Original Question:

My deck has a curve in it. It's like a half circle, except its depth is not equal to its width. From a bird's eye view, the curve is 72" wide and 48" deep. The deck posts are already set.

There are posts at the point where the curve begins and ends and there are two additional posts set at equal intervals, dissecting the curve into three parts.
To cut the top and bottom 3.5" wide rails (2x4 lumber on the straight rail) I will use 2X6 lumber and draw a pattern to produce the three sections of curved railing.
To cut the rail cap (5.5" wide, or a 2X6 on the straight rail cap) I will use a 2X8 or 2X10 to draw a pattern to produce the three sections of curved 5.5" wide rail cap.

I can create the curve for the railing in two ways:

1. Transfer the curve of the facia board (that covers the deck framing's joist ends) to the deck rails?
2. Calculate the railing curve, as dictated by the post placement and curve dimensions.

Here is my question:

How do I do the easiest of the two methods?
Or, is there an easier way to calculate the rail curves so I can draw my patterns?

Click on this calculator link to open a calculator window.

Solution:

Diagram of the representation of the curved deck railing:

Calculations:

GIVEN:

Width of curve of deck 72"
Depth of curve 48"
Width of rail 5.5"

LEGEND:

R = radius of deck curve. [37.5"]
a = angle of arc of 3 equal sections of deck rail. [70.8o]
V = added length for rail width
Z = length of cord of rail. [43.45" + 2* 9.04"]
W = minimum width of board needed to cut curve. [6.93" + width of the rail 5.5" ]
V = additional width to W (to draw pattern for curve) [6.75"]

Reader's response.