Modified Felled Seam

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.

Adie

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.

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.

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.

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…

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.

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.

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.

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:

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.

> 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.
Exactly.
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.

HOWEVER,
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.

Cheers

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

Lawson,

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.

http://artisanssquare.com/sg/index.php?topic=18801.0

Ryan

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.

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.

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.

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 : )

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.

Roger-

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.

Jerry-

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.

"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.

> I think the only need for pins is for long seams like a tent.
Especially when you need to get key points all lined up correctly. Silnylon can be a pain to work with sometimes: it slides around!

Cheers

As Dave said, the marks work great on long runs.

Yep Brendan, I use bonded V69/T70 all the time for silnylon. Its what the military specs for parachutes and what hot air-balloon companies use so I figure might as well use what they use. I have to say though, the thicker thread has alot less give so you have to make sure your bobbin tension and top tension are right. I am not sure what kind of machine you have but the only downside is if your machine has a small bobbin you will be changing it out ALOT more often. If you buy pre-wound bobbins its not a big deal but if you wind your own then get ready to be frustrated.

Lawson sez: "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."

No need for a new name – it isn't faux at all. That is indeed a flat felled seam in all its glory. What people are thinking of as a genuine flat felled seam is not that; it is a lap-fold seam – which as has been pointed out is quite difficult to do without a folder attachement on the machine.

All the discussion of seam strength makes me think back over the last 40 years and I can't recall ever having a seam fail on any gear I have ever made unless it was either improperly done (bad thread tension or too small a seam allowance – been nailed by both) or due to unraveling fabric (mostly on heavier fabrics with skimpy coatings, or on uncoated fabric). So have no fear of the flat felled seam not being strong enough.

This is Judy from LightHeart Gear.

First off, a flat felled seam is different from a true lap seam – which is what is done on a double needle machine with a folder. A flat felled seam is sewn on a dingle needle machine and takes 2 (or 3 if you want) passes under the needle. A lap seam on a double needle machine is sewn in one pass – the fabric is fed into the folder side by side, the folder laps the fabric into each other ( like you hook your hands together).

You cannot do a true lapped seam without a double needle machine, so don't even try unless you want to spend the money. I use a Tex 60 (not bonded) thread on the LIghtHeart Gear tents – I recently switched to the non bonded thread as it gives a tighter seam – trust me, it's a real pain in the rear for me sewing, but on the user end, it is a much better choice. it wont' pull out of the seam if a thread should break. it also swells and fills the needle holes better than a bonded thread. From my end, I would rather use a bonded thread as it's easier to sew with, and doesn't lint up my machines, but from an end user standpoint – I want the best quality in the finished product. I also use a size 90 needle.

Yes, on a felled seam, the first pass is just through 2 layers of fabric, and those needle holes will show more than on a lapped seam because there is nothing under them to support the stitches – this came up recently here on BPL when someone had a cuben tent or tarp he just got and the holes were really showing – I pointed out that it was a felled seam, not a lapped seam –

I dont use pins ever, no iron, you make sure the seams match up with a few well placed notches, and you check several times that the bottom layer is not feeding in too much (this happens on drop feed machines) several times as you sew a long seam.

To sew on lightweight fabric such as sil or cuben, using a heavier thread such as I use and a longer stitch length, you really do need racing pullers on the machines in order to achieve a nice flat non puckered seam. Most of my machines have pullers .

Thanks
Judy
LightHeart Gear

Now you all have got me more confused than ever about the terminology, BUT, am still clear about the substance.

Looks like my text and diagram failed to communicate. Not the first time.

There is no argument about differing strengths. With any of these seams, there is a line of stitching, therefore a line of perforations, that are pulled on when the the fabric is tensioned along the seam. As with force 10 winds blowing at, and sometimes through, a tent, often in bursts that are positively scary. With poor fabric, the line of perforations will operate just as they do on a postage stamp. With good quality nylon or polyester, the perforation holes will elongate a little in the direction of where the tension comes, but the fabric will not fail.

With a flat felled seam, to use the term as defined by Judy here and on the companion thread, when those holes elongate, there will be four layers of fabric penetrated at each hole. And there will be sealer all through the needle holes penetrating those four layers of fabric. Even if the seam is sewn less than perfectly, there will be three layers of fabric penetrated.

But with a faux felled seam, often seen on cheaper tents, it is a different story. The diagram I placed in the above post is of a faux felled seam. Please excuse the term 'faux' if you don't like it. It is the most common way seen in the literature that's used to distinguish it from a flat felled seam. And I think 'faux' is a fair term, because it looks like a flat felled seam, but isn't. For that reason, I can't respond to Lawson's picture. To tell the difference between the real and faux, it is necessary to eyeball it very closely, or touch both sides of the seam with thumb and forefinger.

In the diagram I posted above, the one layer of fabric leaving the stitch line on the right has a row of stitch holes that has absolutely nothing under them except maybe some sealer if the article was also sealed on the inside. Nylon is a highly elastic material. Polyester no where near as much, but try and find good HH coated polyester in the one ounce range. So when the silnylon is tensioned away from the seam, those stitch holes will elongate, and the only thing in that long line of stitch holes between you and wind driven rain will be a miniscule amount of sealer.

I have indeed experienced the result of this when the sealer ages a bit – I got very wet and miserable. Sealing over the aged sealer doesn't work well either. Tried that. Maybe not quite so bad with silicone rather than PU, but still a problem. A problem not worth bothering with if it can be totally avoided with a flat felled seam that puts layers of fabric at all the stitch holes.

Hope everyone groks what a flat felled seam is – just two folded edges interlocked and sewn a mm or so inside the folds with two lines of stitching. Can do a diagram if needed.

With heavier fabrics and smaller items, all of the above may not be an issue, as has been suggested. In packs, simple french seams can be used, sealed, and the binder strip sewn over the seam. That's what Eureka and others do on tents. But there is no reason not to do it on packs in areas where an overlapping seam just isn't worth the trouble. With stuff sacks, though, a more water resistant seam may be worth the trouble. It is for me, having seen what happens once water gets into the pack after hiking in the open in the driving rain all day.

But a word of caution – the Dimension-Polyant fabrics behave more like Cuben than nylon when stitched; in that the stitch holes don't have the 'self-sealing' qualities of nylon. Sure, the material is 'waterproof' before sewn; but once lines of stitch holes are added at the seams … . That's one of the reasons I stuck with nylon, especially when there are lighter sil-coated nylon balloon cloths and other products out there now to experiment with.

I apologize if the above sounds pendantic, which it probably does. Sorry. Maybe some good will come of it, though.