Expert: Nazim Nice - 4/1/2009

Question
Hi Nazim,
We're interested in building movable containers for vegetables on our wood deck.  Obviously, there are a number of issues to consider and I'd like your thoughts, please.  We want to construct three 4' by 4' wood containers, approx. 2 feet high, and lifted off the deck by caster wheels.  These wheels will help us to move the containers around the deck to avoid rotting on the deck.  

In terms of the deck itself, what parameters should we consider re: structural integrity and weight.  By the dimensions above, can you estimate the total weight (adding soil at a depth of approx 18" in each container) and determine if we have enough support, or will need more?

With regard to the containers: If I can determine the specific sizes of wood, should I go to Home Depot or a lumber yard to have them pre-cut the parts?  Not sure who would do better or for less.  I've read that it's good to use pre-treated wood, and a number of varieties are mentioned.  What do you suggest?  Cedar?  

Lastly, any ideas on what to use for the base of the container?  Obviously we need something porous, but overly so, since we need to retain soil.  What materials would you suggest?

I know: Lots of questions.  You don't need to reply to each part, but I would not mind your thoughts and opinions on this project.  Our home is surrounded by tall trees and the deck is the only spot that gets enough sun.  We live in northern Virginia.  Thank you very much!

Answer
Hi Lew,

Well you've already identified that these planters are a structural issue. I can't offer specific advice on what you need to do to reinforce the deck but I can offer you advice on the steps to follow.

I would recommend looking for a structural engineer in your area who can do some quick calculations to help you determine if the deck needs reinforcing.  This may involve doubling up existing joists or adding additional supports.  It may also mean revising how the deck is attached to the house.  Many decks have failed because they were not correctly attached to the house, and the decks literally pull away from the house and collapse when there is too much load on them.  Definitely something you don't want!

To find a structural engineer, you can look online or the phone book, but you might also try calling a small architecture firm and asking for a recommendation from them (for a small project).  

Regarding the weight of the planters, keep in mind once the plants are watered, the weight will increase substantially.  If you do not need the full soil depth in the planters you could consider filling the bottom with chunks of Styrofoam to reduce the weight.  

If you go to a Home Depot to get the lumber and have them cut it, be sure you have a detailed cut list, which means designing these planters before you go and knowing how you're going to put them together.  You'll probably need to do some more research on how exactly they are built (I'm no expert on this), but I would suggest that they either be built of treated lumber or lined on the inside with a waterproof membrane to extend the life of the wood.  

I wonder if using a product like Trex would work for the sides of the planters?  It is a product made from recycled plastics and wouldn't be damaged by the soil or water.  You'll probably need 2x4 or 4x4 supports in the corners (and possibly in the middle of each side) to keep it all together and maybe something to cap the top so it looks finished.  

Also consider using pressure treated plywood if you need a sheet product.  Look for wood products approved for ground contact to get the most life out of them.

For drainage, the old standby is putting rocks in the bottom. You might try this with a couple layers of landscape filter fabric on top, then the soil.  This will help keep the rock free draining.  There are also architectural products that are meant for waterproofing basements which incorporate a plastic drainage mat plus a layer of filter fabric. This might be a high-tech solution to allow for drainage without adding much weight.  You could actually line the entire box with this on the inside so no soil is in contract with the box.

For the actual bottom of the planter I think you'll probably need a sheet product because of the weight, which is why I suggest the pressure treated plywood.  You'll need to add drainage holes though, and I would look into waterproofing the inside with plastic or some sort of roll on waterproofing product.

Other materials you could consider if you want to use wood are Cedar and Ironwood (sometimes called IPE - pronounced e-pay).  You'll probably need to go to a lumber yard (not Home Depot) for the IPE but it will last longer than the cedar but is more expensive.

Hope this helps to get you going in the right direction.  Good Luck!

Nazim Nice
Motionspace Architecture + Design PLLC