How to Sew a Full Flat Felled Seam

Item # X-HT-300442

In this tutorial, we’ll be discussing how to create the full flat felled seam. If you’d like to see some of our other tutorials about seams, check out our blog on how to sew an overlapping seam (#300440XHT), a semi flat felled seam (#300441XHT) and a French seam (#300443XHT). Although this seam may appear to be a little labor-intensive, we’re going to demonstrate it in a way that makes it easy to follow. Let’s begin!

The full flat felled seam

If you want a seam that looks very attractive on the top and bottom side and possesses great strength and water resistance, then you’ll want to consider sewing a full flat felled seam. Because it is quite labor intensive to sew, the full flat felled seam is not utilized very often in canvas projects, but that doesn’t mean it isn’t useful. Only one seam is exposed to the sunlight, making it a smart choice for applications that will be exposed to excessive UV rays.

In the early days of sailmaking, this seam was used to seam spinnakers together. But with modern sailcloth and its dimensional stability, the overlapping seam is now more common. In modern times, the full flat felled seam is reserved for high-end biminis and T-tops where unsightly seams should be avoided. Another positive attribute of the full flat felled seam is that it’s not completely necessary to cut your fabric with a hotknife to keep it from unraveling. That’s because the fabric edges in a full flat felled seam are hidden inside the seam, which will keep them from unraveling over time.

In our video, we created a 1/2-inch seam, so those are the measurements we will use to guide you.

Step 1. Mark your two fabric panels with a 1/2-inch seam line using a fabric pencil. On one panel, the 1/2-inch seam line should be drawn on the side labeled “IN” and the other on the “OUT." Then be sure to label "IN" and "OUT" respectively on the other side of your two fabric panels.

The full flat felled seam

Step 2. Offset the panels by placing basting tape 1/2-inch away from the edge of the bottom panel on the side labeled “OUT.” The basting tape should be right next to the seam line you’ve drawn on the outside, NOT along the fabric's edge. Place the second panel on top with the “IN” side facing you, basting the two panels together. Align the edge of the top panel with the 1/2-inch line on the bottom panel, creating a 1/2-inch flap sticking out from the bottom panel. The two sides labeled “OUT” should both be facing each other but away from you.

The full flat felled seam

Step 3. The fabric assembly should have the top panel with the “IN” side facing you. Then sew along the 1/2-inch line marked on that panel.

The full flat felled seam

Step 4. Add another strip of basting tape to that 1/2-inch flap of fabric that’s sticking out from the bottom panel. Fold that flap over and it should align with the first row of stitches without covering them up.

The full flat felled seam
The full flat felled seam

Step 5. Next, add a third strip of basting tape on that folded-over 1/2-inch flap (the fabric thickness at this edge is now 3 layers of fabric). Then, take the top panel and unfold it so that the two fabric panels are splayed out again (both fabric panels should have “OUT” facing up), creasing down that thick seam (now 4 layers of fabric thick) and making sure the basting tape adheres well to the “IN” or wrong side of that top panel.

The full flat felled seam
The full flat felled seam

Step 6. With the “IN” side of the panels facing up, sew a second row of stitches close to the folded edge, approximately 1/8 inch away from the folded edge. Then you’re done! The inside of your fabric assembly will have two rows of stitches while the outside will only appear to have one.

The full flat felled seam

Here’s a short summary of the advantages and some minor drawbacks to the full flat felled seam.

Full Flat Felled Seam Qualities

Pros:

  • Fabric does not need to be cut with a hotknife to keep from unraveling
  • Very attractive seam
  • Highly water resistant
  • One of the strongest seams (100% fabric strength)
  • Keeps one stitch hidden from the sun

Cons:

  • Labor intensive
  • Time consuming
  • Uses 3 times seam width (1/2-inch seam will take up 1-1/2 inches of fabric)

See below to watch the full how-to video. Sailrite is your one-stop shop for free instructional videos in addition to high-quality sewing supplies for all your DIY projects. It’s your loyal patronage that makes these tutorials possible!