Winding Stairs: Design and Construction

The Beehive Treehouse showcases a stellar set of winding treehouse stairs.

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Hello! Chuck McClellan here. At the World Treehouse Conference earlier this fall, I had the privilege of presenting on the design and construction process for winding stairs. We build these curved staircases for many treehouses: they add equal parts elegance and playfulness to the entry.

After my presentation, I received multiple requests to get this information online for treehouse-DIYers. Without a further ado, I bring you my guide to treehouse winding stair design and construction!

A couple important things to keep in mind about this guide:

  • This is an advanced project, best suited to those with an extensive background in carpentry.
     
  • Dust off your geometry skills, because we'll be diving into a hefty bit of mathematics.
     
  • I've created the following graphics as examples to help walk you through the process; naturally, your stairs will differ from my example based on your unique tree scenario and treehouse design.
     
  • Please note that improperly installed hardware can pose a significant risk to not only the tree, but all parties involved. Using hardware and following this guide are done at your own risk. Nelson Treehouse assumes no liability.

I hope you find this a useful resource for building your own treehouse staircase!


1. Design Considerations and Process

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The design process begins with defining the size of the stairs. Let's assume that you have an entry location on an existing platform in a tree.

  • The height of the stairs is the vertical distance from this entry point to the landing point on the ground.

  • Now determine how the stairs will fit around the tree. Imagine a “virtual cylinder” outside of which the stairs wind (see image below). In our "plan view," this cylinder completely encloses the tree.

  • I will refer to the radius of this cylinder as the "minimum radius."

Virtual cylinder defines inside of stairs

2. Laying Out Stairs and Making Stringer Patterns 

With the minimum radius determined, we can lay out our stairs. This is most easily done on a flat surface. A full-scale plan of the stairs can be drawn on plywood as follows:

  • Draw the minimum radius. We will call this the radius of inside of treads.

  • Draw inside stringer radius. This radius is 6” out from the minimum radius to allow some distance from framing.

  • Draw outside stringer radius. This is typically the inside stringer radius plus 36” (standard stair width).

  • Draw maximum radius. This is the outside extent of stair treads, typically outside stringer radius plus 3”.

  • Draw stair tread lines. At 12” beyond the inside stringer line (known as the “walk-line”), mark off 9” increments (minimum stair tread depth). Draw pie-shaped stair tread lines through those 9” increments from the center point of the virtual cylinder. In this example, the treads form 10° arcs. 

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In this example, I have designed for four treads per stringer set.

  • Next, draw the outside stringer line at the points where stair line intersects the outside stringer radius.

  • Draw inside stringer line as the tangent to the inside stringer radius and parallel to outside stringer line. 

The images below present plan and perspective views of the stringers for clarity. Notice the extra length at the top of the stringers. The top plumb cut will be determined later. 

The stringers can be simply laid out thus:

  • Draw perpendicular lines to the stringer lines where the stringer lines intersect the stair tread lines. These represent the plumb cuts on your stringers (the rise).

  • Draw the stair tread lines parallel to the stringer lines, and offset from each other by the stair rise (in this case, 7”).

You now have the cut layout for your stringers. You can cut the plywood and use these as patterns. 

Next, you will be figuring stringer intersections and knee brace placement.

In the image below, notice the 2x4 risers on the second stringer set. I've left these out until now for clarity. It is simple to lay out beam cuts at this point.

Here is what it will look like: 

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To finish cutting the stair patterns, you now lay out the knee beam cuts:

  • The bottom seat cut on the outside stringer will be 3 1/2” to bear fully on the knee beam. This cut is 11 3/4” below the tread cut.
     
  • Measure 11 3/4” on the inside stringer pattern to layout that seat cut. The top seat cuts are 28” higher.
     
  • The top plumb cut is aligned with the 5th vertical (perpendicular) line from the stringer line. 
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3. Construction Assembly of Stairs Onsite

I prefer to construct my stairs from top down. This involves using temporary 2x4 scaffolding, a “story pole,” and a “clock ring.”

  • The scaffold provides bracing support before knee braces are placed.
     
  • The story pole is used to reference the height of knee brace supports.
     
  • The clock ring is used to reference the horizontal orientation of the knee braces. (Knee braces are installed after the stringers have been placed correctly in space. This sequence assures a more accurate placement of the stairs.) 

STORY POLE

The story pole here is a vertical 2x4 with reference lines that mark the top of the knee brace beam heights. In the example here, you will notice that the scaffold supporting the stringers is aligned with these reference marks. 

CLOCK RING

The clock ring we use is a ring of 1/2” plywood cut to the minimum radius and at least 8” wide. I've shown the outside edge with straight cuts for simplicity. You can see that the clock ring lines reference the knee brace orientation.

  • Note the center of virtual cylinder is offset from C/L of tree.

  • Laser lines from story pole and clock ring are used for knee brace beam placement 

LINE LASER FOR REFERENCE

In the picture below, the vertical and horizontal laser lines converge at the center of the knee brace beam. These laser lines are referencing a story pole and a clock ring, both out of view. 

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PATTERNS

Patterns are made from 1/2” plywood. They are used to orient the knee beams to the paddle tabs and to mark through-bolt holes. 

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The bottom bracket hardware is placed with lasers. Patterns are used to fit the knee brace beam.

From here, you can continue this process to assemble the full winding staircase onsite.

In an upcoming blog post, I'll break down our process for the design and construction of stair rails. Stay tuned!

I hope you've found this guide helpful!

For the heavy-duty treehouse hardware you'll need for this project, head to our online hardware store.

Need help with your treehouse project? We offer consultations for DIY-ers - learn more here.

To the trees!