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Modified Felled Seam
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Adam Thibault
(apthibault) - M
Modified Felled Seam on 02/20/2013 07:54:14 MST Print View

After looking at a couple of tents lately I got to thinking about the flat felled seam. You know how you usually put the two wrong sides together and then sew your seam you open up your two pieces, roll over the seam allowance and sew one or two lines of stitching down the seam allowance? The first seam you sew is only through two pieces of fabric, while the other seams are through four pieces of fabric, it seems to me the first seam you sew is inherently weak when you sew this way. In the tents I was looking at all of the seams went through four pieces of fabric (I'm sure they use some kind of foot that does it all for them).

So my question is, does anyone know a good way to sew a felled seam skipping the first seam that only goes through two layers without a special foot? I was thinking some kind of basting stitch for the first line of stitching, then sew two lines of stitching through the folded seam allowance, then coming back and ripping out the basting stitch (however this might leave some holes that would leak). Thoughts?

Let me know if this is really confusing and I'll draw a picture.


Locale: Northwest Mass
Re: Modified Felled Seam on 02/20/2013 08:01:18 MST Print View

By the time you have ripped out your first line of stitching, you probably end up with a weaker seem than if you just left it in. You could perhaps just use a really long stitch length, which would not over-perforate the two layers of fabric, then use a normal stitch length for the 4 layers of fabric. but unless you have had problem with the flat felled seam in the past, this just seems like you are solving a problem that isn't there.


jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Modified Felled Seam on 02/20/2013 08:10:50 MST Print View

Very clearly said and good question

Just sew a third row of stitching (through 4 layers) and you'll end up with 1 row of stitching through 2 layers and 2 rows through 4 layers.

No reason to rip out the first row of stitching.

When you're sewing the 2nd row of stitches, make sure you're not sewing it such that the 1st row of stitches will take all the load - flatten out the folded fabric good. Sometimes, I'll screw up a little and the 2nd row of stitches will take most of the load but the 1st row won't take much load but that isn't important, especially if I do a 3rd row.

If I do a third row, I just run the fabric through at constant speed, so the quality and strength of the seam is best. And the 2nd and 3rd rows will share the load good.

Brendan S
(brendans) - MLife

Locale: Fruita CO
Re: Re: Modified Felled Seam on 02/20/2013 08:25:18 MST Print View

One option is folding over the longer seam allowance and sewing through that first (rather than just tucking it under after making a stitch through 2 layers), so you're going through 3 layers instead of 2.

Jerry, I haven't tried a third stitch, but I don't see how a third stitch in the middle takes any load off of the 2-layer stitch. It seems like the load is still all on the outside 2 stitches regardless of what you put in the middle. Is that flawed thinking?

I'm about to start a tent so this is of interest. I actually bought a lap seam roller to see if I could rig it to do the initial 4 layer fold (sewing with a single needle) but no luck. I have an industrial machine that could do it right but I'd need a different needle bar for twin needles and can't find one anywhere.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Re: Modified Felled Seam on 02/20/2013 08:41:14 MST Print View

When you're sewing the 2nd row of stitches, if you pull apart the 2 pieces of fabric a little to put load on the 1st row of stitches, and carefully fold over and flatten the fabric, you can get it so the 1st and 2nd row of stitches will share the load.

Often, I screw up a little and the 2nd row of stitches takes more of the load, but no worry.

Then, the 3rd row of stitches goes between 1st and 2nd row. The 2nd and 3rd row of stitches will share the load good.

It really doesn't matter that much if the 1st row of stitches doesn't share the load, because there are two good rows. The purpose of the 1st row is more just to align the 2 pieces of fabric.

Adam Thibault
(apthibault) - M
Re: Modified Felled Seam on 02/20/2013 09:09:08 MST Print View

Yeah, I was thinking if you could reduce that first row of stitching the seam might be more waterproof. It seems that water would most likely enter there where there is only one maybe two layers of fabric at the seam (when it is pitched fully taut). It doesn't matter much on a silnylon shelter because you can just seam seal it however I was going to use a lightweight canvas so I can't really go back with seam sealer afterwards. It sounds like maybe you need an industrial machine and a special foot to accomplish this though...

Brendan S
(brendans) - MLife

Locale: Fruita CO
Re: Re: Modified Felled Seam on 02/20/2013 09:31:55 MST Print View

I guess I don't see how the third row does anything unless one of the other two rows fail. If you have load pulling out from each side of the seam, isn't all of the pulling on the outside seams, meaning that between the outer two rows of stitches, there isn't any load at all? I guess the stretch of sil might mean there is some.

My thinking: no matter how many layers you sew through, the weak spot is still the top single layer, and that's were you'll get seam hole elongation, which seems to be the biggest thing to try to avoid. It almost seams like a real lap seam is more for speed/convenience and having two perfectly aligned stitches (which IS important) than for strength of the seam. The modified, two-step felled seam seems plenty strong and the only disadvantage I see is that the first seam is more "exposed" (doesn't have the tiny bit of folded fabric behind it), so hole elongation is more visible.

I may be totally off and will do some experimenting later.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Modified Felled Seam on 02/20/2013 09:37:10 MST Print View

Maybe the 2nd (and 3rd) seam(s) leak more water because it follows the threads.

Maybe removing the 1st row of stitches would make it more or less waterproof.

Canvas? How curious. Then it's not waterproof anywhere so it doesn't matter.

You could waterproof seam of canvas with either silicone or polyurethane, I would think. Try it on a test sample, let it dry for a couple days, examine it/try to rub off the seam sealer/possibly test waterproofness but that might be difficult.

David Scheidt
(dscheidt) - F
Re: Modified Felled Seam on 02/20/2013 09:47:32 MST Print View

It takes more than a special foot to do this the way factories do it. They use a double needle machine, which has two needles, two threads, two hooks, two bobbins, and makes two entirely independent rows of stitching at the same time. A folding attachment is typically used to fold the pieces as they're fed into the needles, so the seam is perfectly even.

Seam strength shouldn't be an issue, but if you're worried about it, a third pass through the machine will get you that parallel row of stitching. I wouldn't think that 'zipper' failure would an issue, if you're using a reasonable needle and thread for the material.

Sam Farrington
(scfhome) - M

Locale: Chocorua NH, USA
felled seams on 02/20/2013 20:29:54 MST Print View

This issue was covered within the last year or so in this forum, so is deja vu all over again; but that may be just the way it is in this medium - just takes some getting used to.

First a dry iron is set just hot enough to make a good folded crease that will last a few minutes without damaging the material. Careful experimentation is required on scraps to get the right setting on the iron.

Next, the edges of the fabric panels must be marked with a line where the fold is to be. The width of the thin strip to be folded over depends on how wide you want the felled seam to be.

Next, the material is folded on the line, and just the edge of the fold is pressed with the edge of the iron. The working surface underneath must be something that will not melt or ignite. I use a ping-pong table made of a wood composite we used to call homasote. Next, the two panels are joined with the folds overlapping as they would on the finished lap felled seam.

Pins are inserted to hold the seam together, and are placed perpendicular to the seam, and between where the two stitch lines will be. With practice, the material can be laid flat on the table, and the pins inserted using the fingernail on one hand as a thimble to press against the fabric and pinpoint in order to get the pin to come out of the fabric at the desired point.

Next the two stitch lines are sewn. Each line penetrates four layers of fabric.
Stitching the first line, I pull the pins out just before the work enters the presser foot. Usually I can sew the second line without having to re-pin, because the fold in the fabric has been pressed by the iron to stay in place long enough to finish the seam. I sew very slowly, to insure the material is just where it should be when it passes under the needle. This requires a good machine motor and variable resistor in the pedal to operate at slow speeds.

The result is a true flat or lap felled seam that has no needle or pin holes outside of the stitch lines. There are the pin holes between the stitch lines that go through four layers of fabric. After seam-sealing, they present no problem.
The seams on this silnylon fly for a Hubba were sewn in this fashion:

With a "faux" felled seam, as was already pointed out in earlier posts, there is a stitch line where holes in one layer of fabric are exposed:
F-fell seam
The stress on these holes is likely to defeat the sealer and allow rain to enter.

Wish I could sew like a pro on a pro machine with a seam folding device. The chances of that happening are between zero and none; so resort is had to the above method. For purposes of measuring pattern panel sizes, it is a little tricky at first getting used to the idea that the fold lines are offset to each side of the center of the seam, but it gets easier with practice.

It seems worth the effort to get a much more durable and water resistant seam.

Edited by scfhome on 02/20/2013 20:48:47 MST.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Modified Felled Seam on 02/20/2013 23:50:43 MST Print View

> It takes more than a special foot to do this the way factories do it. They use a
> double needle machine, which has two needles, two threads, two hooks, two bobbins,
> and makes two entirely independent rows of stitching at the same time. A folding
> attachment is typically used to fold the pieces as they're fed into the needles, so
> the seam is perfectly even.
In fact, my ancient Singer comes with a foot a bit like that. But I have never used it, so I am not sure how well it might work.

there seems to be a myth that using a flat felled seam means that you don't get the load on a single layer of fabric. WRONG. Completely wrong.

Look carefully at Sam's last diagram. All three seams go through a single layer of the left-hand fabric. The first line of stitches is no weaker than the other two. But having 3 lines is obviously stronger than one.

So why use a flat felled seam? Two main reasons.
The first goes back to the days of uncoated fabrics. Such a seam conceals the raw edge of the fabric and stops it from fraying. This is a good reason, with uncoated fabrics. We are not using uncoated fabrics for tents and rainwear.
The second reason is customer perception: the way the top fabric has its edge tucked under looks neater. Commercial machines can do this with the special foot, so they do.

But you would get the same performance if you just folded the sewn fabric over once and put the extra 2 lines of stitched through them, leaving the edges showing. Put this seam on the 'inside' of the tent or jacket, so all you see on the outside is just the join, and seam seal the stitching. Same performance.


Lawson Kline
(Mountainfitter) - M

Flat Felled Seams on 02/22/2013 09:16:51 MST Print View

True flat felled seams are easy to sew if you have a double needle machine with a custom folder. But if you don't. Then don't bother.

Save yourself a headache and do a faux flat felled seam. Sew an extra line of stitching if your worried about strength. Also disregard everything you have ever hear about using Irons and Pins. DONT USE THEM. They slow you down, make your seams look like crap, and are NOT NEEDED.

I am going to attach a photo of a faux flat felled seam I just did to show you what is possible. I took two pieces of silnylon about 3 feet long and used a single needle machine. For demonstration purposes I did three lines of stitching, used no pins, no iron and it took me less than two minutes to do from start to finish.

Let me know if you have any questions.

Top of seam

Bottom of seam

Ryan Smith
(ViolentGreen) - F

Locale: Southeast
Re: Flat Felled Seams on 02/22/2013 10:32:22 MST Print View


Out of curiosity, what do you use in place of pins? For everyone else - check out the link below. It has a lot of great felled seam info, pics, and videos. Warning - You do have to wade through a little forum chatter in that link.


Lawson Kline
(Mountainfitter) - M

Nothing on 02/22/2013 11:57:34 MST Print View

Good question. But the answer is Nothing. The only thing that I use to hold the seam together while I am sewing is my big burly hands. Seriously my hands were not meant for sewing so if I can do it, anyone can. My point really was that using a single needle machine anyone can make a perfect looking seam with even three rows of stitching that are straight. Basically stop dreaming about a $1,500 double needle machine with a $300 folder and true flat felled seams. You can you can get the factory look and strength an your house using a $150 single need machine.. Let me see if I can come up with a better way to explain how I made the above faux flat felled seam.

Brendan S
(brendans) - MLife

Locale: Fruita CO
Re: Flat Felled Seams on 02/22/2013 12:13:42 MST Print View

I tried out several different variations (true felled seam as Samuel describes, faux, faux with a third stitch in the middle, and faux without the extra fabric folded under like Roger describes) yesterday on some sil and yanked on them some and there was no difference between any of them as far as hole elongation. Also, in the faux variations that have stitches through a different # of layers on the two stitches (such as 4 layers on one side and 2 on the other), the holes grow at the same rate for the exact reason Roger describes above. Bottom line: don't think it's worth making it more complicated than it needs to be.

Lawson Kline
(Mountainfitter) - M

Double/Triple Top Stitch on 02/22/2013 13:35:17 MST Print View

As Roger and Brendan said. There is no difference in strength between the two if the seam is sewed properly. I think instead of calling it a Faux Flat Felled we should come up with another name as Faux implies that its a lesser quality version. This is not true as its a different type of stitch all together and is sewn in a completely different manner and in my testing I have found it to be just as strong as a true flat felled seam. Some people disagree because they can see the needle holes opening up. The flat felled seam does the same thing but its just harder to see because the holes are on top of the seam and not against the edge. After you have everything seam sealed it is one hell of a strong seam. That said, lets call it the double or triple top stitch depending on how many rows of stitching is done.

One other thing I forgot to say is to make sure you use the right size thread to needle combination. For silnylon I generally use a bonded nylon T70 thread and a size 100 needle. I know some people (Roger) are going to say these are too big. I have tried using T30 with a 60/70 weight needle and T45 with an 70/80/90 weight needle and while they work OK I think the T70 with the 100 weight needle works the best. It will give a seam that is super strong.

Another thing most people mess up on is there machine tension. Too much tension and it will pucker the material. Too loose and its not going to work. Before I sew anything I take some scrap pieces of what I am about to sew and use just one layer and get everything set perfect. Thick materials are not as finicky, but thin stuff like silnylon and the .90oz and lower bag materials are. You have to set everything right or your going to have some problems.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Modified Felled Seam on 02/22/2013 13:50:54 MST Print View

I think the only need for pins is for long seams like a tent. The top piece of fabric sticks on the pressure foot a little so by the time you're at the end of the seam, the two pieces don't line up any more. I just do hand stitches but you could use pins. I just do it every 2 feet or so, and in between just use my fingers to keep the two pieces aligned.

If you were doing this for a living, then it would make sense to have the two needle machine and the folder. Running it through machine just once instead of twice would save time.

Since it's just as good as a flat felled seam, maybe we should call it a flat felled seam : )

Brendan S
(brendans) - MLife

Locale: Fruita CO
Re: Re: Modified Felled Seam on 02/22/2013 13:57:09 MST Print View

With long seams having having exact tension matters, and I've found that often the bobbin tension is a bit too tight, leading to the two pieces not lining up like Jerry mentioned. Pinning a bit can help compensate (coming from someone who avoids pins like the plague).

Damn Lawson, V69 on sil? I've never even thought to use bonded nylon on sil. I have some 46 and 69...might have to give it a try.

Edited by brendans on 02/22/2013 14:02:08 MST.

David Olsen

Locale: Steptoe Butte
Re: Re: Re: Modified Felled Seam on 02/22/2013 14:02:51 MST Print View


What you are talking about is stitching and then top stitching?

Many manufacturers that seam tape use this, since the raw edges of the fabric will be under the tape anyway.

There is a small strength gain in a flat felled seam. You can test it and see for yourself. I believe it is just the extra friction
you gain from the several fabric layers rubbing against each other. But not enough to make any real difference.


Rather than pinning a long seam, one can mark with a sharpee every foot or two on both pieces to make sure fabric
top and bottom is going through the feed evenly.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Re: Re: Modified Felled Seam on 02/22/2013 14:11:05 MST Print View

"Rather than pinning a long seam, one can mark with a sharpee every foot or two on both pieces to make sure fabric top and bottom is going through the feed evenly"

Good idea, that would actually be easier, thanks

I remember my mom sewing clothes. The patterns periodically have a little triangle cut. I assume you line up the two pieces of fabric so the triangle cut on each aligns. I suppose you could do that. Probably sharpie mark is easier.