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Does a second layer of 1/8 in ply make a real difference
Posted:
Sun Jul 05, 2009 11:46 amby Marck
I mean how besides the obvious things like something punching through, how much more structurally "stiff" does the second layer make the trailer???
I am using 2 layers , just got to wondering is all
Posted:
Sun Jul 05, 2009 12:47 pmby 48Rob
Marck,
If the two are laminated/glued together, it makes it far stiffer.
I have two sheets of 1/8" on my roof, and can walk on it, carefully, with all my 180 pounds.
Rob
Posted:
Sun Jul 05, 2009 1:18 pmby Geron
Second layer hides the "dimples" from staples/nails/screws/fasteners for a "smoother" dimple free finish. That is if it's glued on top of the first layer.
g
Posted:
Sun Jul 05, 2009 5:03 pmby Roly Nelson
My little 1/4 Nelson Stacker, has only one layer of 1/8 inch plywood on the roof, and I can push on it hard enough to make it "oil-can", which isn't good. If I were to do it again, it would be a 2 layer roof, and gosh, maybe the wooden cabin would then weigh 105 lbs instead of the present 95 lbs. Oh well, after all building teadrops is an experiment, right?
Roly
Posted:
Sun Jul 05, 2009 7:37 pmby Arne
mine has one 1/8th layer on the roof. where it is curved, no problem. I can depress it slightly where i flattened it for the vent... I'd still go with one layer (fully filled with foam insulation on the ceiling and I would not leave the flat area for the fantastic fan... it would all have a curve, and I'd just caulk the gap where the fan mounts as the curve would be so slight on the top.
Posted:
Mon Jul 06, 2009 12:58 amby Mauleskinner
I could definitely feel the difference before and after adding the second layer of plywood to mine...I'd definitely recommend two layers.
You just never know...
Posted:
Mon Jul 06, 2009 3:26 amby Marck
which kind of glue did you all use between the 2 layers of ply??
I used good ole gorilla glue (poly glue) to glue it down to the spars..(along with a healthy supply of nails/screws. But was leaning towards probond 2 for the next step of glueing the layers together
Posted:
Mon Jul 06, 2009 10:23 amby Mauleskinner
I used West System epoxy, thickened.
Posted:
Mon Jul 06, 2009 11:21 amby halfdome, Danny
Like said before, two layers of 1/8" makes a stiffer roof and smooths out the previous seams. I start the Baltic Birch from one direction for the first layer and the other direction for the second so that all first layer joints are covered. I use Tightbond ll for the glue, spread with a plastic Bondo spreader. Makes for a strong teardrop with minimal extra weight. 
Danny
Posted:
Mon Jul 06, 2009 4:42 pmby bbarry
I was surprised how stiff 1/8" ply is when bent into a curve or properly braced. When flat, as Arne says, it has some flex to it. I went with only one layer (skinned in aluminum) and am quite happy with the result. If epoxying and painting, I'd probably go for two layers.
Brad
Posted:
Tue Jul 07, 2009 9:13 pmby Marck
I start the Baltic Birch from one direction for the first layer and the other direction for the second so that all first layer joints are covered. I use Tightbond ll for the glue, spread with a plastic Bondo spreader.
EXACTLY what I did yesterday and today I see exactly how much "stiffer" it made things to have 2 layers .... now if I was going to go with aluminium (which I would love to but can't on this one) I think one layer might just be enough
Posted:
Thu Jul 09, 2009 8:23 pmby kennyrayandersen
The stiffness and strength change as a ratio of the thickness cubed
(.25)^3 / (.125)^3 = 8
It’s about 8 times as strong when the thickness is doubled. That only applies if the two sheets are glued together. If they are not fastened or glued then the strength only doubles.
Posted:
Thu Jul 09, 2009 8:33 pmby halfdome, Danny
kennyrayandersen wrote:The stiffness and strength change as a ratio of the thickness cubed
(.25)^3 / (.125)^3 = 8
It’s about 8 times as strong when the thickness is doubled. That only applies if the two sheets are glued together. If they are not fastened or glued then the strength only doubles.
Does that include the radius too? It seems it would be greatest at the tightest radius. Danny
Posted:
Thu Jul 09, 2009 8:39 pmby doug hodder
I used 2 layers of the 1/8 baltic on all the radaii on the Voyager and 1/4" ply on the rest. Like mentioned, the 2nd layer hides a lot and on those tighter curves, it's way tough. Doug
Posted:
Thu Jul 09, 2009 11:18 pmby kennyrayandersen
halfdome, Danny wrote:kennyrayandersen wrote:The stiffness and strength change as a ratio of the thickness cubed
(.25)^3 / (.125)^3 = 8
It’s about 8 times as strong when the thickness is doubled. That only applies if the two sheets are glued together. If they are not fastened or glued then the strength only doubles.
Does that include the radius too? It seems it would be greatest at the tightest radius. Danny
That calculation is just for the base material. Curves make the panel stronger (curved plate is stronger than a flat one). I thought the whole curved panel stiffness derivation might be a be beyond the call of duty!
I was just giving a simple way to compare how changed in thickness change the strength. Anyway, 1/4 is WAY stronger than 1/8, not whether or not it is needed is an entirely diofferent question