a tale of woe - how much strength do laps add?

I’ll start with a general question and then tell my story of when things go wrong.

Question: How much strength does a bottom lap add?

Story: I’m making a Walker Foam blank 10 footer, 3.25 thick, 60/40 rails with a tucked under edge meeting 3" rail chines on the bottom using UV cure poly. (I’ve made plenty in epoxy/eps sandwich etc., so you will be preaching to the choir if you say it could have been lighter/stronger.) Now my shed is 8x12, so doing a 10 footer is a squeeze. Anyway, the bottom lam with 6 oz goes fine, standard lap onto the deck. With the UV resin, I haven’t been measing it out, just pour straight out of the one gallon container. After the bottom, I knew I had at least a couple of quarts left in the jug, so 2x6 oz with a 4 oz patch on the deck and start pouring down the center, get a nice puddle going and set the jug down to squeegee out to the rail. After a couple of passes around the board, I go back to get the jug for extra resin down the rails and much to my horror, find that I have some how knocked it over into my trash pail (ok the music might have been a little to loud). Thankfully I had done a serious first pour, but the sad part is this is the last of my lam resin, on a Sunday afternoon with the sun going down. With some furious squeeqee work, I am able to wet the deck out, but don’t have enough to fully wet the laps just those steaky runs. So I squeege down to the tucked edge, out in the sun and then “zip lap” around the bottom tucked edge. All things considered, it came out pretty nice, but now I’m wondering how much strength will I lose by not having a bottom lap? Sure will make the bottom sanding easy though.

Any thoughts out there?

Howzit native, By cutting the lap to short you compromise the tubular rail strength of the glass job. This is important since it is an intregal strength area that can cause the board to break easier. Ever notice how a ding in the rail can be the starting point of a buckle or a break in a board, that's because the rails have the most glass there for the most strength. One of the boards I'm glassing now has doulbe wings and after I did the bottom I glass over the deck area where the lapped glass was cut at the wings since the cuts leave a non glassed area on the lap other wise the board would have 4 weak spots where you don't want weakness.Aloha,Kokua

Defintley agree with Kokua on teh tubular strength. The rails, when glassed corretly, act much like the rims on a spoked wheel. Sure, the tensile strength of the spokes is what gives the wheel its rigidity, but without a strong outer structure to anchor those spokes, the wheel would lose torsional strength. Obvioulsy the stronger the outer anchoring layer is, the stronger your overall board will be.

If I were you , I’d look inot anohter adding another layer of glass wrapping the rails. Either top ot bottom. Even a a layer of 4oz would make a big difference, but hey, it’s a 10 foot log, so go heavier.


Get some 4" wide fiberglass tape & go around the perimeter with that. You can lam & hotcoat each side in your shaping stands without flipping. Strength without weight.

Drew and Kokua are right. I’ve been adding a 4 oz. perimeter lam (2" each side) on my lighter weight LB’s and found that it also adds a lot of rigidity to reduce flexing. On 2-1/2 thick x 9’ + boards, this an important feature IMHO. Also, most 10’ + boards are ding magnets on the rails and the extra glassing helps.

most 10' + boards are ding magnets on the rails and the extra glassing helps.

Why is it that big boards ding rails so easy?

Is it beacuse your shapping down into much lower density foam on a longboard blank when doing the rails? Especially on a pinched rail from a thick blank?

ding magnets - I hear ya. My former 10’3" tandem board was ALWAYS banging into something. I could carry any 9-footer anywhere, but the big board just had to whack one or the other end on every other corner. Not like I was careless, either; you’d be too if you had all that to patch. They just do it.

That said, I gotta disagree about ultimate strength. Look at buckled or nearly snapped boards. The first sign of near-failure is often a bunch of stress cracks in the finish coat, running across the board. The board bent enough that the brittle finish coat couldn’t take it. This is a near miss.

But a real close-to-failure event has a ridge across the side in compression, where the board couldn’t take the shear, and the compression side glass has separated from the foam. It’s the foam/glass bond that has failed, and a board exhibiting this structure probably has serious (terminal) interior damage. If it’s not actually floppy, after another thrash it will be.

So, I don’t think that not lapping will have your board prone to breaking in half. You will, however, have weaker, easily dinged, rails.

Thanks for all the great input. I hadn’t really thought about the ding factor as I don’t have a board over 8 foot for myself, but just moving this log around I have banged it several times and that is being super careful. So, I think I’ll add a lap just for some more protection. I was already thinking of Benny’s 4" tape idea.

Does anyone know if the narrow fiberglass tapes go off as clear as Hexvell or BGF? This board has a clear bottom and I don’t want a nasty looking lap line.

Thanks again for the help.


Howzit native, The tape will be invisable after laminating, I've used it plenty of times as a strengthener down the middle of the bottom of boards.Aloha,Kokua

Mahalo Kokua…you’re the man