How to Stabilise Shoulder Seams on Knit Tops

If you look inside RTW knit tops and inspect the shoulder seams, you’ll notice that most of the time there is some type of reinforcement at the shoulder seam. And there is a good reason for that. Since the shoulder is cut cross-wise, with the greatest degree of stretch horizontally, they run a big risk of being stretched out, both when sewn and when worn. 

So you might end up with a finished shoulder seam that’s substantially wider than the original pattern. And to make matters even worse, shoulder seams are usually also slanted, especially the front shoulder piece, which increases the stretch even more.

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This is why it makes a lot of sense to keep the fabric in check when you are sewing shoulder seams on knit tops and t-shirts.

But what method is the best for this? Well, like with most things, it depends. But in this tutorial, I will go talk about five good ways to reinforce the shoulder seam, and when to use them.

From top: Clear elastic, Power net, Fusible interfacing, Fabric strip, Fusible bias tape

Materials for stabilising shoulder seams

Clear Elastic

This is one of my personal favourites and is quite common in the garment industry too. What makes it so great is that clear elastic has similar stretch and recovery abilities as 4-way stretchy Spandex knits, so it has the perfect amount of stretch for any type of close-fitting stretch tops. 

The width is also around the same as most serger and sewing machine overlock seams, so you can easily cover the entire elastic with the shoulder seam. Another advantage is that you don’t need to fuse it which makes it awesome to use for synthetic knits.

Pros

• Great stretch and recovery
• Same width as the shoulder seam overlock stitch (roughly)
• No need to fuse it
• Very easy to sew on a serger if the presser foot has a slot of narrow elastic

Cons

• Doesn’t work well for necklines that are sewn with a binder attachment, since one one shoulder seam needs to be open, and that makes adding the clear elastic afterwards kinda hard
• Can be a bit flimsy to sew, there is a slight learning curve
• Some people don’t like the silicone (“plastic”) feel of clear elastic

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How to sew clear elastic

Sewing clear elastic entails a slight learning curve, as it can slip and curl. Luckily, I’ve already done a pretty extensive tutorial on how to use clear elastic to stabilise shoulder seams that you can check out.

Power Net

Power net is a heavier stretch mesh, often used in lingerie and shapewear. It is similar to clear elastic in that it has a good balance of stretch and recovery, so it’s great for stretchy knit fabrics 

Pros

• Great stretch and recovery
• No need to fuse it
• Very versatile works with pretty much any fabric or assembly method

Cons

• Adds some bulk to the shoulder seam

Fusible interfacing

On paper, this might seem like a great option and it’s also something most of us have at home already. But it’s actually my least favourite method of stabilising the shoulder seams. The main reason is that many fusibles are either too stretchy, with bad recovery or too stiff, thus making them a bad choice for many knit fabrics, especially those with a lot of stretch, such 4-way stretch fabrics and rib knits.

Pros

• Easily available
• Easy to sew, since it’s already stuck on the fabric when you sew the shoulder seam
• Hardly any bulk

Cons

• Hard to find fusible interfacing with the perfect amount stretch
• Doesn’t fuse well on synthetic fibres
• Can break if stretched too much

Fabric strips

This is probably the most common method in the garment industry and it’s a great way to use up fabric scraps. Hence why I’m a big fan of this option. The knit you use must be pretty thin and have just a smidgen of length-wise stretch and the strips should be cut vertically. So super stretchy 4-way knits are usually a bad choice for this method. But regular 100% cotton jersey  (i.e. t-shirt fabric) or cotton/poly mix jersey are awesome for stabilising shoulders seams, so don’t throw away those scraps!

Pros

• Adds stability and just the right amount of stretch on most knits
• No need to fuse it
• Very versatile, will work with pretty much any fabric or assembly method

Cons

• Adds bulk to the shoulder seam, try to pick a thin knit

Fusible Bias Tape

Yes, I did say that fusible interfacing is often a bad choice for stabilising shoulder seams. But there is one exception to this rule, and that is fusible bias tape, which is interfacing cut on the bias and then stabilised with a chain stitch. This gives the tape a great balance between stretch and stability.

Just make sure you don’t get the rigid version of this tape, you need the bias one with some stretch.

Pros

• Easy to sew, since it’s already stuck on the fabric when you sew the shoulder seam
• Great combo of stretch and stability
• No need to pre-cut strips

Cons

• Doesn’t fuse well on all synthetic fibres
• A bit too wide for my liking, but the tape can be trimmed of course
• Kinda expensive

Vilene fusible bias tape (the best brand) on Amazon (affiliate link)

But what about stay tape?

A firm stay tape is a really bad choice when sewing stretchy knits if you ask me. The woven ones are way too rigid and bulky and the fusible ones have zero stretch, which will make shoulder seam feel really odd. You do need a little stretch when working with stretchy knits, so stay away from stay tape! (pun intended).

Principles for stabilising and sewing shoulder seams on knit tops

The reinforcement strip should always be placed on the back shoulder piece.

For fabric strips, you can attach them using stick glue or a zigzag stitch to attach them to the fabric before sewing the shoulder seam. Doing this prevents the strips from slipping, which can a nuisance.

Sew both shoulder seams in one go if you have a serger, this saves time and minimises hassle. I do this when sewing clear elastic too, I don’t cut the elastic until I’ve sewn both seams.

Fold the shoulder seam towards the back. This is the standard method when sewing knit tops and the seam allowance also cover the reinforcement that you did on the back piece. 

Press the shoulder seam gently if the fabric can handle an iron.

So these were my best tips on how to stabilise shoulder seams on any type of knit top, including sweaters, cardigans and of course t-shirts. BTW, I’ve also done a video on this how to sew and stabilise shoulder seams: