Workhorse fitteds are 100% cotton diapers sold by the company Green Mountain Diapers. They're incredibly popular for their low price and excellent quality. I've bought a few used ones in preparation for baby #2, and one of them needed the elastic replaced. After Googling around, I couldn't find a tutorial, so I made up my own process and documented it in case anyone else needs help.
Above you see two workhorse diapers, both size small. On the left is a diaper in good condition, on the right is a diaper with worn out elastics in both the leg area and the back. You can see that even though they are both size small, the one with worn out elastic is much bigger and won't properly fit since the elastic is relaxed.
Relaxed elastic on right
Relaxed elastic in top of photo
The elastic in my diapers has been sewn inside the cotton layers with a straight stitch. From what I've read, there may be other production methods but this is how mine are sewn.
Since I cannot "get" to the elastic (it's not in a casing) I'm not entirely sure what kind was used. I decided to replace it with 1/4" polyester elastic that I had on hand (from Wawak). You will also need a bodkin or safety pin to thread the elastic, a seam ripper, small sharp scissors, and a sewing machine. The photos show the elastic being replaced in the back of the diaper, but the process is the same for replacing leg elastic.
First, I made two small snips in the fabric in order to insert new elastic. The snips are on the outside of the diaper, just below the original elastic, at both ends, and only in the first layer of fabric.
The above photo shows where I snipped a hole and how small it is (just big enough to insert 1/4" elastic). Make another snip on the other end. Next, locate the two lines of straight stitching that run perpendicular to the original elastic (in other words, from front to back instead of side to side). These two lines of straight stitching need to be cut in order to create a casing for the new elastic. Just below the stitching that holds the original elastic in place, use your seam ripper to cut one or two stitches.
At this point, you should be able to take your bodkin, or anything long and skinny, and push it through one of your snips until it comes out the other side. This is where you will insert the new elastic.
I measured a piece of elastic at 4 1/2". Again, this is the SMALL workhorse and it is the back elastic. Always cut extra so you can handle the elastic! I didn't know how much I needed and I ended up cutting off 1", so the finished elastic piece was 3 1/2".
Using the bodkin or a safety pin, thread the elastic through one hole and out the other. You will now need to tack down the elastic and deal with the holes you created in order to insert it, since the fabric will fray otherwise. I found the easiest way to do both of those things was to use a zig-zag stitch and go over the hole/elastic end all at once. Completely cover the hole so the fabric will not fray.
If you use matching thread you can hardly even tell. Next, pull the elastic at the other end until it gathers the fabric and tightens up the back of the diaper. I used my good condition workhorse to compare and eyeballed it until they looked similar. Once I had it stretched as far as I wanted, I secured the end with a safety pin.
Now I had my hands free to zig-zag on the second end of the new elastic and close up that hole.
This end doesn't look as pretty as the first because the elastic was under tension while sewing it down. You can snip off the end and sew over it again if desired. The final step is to sew the bottom of the new casing (the top is the old line of stitching).
Sew a line of straight stitching parallel to the old elastic stitching line, but beneath your new elastic. Do not catch the elastic in your stitching. Stretch the diaper as you go so that the fabric is flat. Make sure that you sew over the two perpendicular lines of stitching that you previously cut, this step will secure that those lines of stitching so they do not unravel.
Repaired diaper in top of photo
Then you're done! The only visual difference between the repaired diaper and the good condition diaper is the second row of stitching which created the casing. The work has all been done on the outside of the diaper, the part that does not touch baby. Ideally, the elastic would be in the middle of all the layers of cotton, but I didn't want to risk cutting through too many layers and making a hole straight through. Baby #2 will not be here for a few more months, so I haven't had a chance to test the durability of this repair, but I will update this post if it all falls apart :)
Questions? Please leave them in the comments and I will respond!