Four Tips for Serging Bulky Knit Fabrics

Four Tips for Serging Bulky Knit Fabrics

This content was originally written for Indiesew and has been adapted for this blog. It was originally published on October 23, 2015.

At my family’s sewing reunion some time ago, my sister ran into a sewing hurdle I couldn’t begin to help her with. She’s an experienced sewist, especially when it comes to knits, but serging the seams of her raglan top just wasn’t working.

She was using a lovely, soft French terry she purchased on our first trip to LA together. It was a thick knit fabric with a lovely looped texture. But the seams of her raglan tee were appearing wavy and stretched out. We both agreed there must be a tension issue with her serger, but when she asked for more help I responded with an unhelpful shrug.

In truth, I had never seen a wavy serged seam come off my own trusty serger. I also hadn’t done much serging of bulky knit fabrics like sweater knits, French terry, and sweatshirt fleece. But over the next few months I noticed a few other sewists having this same issue my sister had. And then one fateful evening when I was attaching the waistband to a sweatshirt, it happened to me.

I played around with the knobs and dials on my serger for what felt like hours until I finally found a solution. It turns out the issue didn’t have anything to do with tension.

In my experience, the dreaded wavy seam is most often a result of serging more than two layers of fabric together like when attaching cuffs, waistbands, and neckbands. In this tutorial, I’m serging four layers of a thick double knit fabric together.

Serging Thick Knit Fabrics

For this tutorial, I’ll be demonstrating on my Brother 1034D. Be sure to read your serger manual to best understand how the different settings on your serger affect the stitch.

Anatomy of a Serger

First, let’s learn a bit about the anatomy of the serger. Below is an image of what the three dials on the side of the Brother 1034D are used for. For most fabrics, I keep these dials at the default settings as noted by the numbers within grey rectangles.

Anatomy of a Serger

Below is a photo of what a thick knit seam looked like after being serged using the default settings. Not pretty. 

Stretched out Seam | How to Sew Thick Knit Fabrics

Now, let’s adjust these settings for a better serged seam.

1. Increase the stitch width.

For bulky knits a wider stitch makes a noticeable difference in how flat the seam will lie after serging. I increase my stitch width to the second-to-widest setting for thick knit fabrics.

Increase stitch width when sewing bulky knit fabrics. 

2. Increase the stitch length.

I also find that the serger's longest stitch length results in a smoother serged seam when sewing with bulky knits. I tested out this theory by decreasing the stitch length to its lowest setting and found that it increased the waviness of the seam drastically.

Increase stitch length when sewing bulky knit fabrics.

3. Increase the differential feed ratio.

The differential feed ratio is perhaps the most important setting to play with for different fabric types. If your serger has differential feed, that means that its two sets of feed dogs can move at different speeds. The ratio is the rate at which the front feed dogs move over the rate at which the back feed dogs move. So a higher differential feed ratio means that the front feeds will feed your heavy knit fabrics through the needles a bit faster than the back ones. This factor alone will prevent stretching of heavy knit fabrics. I increase my differential feed ratio between 1 and 2, right around 1.5.

Increase the differential feed ratio on serger.

4. Press the seam with a hot steam iron.

For those brand new to serging with knits, this last step might not be as obvious as it sounds. I’ve received many a frustrated email from sewists who simply cannot achieving the perfectly flat serged seam they’re looking for. I always suggest pressing the serged seam with a hot steam iron (use the appropriate settings for your fabric) as a last ditch effort. And in my experience, it almost always does the trick. Hot steam has the magical ability to snap stretched fabric back into place and smooth out those wavy seams.

Press seam on thick knit fabrics to snap back.

For the steps above I kept my serger’s tension dials as pictured below.  You may need to adjust your serger’s tension (especially the upper and lower looper) depending on your fabric type.

Serger Tension Settings for Thick Knit Fabrics

And this is what my new serged seam looks like:

Flat Serged Seam | How To Sew Thick Knit Fabrics

Since every serger and fabric is different, I recommend playing around with these settings until you achieve the seam you’re looking for. Be sure to take note of the settings so you don’t forget them when you return to a similar project in the future.

Happy serging!