Lightweight decking for small utility trailer?

I have a small 40x48" utility trailer that I'd like to use behind my ND2 Miata. It's an Ironton brand (Northern Tool) that is just the frame and I'd like to fabricate a floor and sides for it. I'm looking for a lightweight material that doesn't cost a fortune. I've found plastic chicken coop floor panels that seem like a decent solution. My concerns are cost and weight and possibly air resistance for the sides. I'll be using the trailer for stuff that I can't fit in the Miata, so like bags of potting soil or gravel, furniture, cases of water, etc.

What would you guys choose? 

I have a similar 4x8 harbor freight trailer, purchased about 10 years ago. I used 3/4" pressure treated plywood. It has been stored outside and still looks good. With current wood prices it might be kind of expensive though.

Expanded steel for the floor, maybe some reinforcement plates to keep it stiff.

Lightish, sturdy enough, easy to weld in place. 

At least that's been my plan if I ever get my Harbor freight 4x8 back on the road. 

I've been looking at expanded steel mesh. I'd use bolts and fender washers to hold it down. It's relatively lightweight and doesn't produce a lot of aerodynamic drag.

But a 4x4' piece of sanded birch plywood is like $23 at my local home improvement store. It's not exterior rated and only 1/2" thick but I think it would work ok. The plan is to make the sides removable so I can lean it up against the wall in the garage, so weather resistance isn't a huge concern.

Maybe I'm overthinking this? That is entirely possible lol

3/4" plywood.

The neat thing it has over metal/plastic is you can screw chunks of lumber to it, no problem.  So that transmission or engine or crossmember can be blocked up and blocked in, so the tie down straps only have to tie down, not keep located.

I have a 4X8 HF trailer, I used pressure treated plywood for decking and sides (I'm thinking the sides are about 34" tall - I can post a picture tomorrow). Mine has 5/4 stake pockets on the side, so I bought some 5/4 wood to put in those for the uprights.  I may it so the front, gate, and sides were all removable, but I've never removed anything but the gate in the 10 years or so I've owned it.  The pressure treated stuff has all held up exceptionally well, it's lived outside its entire life.  Some of the 5/4 wood is started to get a little punky and should be replaced soon, but that was regular (non-pressure treated) wood.

I think it's tough to beat plywood for the decking, and if you're not too hung up on the aerodynamics it'll probably work well for the sides, as well.  Mine does mostly in-town trips to the dump, local Home Depot, etc. so I can't speak much about the aerodynamic drag on the highway.  It's been towed behind our old RAM pickup and our Rav4, so it's still significantly shorter than the tow vehicle.

Plywood, 1/2" is probably okay, but you really want something outdoor rated. Or at least do something to seal it. If you're using it to actually haul stuff  (versus the usual tire trailer application) I wouldn't want the floor to have a bunch of holes in it.

My trailer is 85 x 57 approx and uses aluminum  checker plate glued and pop riveted to the frame.  You may need to consider span especially if you have high point loads. I regularly cart a yard of top soil which is about 2,000 lb. 
 

For your amusement my current car is a diesel Focus 4 ...

Head to your local metal scrap/recycler and see if you can walk around the property. You might be surprised what you find. At my local spot, there's always scraps of expanded materials(steel, galvanized and aluminum), cut-off pieces of drainage grates, perforated metal/aluminum, angle and other "treasures". Looking inside old box trucks and work vans usually reveals some sort of shelving that can be used. Also, there is a pile of slightly used chrome restaurant-style shelf units which can be easily adapted to the sides and possibly the floor and would give you a very light weight trailer when done.

Just a few thoughts......

In reply to dps214 :

From experience, the wood doesn't get holey, it starts to delaminate.

My wheel spindles are unstressed in use and bolt through the deck to a 30x3x3/16" plate that does a good job of keeping it together, but I discovered that the decking has a five year lifespan if left unsealed when I was picking an engine up and the screws for my cribbings just fell into the decking.  I was able to screw into them from underneath and turn them into structural cribbings to get me home.

3/4 treated plywood will last a long time. It's what I have used in my 4x8 utility trailer for the last 30 years. I'm on my second sheet.

Personally, I don't like expanded metal in a utility trailer. It stops you from hauling sand, gravel, or mulch for the yard unless you are going to put down a tarp or plywood first.

I built my 5x8 trailer about 10 years ago, and used 3/4" PT plywood.  It's just now starting to delaminated on the bottom, and should be replaced.  The sides are holding up fine, though.  I also use mine for mulch, dirt, yard debris, etc and sometimes wet stuff sits in it for long periods.  I've also used it to haul scrap metal and stone.  So it's lived a brutal life. 

Aluminum will corrode, and is otherwise somewhat difficult to work with.  Steel is bloody heavy.  A half-sheet of 3/4 plywood weighs about 30 lbs.  I know you're worried about weight, but in terms of weight to strength ratio, flexibility, easiness to work with, cost, etc...wood is where it's at. 

EDIT: for giggles I looked up ABS plastic...1/4" ABS sheet would be about 2/3 of the weight; 1/2" would be heavier.  Strength and cost would be in the same ballpark.  Not sure about long-term longevity, if you don't leave it out in the sun it would probably be fine.   You could get it in any color you wanted, molded through, so that's a plus. 

In reply to Pete. (l33t FS) :

Maybe my wording wasn't clear. I meant if I was going to be hauling gravel/dirt/loose,semi-fragile items, I wouldn't want a deck that's 50% open space (ie expanded metal).

I had a 1/2" piece of plywood on the bottom of mine. Worked well, but, when you're getting the wood, swing by the paint department and snag a quart of their "ooops" paint. Just look for an exterior paint to cover the plywood and make it last longer. At my HD the ooops paint cans are like $4. You can also just get a quart of rust oleum.

Mine was store outside for a couple of years and driven in rain regularly and never delaminated or warped. 

-Rob

I was given one of those trailers and it had plywood floor and sides, but it didn't age well, and the plywood looked scabby. So I built a bolt on frame for the sides and used expanded metal pieces I got free from work. For the floor I'd planned on using 5/4 decking but when I went to get some they didn't have what I wanted so I looked around and settled on pressure treated fence boards. They're thinner and very light - and very inexpensive, so I wasn't sure how they'd hold up, but that was 10 years ago and they still look good. I guess you could also use them for the sides as well.

It might be too expensive these days, but I used PT inch boards (3/4 thick by 7.5") instead of plywood, having seen delaminated plywood on so many trailers. 
I have used fence picket boards for a few other projects, but they are usually somewhat thinner, and a rough cut finish. A friend used 5/4  x 6 x 16' horizontal fencing on his car trailer. 

1/2" plywood lasted for a long time on my utility trailer, it was used to haul everything from dirt and lumber to motorcycles to engine blocks.  For a trailer the size of yours, you should be able to store it inside the garage tipped up against the wall so it won't see that much weather, but a couple coats of paint will certainly help protect it.

I've had 1/2" treated plywood on my HF 4x8 trailer for a couple years and it's holding up well, including a year plus sitting outside when I didn't have room for it in the garage. Letting leaves and brush sit in it for too long discolored it, but structurally it's still solid. I should paint it or something eventually. 1/2" was cheaper and lighter than 3/4" and is plenty strong.

I used carriage bolts along with these "torque washers" to attach the plywood to the frame. Carriage bolts are nice for this because they don't have as much of an edge for anything to catch on as regular hex bolts.

I approve of towing a trailer behind a sports car.

In reply to obsolete :

I had not seen those washers... I like!

Ive also found that protecting the edges of the plywood helps with longevity. 

No clue on trailer decking, but if we're posting pictures of sports cars towing trailers, here's one I saw a few months back. 

No clue who the driver is but I give them props for being awesome.