Modified Felled Seam

Here is a drawing I did comparing the seams. The right way to do a flat felled seam, the wrong way to do a flat felled seam, and the knock off seam. The red lines signify the stitching.

In my opinion, I think the right way to do a flat felled seam and a knock off seam with a third row of stitching are very similar in strength and I would take a piece of gear with either of them. If you have a double needle machine then you should do a true flat felled and get the seam done in one pass. If you have a single needle machine then you should do the knock off seam and for added strength its always a good idea to do a third row of stitching.

Good Luck.

the triangle cut is called a notch. They are, as you surmised use to ensure pieces are lined up correctly, and sometimes to make it clear which is the front and what's the back (consider a one piece sleeve, it's hard to figure out without thinking hard. Notches are easier.) They solve a different problem than the proposed sharpie marks. (Also, I've never seen a gear pattern that's included any, but that's another problem.)

When I'm doing things that I really care to get lined up exactly right, I will draw two lines, one on the top piece, showing my where the first line of stitching belongs, the second on the bottom showing where the edge of the top piece should be. I don't use a sharpie, but a pencil or chalk, something that ill come out. (and which makes finer line. Even a fine tipped sharpie line is huge.) I've also got a collection of compensating feet (sadly, not generally available for home machines), which make the job easier.

I can't imagine using #69 bonded nyon on even 70d silnylon. Most of my work is with tex 30 and 40 threads and 11 and 12 (75 and 80 metric, I think) needles. I don't have a problem getting tensions set up right for those combinations.

> If you have a single needle machine then you should do the knock off seam and for
> added strength its always a good idea to do a third row of stitching.
+1

Cheers

I usually put a line where I want the first row of stitches to be, on both pieces of fabric.

I use Sharpie. It tends to come off over time, and I don't mind layout lines on finished product. Maybe not on a piece of clothing, but on a tent, I don't mind being reminded how I sewed it : )

I played around with this a bit last weekend and a lot of what I said above didn't end up being true. Adding extra layers does improve strength, as does a third stitch, although provided your stitching is straight and parallel I think the modified felled (or whatever we're calling it now) is plenty strong.

If the extra bit of fabric backing up the holes matters (not sure it does, especially if using a non adhesive sealer), here's a solution that's not really any more difficult that the regular modified felled seam, plus adds an extra layer for the first stitch:

Basically, rather than sewing the two pieces together flat, fold over the seam allowance that you normally fold under in step 2 and sew through that in step 1

Step 2 same as usual

You have a raw edge, but doesn't really matter with sil, especially after it's sealed. Folding the "flap" to the left in the above photo in step 2 hides the raw edge and gives the extra strength (which does reduce the hole elongation) but doesn't have the little bit of fabric covering the hole.

Brendan's way looks very promising as well and as you can see its a nice looking seam. I will have to give this way a try.

I completely agree that adding extra layers increases the strength. Its the reason parachute and hot air balloon manufacturers add tape to the bottom side of seams. Extra reinforcement = extra strength.

Thanks a lot guys for the thorough research. This one is getting bookmarked!

Brendan,
Is that a singer rocketeer? I picked up a 503a a couple months back, life has not been the same since :)

Good eye, it's actually a 401a. I have two industrial machines and that singer's still my favorite to use, especially for light fabrics. I found it at a thrift store, cleaned it up to sell, and that was three years ago. Don't think it's going anywhere.

To get a flat seam where the strength is spread over the two seams, I glue the silnylon with diluted silicone.
A short movie

Ivo, did you have any trouble with the sewing machine needle or thread sticking to the silicone while sewing?

BTW, after you are done with the seams, you will have a lot of silicone on each- three layers. Wouldn't it be more efficient to just do it at the end? Only one layer and the time saved?

I did appreciate the movie.

Tad,
After about 15 min, the silicone has hardened and no longer sticky.
(Depending on how diluted.
and time it was created.)

In the end,
The silicone is very thin and final not that heavy
I do not like pinning the fabric.
And I want a very flat seam

For a long side, the fabric must (In the second phase) be aligned again with a line of pins.

Then, I use a long aluminum bar to put pressure.

Ivo, what brand of silicone are you using- to get the 15 min drying times? Sounds like I need to get some.

Ivo,
I've found with silnylon that bonding the seam takes away much of the elasticity of the fabric, but just at the seam. This prevented the tent canopy from becoming taut.
When the seam was redone without adhesive, just thread at tension low enough to allow some stretch, the canopy became fully taut when tightened over the pole frame.

But I was using undiluted GE silicone glue. Perhaps your diluted mixture will yield a different result.

Lawson,
Thank you for those excellent seam diagrams. They really cleared up for me what you were saying.

I really wonder if the second, or wrong way diagram is really all that wrong. The reason being that a stitch line on the right way diagram that is less than a mm from the raw edge of material may not contribute all that much to the seam's overall strength. If one just sewed an edge to another piece of fabric with a stitch line no more than a mm from the raw edge, it would be very easy to rip the edge off, as the thread would just pull out through the one mm of material. I don't think the elastic silcoat on silnylon is going to retard that very much from happening. On the other hand, if sewing the seam the right way is going to add significantly to strength, I don't mind taking the extra care.

Your third diagram, the knock-off, appears to be a modified, or rolled seam along the lines that the OP had in mind. Without knowing exactly how strong it is, i agree that it is plenty strong for its purpose. The only issue I have with it is stitch holes opening up at one end of the seam, and thereby compromising the water resistance of the canopy material.

Your first, or right way diagram, appears to be the flat felled seam that is frequently used on high quality tents and tarps. Hope that is correct.

Brendan,
Without being sure that my understanding of the seam you pictured is correct, it appears to be a very savvy way of getting the benefits of a flat felled seam without a double needle machine, or having to use pins. As best I understand, it does add a few extra layers of fabric to the seam, but the weight added may be negligible. One reason for pins, is to help insure that the fabric on one side of the seam doesn't stretch more than the fabric on the other, so that the fabric panels at the end of the seam don't line up as intended.

One dilemma for me is that when there is no pattern, and I am tailoring the netting door and vestibule to the front edge of the canopy with pins, the rolled seam seems to be the only approach that comes to mind. This does allow the stitch holes at the front of the canopy/vestible seam to elongate and potentially leak. I'm still trying to come to grips with that one.

That knock off diagram seam has 5 layers leading to unequal seam allowances .

'Faux' flat felled has 4. Equal seam allowances.

I know which I'll use when measuring panels.

KISS anyone?

Actually, the reason there are five layers is because the seam allowances are the same for the first seam; the two edges of the fabric are aligned, then the seam is sewn with a large seam allowance, then the seam is flattened out, and the seam allowances are folded in half and tucked under itself. Then you sew two lines of stitching through the five layers.

It looks like the seam allowances are different because the diagram is a much-expanded view; in reality the 'vertical' distances are minimal.

You can use double and single seam allowances to make a double-lapped seam, in which case the single allowance is on the bottom of the stack as shown, and the double allowance is folded under it.

Ivo, Great idea to avoid the hassle of pinning. Would you share more information about your diluted silicone mixture (like mixture ratios , etc.) ?

I said:
That knock off diagram seam has 5 layers leading to unequal seam allowances .

'Faux' flat felled has 4. Equal seam allowances.

I know which I'll use when measuring panels.

KISS anyone?

Kevin said:
Actually, the reason there are five layers is because the seam allowances are the same for the first seam; the two edges of the fabric are aligned, then the seam is sewn with a large seam allowance, then the seam is flattened out, and the seam allowances are folded in half and tucked under itself. Then you sew two lines of stitching through the five layers.

It looks like the seam allowances are different because the diagram is a much-expanded view; in reality the 'vertical' distances are minimal.

You can use double and single seam allowances to make a double-lapped seam, in which case the single allowance is on the bottom of the stack as shown, and the double allowance is folded under it.

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I think you don't understand what I mean.

If 2 equal sized panels are cut and sewn together (e.g. flat tarp ridgeline):

With the 'faux' flat felled (traditional)seam the centre of the seam (where you might sew a 3rd line of stitches) is the centre of the full tarp. (Because the pieces are first sewn with one edge one seam allowance away from the other)

With the Knock off, the centre of the tarp will be now at one edge of the seam not in its' centre – so an extra seam allowance width of fabric will be necessary measured on one piece to produce equal sized panels. (If position either side of centre of the seam as is usual)

I just made the knock off seam fold several times with 2 equal sized pieces of paper and found this to be the case.

If I'm mistaken here, please show me how! (Scratches head!)

But if you're making a tarp does it really matter that the left side is 1/2" narrower than the right side?

Wade Ford says:
"But if you're making a tarp does it really matter that the left side is 1/2" narrower than the right side?"

Probably not, but it will matter a lot in something with panels that need to fit together properly, as in a tent or pack.

The usual way to make a mock felled seam is not with the edges of the fabric alligned, but with uneven allowances like this[1]:

That's half inch on the bottom, quarter on the top. Then you fold the longer allowance around the shorter one, and stitch. That gives two visible lines of stiching on the side with the added bulk, one on the other.

And third pass or not as fancy strikes you.

You will see, particularly in home sewing patterns, instructions telling you to sew the allowances even, and then trim one of them. That's a pain to do, but it's another option. I don't trust my self to cut the right allowance, either, nor to not cut too much or too little.

[1] that first picture also shows why this is not considered a lapped seam. the two pieces don't over lap, they're superimposed, like a normal plain seam, but with the offset.

Samuel,
you are right, using undiluted silicone takes away much of the elasticity but with a catenary cut there is no problem.
Undiluted silicone makes the shelter heavier .
My first trailstar copy was about 100 g heavier than needed . There I used undiluted silicone

Now, the only intention was a temporary and no permanent bonding. It makes the silnylon less thin and easier to handle.
If strength is important then you should use undiluted silicone I think.
See: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=51361
I used pure silicone without additives. Dilution is not so important.
Slightly thicker than olive oil. See movie.
Too thin, then it does not stick.
Thick, heavier than necessary and you have to work fast and the solution has a short shelf life

" the only intention was a temporary and no permanent bonding

Didn't get that till now. Thanks.

> I think you don't understand what I mean.

You're right; I didn't. And, as you say, the resulting seam will be off-centre (the initial seam is roughly the centre of the panels). So, if you want the centre of the folded seam to be the panel centre, you will need to adjust the both panel designs to account for this, or, at least, move the initial seam point by half the folded seam width.

Making a four-layer, equal seam allowance, faux lap seam has other difficulties if you want to run two seams through all four layers on both seams. Not insurmountable, but about the same degree of non-KISS as the five-layer seam…