Deck span?

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hardwoodcd

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Trying to decide what thickness deck to put on my 1648 jon. My largest span between supports will be right at 17". Can I get away with 1/2" Marine grade plywood? Maybe with some angle backing? Or should I opt for 3/4"?
 
1/2" exterior grade A/B or B/C plywood is the norm.
there is no need to go with "marine grade" plywood unless you
have the funds and really want to go that route [for peace of mind].
but even marine grade plywood must be sealed appropriately.
photos of your deck supports or drawings of what you envision will help a lot.

if you want to go with the A/B exterior, you can seal it good and apply a
couple good coats of varnish or oil base paint and it will last for years.
Then there is a rub: depending on what deck covering you will use.
some carpet adhesives do not adhere well to painted surfaces.
So, check into that before you spend a lot of time and money
on a procedure that may not serve you satisfactorily.

My past experience was a bass tournament boat that I had purchased
that had the plywood painted with cheap latex house paint and the
carpet adhesive was not compatible with it. It worked ok, but you
could tell when you walked on it that it was coming loose in spots.

research all the products you may use to ensure everything is compatible with each other.
 
What are you trying to save with the thinner plywood, cost or weight? Standard softwood plywood weights are :
1/2" 1.25#/sq ft
5/8" 1.5#/sq ft
3/4" 1.9#/sq ft
So a standard sheet of 1/2" is about 20# lighter than a standard sheet of 3/4". If you were using 10 sheets for your boat that is a lot of weight. If one sheet not so much. If boat manufacturers are using 1/2" I'm going to guess that's about maximizing profits rather than using what's best. They're buying a gillion sheets of plywood and by using thinner are saving a bundle on cost. You'd prob be using two sheets.

You also need to consider the weight distribution of what's going on top. If you weigh 200#, that weight is distributed by your two feet when standing on the deck. Considering the size of your feet (for the convenience of the argument) you are distributing that weight on approximately 1 sq/ft (using each footprint as app. 6"x12") or 100# of weight being distributed by each foot. The structure underneath has everything to do with the thickness of the deck. If you want to spend the money on a lot of structure you could reduce your ply thickness even more. The trade off is more structure vs thicker plywood. I used 1/2" on my floor screwed directly to the ribs. It's a little spongy. I used 3/4" on my deck and it's not.
 
I'll throw in these 2 cents just to be sure you got it covered. When you are spanning plywood over supports be sure to run the grain perpendicular to the supports. It's much stronger that way. Never run the grain parallel to them. 1/2" should be no problem if you do that.
 
bobberboy said:
What are you trying to save with the thinner plywood, cost or weight? Standard softwood plywood weights are :
1/2" 1.25#/sq ft
5/8" 1.5#/sq ft
3/4" 1.9#/sq ft
So a standard sheet of 1/2" is about 20# lighter than a standard sheet of 3/4". If you were using 10 sheets for your boat that is a lot of weight. If one sheet not so much. If boat manufacturers are using 1/2" I'm going to guess that's about maximizing profits rather than using what's best. They're buying a gillion sheets of plywood and by using thinner are saving a bundle on cost. You'd prob be using two sheets.

You also need to consider the weight distribution of what's going on top. If you weigh 200#, that weight is distributed by your two feet when standing on the deck. Considering the size of your feet (for the convenience of the argument) you are distributing that weight on approximately 1 sq/ft (using each footprint as app. 6"x12") or 100# of weight being distributed by each foot. The structure underneath has everything to do with the thickness of the deck. If you want to spend the money on a lot of structure you could reduce your ply thickness even more. The trade off is more structure vs thicker plywood. I used 1/2" on my floor screwed directly to the ribs. It's a little spongy. I used 3/4" on my deck and it's not.
Click to expand...

If I chose to go with 1/2" plywood, It would be to save weight. I'm not extremely worried about the extra weight going from 1/2" to 3/4" but if it's not needed then I'll save the weight.
 
Shaugh said:
I'll throw in these 2 cents just to be sure you got it covered. When you are spanning plywood over supports be sure to run the grain perpendicular to the supports. It's much stronger that way. Never run the grain parallel to them. 1/2" should be no problem if you do that.
Click to expand...

Hey Shaugh. I'm not sure I follow you on this one buddy. It's plywood, therefor it's non directional. Shouldn't be any stronger one way or the other. Maybe I'm not following you thought path.
 
plywood.jpg

when the plywood is standing on its side edge, 4 foot dimension up and 8 foot from left to right,
that is how the grain lays - this is where you will achieve the most strength from left to right.
so pay very close attention to the orientation when you cut out your structural pieces and install them.
especially in the thin sheets such as 1/4" and 1/2" thickness.


OSB - Oriented Strand Board, flakeboard, particleboard and MDF do not have grain. so only those are omnidirectional





.
 
Thanks Johnny... It's very easy to demonstrate if you have a 12 x 12 square of plywood... just put it in a vise and try to flex it in both orientations.....You'll see. It's actually one of the best characteristics of plywood, and what allows you to make solid decking from 1/2" material.

I mentioned it because I see it done wrong all the time. Your natural inclination is to use the longest pieces you can, and that invariably leads to running the grain along the supports. What you get then is a soft spot in the floor.

You need to fight that urge and cut the pieces smaller to insure they run across the supports.
 
Thanks for both of your responses. I'm not a carpenter but have been in the construction industry my entire life. I'm a hardwood flooring tradesman. So your illustrations make sense compared to how the subfloor would be laid on joists of a house. I just never gave much thought about a smaller piece of plywood being stronger one way over the other. I will be doing my own little 12"x12" test!!! In addition, even without this insight, I would have laid the deck the "right" way so my seam went across the bow of the boat.
 

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