems required me to know where exactly the stair inside and outsides would be, so I moved on to...

Step Design

In the spiral pictures I googled, the steps are all different shapes and sizes.  I took their lead and started experimenting with different step shapes.   These early drawings turned out looking very strange:

Awkward angles abound no matter what I did.  My main design concerns were keeping the steps easy to support and easy to build with my available materials.  As with the treefort, the materials consisted of wood scavenged from construction sites.  The widest lumber I could find was a 2x12, and it still wasn't wide enough for these step designs.  No matter what I did the steps would need to be made of multiple pieces of lumber.  At this point I had 3 differently shaped steps that would each require more than one specific shape of wood be cut out and combined together.

Strange looking early stair shape ideas

This would be wasteful and a lot of work, so I looked into making the steps all the same like they would be in a regular spiral.

Circle divided and overlaid on staircase square on left, steps are trimmed on right

Immediately this looked easier to work with.  With some simple math/drawing, I found a way to cut out these shapes from 2x12 boards:

Starting with the 2x12 board on the left, a diagonal cut is made and the pieces are arranged with the cut side out to form steps

As soon as the drawing was done, I knew I wanted these steps.  Even though they were essentially circular, I was determined to find a to fit them onto the square support system.  Structural support design is up next.

Design- Defining the Dilemma

As with any design process, the first step is always google the problem.  Searching for "how to build a spiral staircase around a tree" didn't yield any useful results, so I had to create a design myself from scratch.  Even if I had found a suitable design, it would have to be modified to fit my needs.  And what specifically were my needs?  At this point I set about creating a clear definition of...

The Problem
 I want to build a direct route to the 4th floor<insert hyperlink> of the treefort.  It has to bypass and be completely independent from levels 1, 2, and 3.  My first decisions were to determine where and how this would be possible (if at all), so I took a notebook and tape measure to the 4th floor and started sketching.  My early ideas varied from simple ladders to complex systems of platforms with floating staircases, but I was enamored with the idea of a spiral.  The fourth floor is supported by 4 trees that all lie within the interior of the floor, and each of these trees has at least one knee brace supporting the floor.  The braces made it impossible to use any of the 4th floor supporting trees as a spiral staircase location, so I looked around to see if any close by trees could be used.  I took some measurements and decided a nearby double leader tulip poplar was the best candidate.

Looking down from the 4th floor, spiral will go up tree at top of picture

With my location determined, the next step was to find a way to connect it to the 4th floor.  There's a gap of approximately 8' between the proposed spiral location and the nearest corner of the 4th floor.  I balanced a 2x4 in the gap to help visualize a solution.

2x4 balanced from the 4th floor corner to future spiral location

Eventually, I came up with the idea of replacing the top stair of the spiral with a rectangular bridge straight to the 4th floor.  It could then be supported by the spiral structure on one side and the 4th floor on the other.  Thinking back to my structures engineering courses, I planned to fix the bridge to the staircase end and use a roller type connection to the 4th floor.  I wanted to make sure the bridge and 4th floor were structurally independent to allow for tree movement.  Unfortunately I had neither the know how nor means to build a 'roller'.  Resting the bridge on the 4th floor would allow movement but could possibly damage both.  Some sort of low friction material could be used in between, but I didn't have anything like that.  Plus, it would change the whole look of the 4th floor.  This problem took quite a while to find a solution, but eventually I came up with the idea of hanging the bridge below the 4th floor using steel cables.  Going from the bridge to 4th floor would require a step up, no big deal.  This detail was very important going forward in the design because it set the elevation of the top step and effectively determined the height of the spiral.

4th floor on the left (purple), bridge (blue), and approximate spiral location (light blue) with double tree on right

As shown by the orange arrow, I had inadvertently decided which was the spiral would rotate.  Next up was to figure out how exactly the staircase would work.