Step 1: Replacing the Worn Sling
If you would rather replace with fabric slings, an Internet search will give several hits like these.
If you want to do what I did, then for each chair you will need approximately (depends on your chair style and number of chairs--I have 4 chairs):
1"x3", I bought 20 for $1.92 ea.
1-1/4 self drill screw, 1lb box for $5.37.
1-3/4" or so length screw, 4.
Paint or stain. I bought 1 gal. exterior for $39.98.
All supplies came from Home Depot and totaled about $100. If you already have paint or stain, you can do 4 chairs for about $60. I like the savings, the weather resistant wood and I got to paint them with my wife's favorite color. That last one buys benefits well worth the $100 spent.
Ok, if I convinced you then let's move on.
Step 2: Measure and Calculate
Note that a 1"x3" actually measures about 2-1/2" wide. So take the length of you chair and divide by 2.5. This gives you how many slats you need. For mine, 51 / 2.5 = 20.4. Round up to 21 slats needed.
Each slat will be cut to the width of your chair. So multiply your total slats previously calculated by the width. For mine, 20-1/4 x 21 = 425.25. This is how many linear inches of 1x 3 slats I needed.
But I need to know how many boards to buy. Let's convert to feet and then get the number of boards. 425.25 / 12 = 35.43 linear feet needed. 35.43 / 8 foot board length = 4.4 boards to buy.
So I needed to buy 5 boards (rounded up) for each chair. This was all done rounding up and without factoring the gap distance between each slat, all of which will reduce the amount of boards to buy. But at $1.92 each, I'd rather have a little extra for cutting out bad spots or cutting wrong.
Step 3: Cut, Paint, Drill and Counter Sink
This jig trick is one I learned from a wise cabinet guy. He taught me this trick so I could locate the exact drawer knob and handle position on my new cabinets.
To make the jig, use scrap wood (3 pcs). Screw an end stop and side stop onto a larger block (see pics). Then locate your slat center and edge distance that you will use for your slats. Mark and drill this position. Hopefully the pics help explain this.
Now place a slat into position and drill the thru hole. The diameter drill is selected based on your screw size, but should be just large enough to slip the screw thru the hole. I used #6 screws and #32 drill.
I then counter sunk one side so the screws will sit below the surface.
Step 4: Screw the Slats in Place
I made an attempt to keep the slats level and learned I needed to try harder, or be content with imperfection--because I didn't succeed at keeping them level, I guess I chose less than perfection is ok.
The screw length is critical to this process. I made sure I had enough length to penetrate into the chair frame. On concave radius bends, a longer screw is needed(see the pic).
After I was done I decided to remove the first slat at the bottom. I remembered how shorter people complained at how the bottom slat pressed against their leg when sitting in an Adirondack chair I made.