Which lap is stiffer?

Ok, theres been a lot of discussion going on about flex and stiffness and I’ve been thinking a lot about it too.

What I’m wondering now is; if I only where going for one rail lap a a certain place of the board which one would be stiffer? The lap from the bottom glass or the lap from the deck glass?

Or maybe there’s no difference between the two?

Thanks, Jimmy yoshio shibata.

Back in the late 60’s and early 70’s there was a lap called “zippered” which was around for sometime.To accomplish this the bottom was glassed as normal (usually a cut lap) and the deck was laminated leaving the glass hanging without tucking the lap under.When the resin jelled you just trimmed the overhang and hotcoated the board.This eliminated the bumps caused by the lap on the bottom.I never had any problems but consider we were using foam that was a bit denser and 6 or 7.5 ounce glass.I realize this doesn’t answer your question but I just tossed it in anyway. RB

Dam Mr. Clean it great to have you back…I just did a zipper cut on a board the other day. rail cut lap, deck inlay, then Zipper cut in the over lay on the deck lap, all in 6 oz. No sand, just hotcoat.

Welcome back Cleanlines,

I’ve did a zipper lap too. My lam went off too soon and I had no choice. mike

Cleanlines, Good to see you back!! Hey Rooster, thats freakin funny and yep I’ve been there too.


Stiffness, the lap from the whole bottom into the deck makes the board stiffer, since it’s perpendicular to the direction of flex.

how about the lap at the apex of the rail

im with you guys on the zipper lap

just seems the natural way to do it

Cleanlines, I’ve done a couple of hempboards in the past and have used the “zippered” lap as you mentioned. The glassed hemp doesn’t sand good at all so I wanted to glass without sanding the laps, so a “zippered” lap solved my problem. I did trim the hemp exactly at the first cut lap so it got totally flush.

Jimmy yoshio shibata.

Thanks Lee for answering my question.

I was thinking like this, a normal lap around the middle of the board for stiffness,and then a more flexier lap at nose and tail which would come from either bottom OR deck depending on how much flex you wanted.

Silly, do you mean that the laps meet at the apex of the rail?

Jimmy yoshio shibata.


how about the lap at the apex of the rail

Shouldn’t bee too bad, less shear there than on the bottom. I’d say rating from flexy to stiff:

  1. deck

  2. rail

  3. bottom

Since the lapped area is thicker thus stiffer, keeping it away from the bottom that experience the most shear makes sense IMHO (everything else being equal of course). Not that I think it matters too much though.



" Which lap is stiffer ? "

W-E-L-L , let’s see …

I think MY … er … “lap” would be , after THESE “laminators” had done their “work”…

[So, THERE you have it eh ?

…another example of why people say “Chipper , get your mind ABOVE your navel”]

Good one Ben!


if your using epoxy resin you can control flex better with different resin mixes. If you do not have access to RR epoxy than you can dilute your resin with a solvent like denatured alcohol or xylene to make the resin more flexible. 10 to 20% dilution works good. But the laminate will be softer…ok if your using it on balsa but on eps foam it may damage easy…

i wouldnt make the front half of the board too flexible if i were you…proceed with caution


We used to taper the laps on some of our smaller wave boards, so much less at the nose and tail, even zipped, and lots of lap just past the widepoints where they tend to break.

Coupled with flat decks and boxy rails, you can actually feel the flex when you turn hard in small waves.

Not saying it’s a particularly great idea, as we stopped doing that after a while.

what in trying to say is that on a lap there is two layers of glass on the apex of the rail and this has a lot do with the stiffnes of the laminate.I dont really know how to apply this concept to flex, but have been playing with tape and using one layer at the apex .I like to be able to control the stiffnes by the way i lap or use tape in places over a lap

in saying that though using high density balsa and 4 oz under changes the whole equation

I found that the higher density balsa and thicker stringers

i can get away with less glass on the rail… flex and stiffnes from the wood itself is far more desirable than stiffness from the outer glass ,especially on the rail apex.

just my opinion though

Hi meecrafty, I have got access ro RR epoxy here in japan, but I think they only stock RR 2000. Will give the distributor a call to check that out.

But thanks for the trick about diluting the resin with alcohol…will test that if the distributors do not stock RR 2020 and RR 2040. And yes will be using over balsa and other woods.

LeeD, so you must be saying that your boards broke after a while,just where the laps got lighter?

Silly, I’ve also played a little with different densities of balsa on rails and deck.

I think tuning flex is a combination of materials,design of board and how much resin and glass you use. The combinations are endless…which makes sandwich construction even more FUN!

Jimmy yoshio shibata.

Actually, I got tired of taping off the tapered line for each cutlap and making pinstripes to be really even on both sides.

And when we went into production, our two glassers just looked at my boards and laughed, saying they thought it was a waste of time and who really notices a slight difference in flex.

Oh well, I was just the shaper, not the boss…

The zipper lap is interesting. (Nice to have you back CleanLines!)

In my opinion, the lap is the secondary flex/stress area to the stringer. I think you should lap two layers, and add/subtract the third inner deck layer rail lap depending on your flex requirements. There are also many small things a shaper can do to engineer flex.

it does make the resin considerably weaker though,to use a thinner to make a resin flexible.

after a while the thinner dissapears leaving an incomplete matrix

there maybe other ways but i havent looked into it

okay say the resin on the rail is flexi.

whats the point if your flex patterns are satisfactory already?

does it make it more impact resistant/tougher?

tougher like rubber.

I think your thinking is flawed.

Without solid laps, the board is allowed to flex a certain amount, breaking down the resins that hold the fiberglass together, leading to certain snappage failure.

Wood stringers tend to give right to the point of glass failure, so you can’t rely solely on the stringer to provide antibreaking.

Both have to incorporated, and ratio still being experimented with…

I’d rather have too stiff a board that resists breaking than a flexi board that might ride great in medium waves, but fracture from lack of glass.

Big laps have proven over time to resist breakage and still somewhat work.

Then perhaps you should reread the thread.

He was asking about lapping only one and whether it would affect flex, if one bottom or one layer top would be better.

I suggested he lap at least two, which is the shortboard standard. (Trimming the 2nd deck glass layer underneath on the upper rail. Which is the way it is typically done.) If he wants it stiffer then he could fully lap all three layers.