They need to be deep enough to accommodate roots - depends on what veggies you want to grow. I don't think 10" is deep enough for many, for best growth. What are you planning on growing? Any mix will dry up faster in shallow containers, and even faster if it is windy.

You can *significantly* cut down on evaporation by "plasticulturing" your container. Ie: you put a plastic liner (ie: a bag) over top and plant your plants through them. Helps keep the water in the soil, not the air. Unscientific experiment (tomatoes in 5gal paint bucket, +28C out, balcony, southern view). No plastic: water every other day, container weighs very little. 1 white plastic grocery bag on top: water every 5th day. Lots of ways other than plastic film, plastic is ugly after all, but none are as thin.

From the pictures I would think herbs, strawberries, etc. with small roots would be okay. Tomatoes not a chance.

I was at Costco the other day and they had raised bed planters. I didn't measure, but the soil depth looked about 10-12" (tapering to a point, so perhaps as shallow as 8")

One issue on a balcony: when you water the person on the 1st floor will get all the run-off splashing onto their legs when you water. Water collection keeps you a nice neighbour.

Our balcony drains on a gardenia hedge so no issues with neighbours. The gardenia enjoys the water I think because our side of the hedge is more lush than the neighbours.

The timber I plan to use is 7 inches wide so the soil will only be 6-6.5 inches.

I want to grow lettuce, broccoli, kale, lemongrass, basil and coriander.

If the bed is going to be too shallow, what are my other options? What is the depth I need to be looking for?

The container needs to be elevated because the balustrade is solid and blocks out the sun if the containers are on the floor.

Make the bed deeper (14") by stacking two 1x8's on top of each other? or use 1x12's instead? From the picture it looks like you have about 3' of space to go 'down' while keeping the same bed height. The roots don't need light :)

Another possibility is instead of having a 'bed' only build the frame up to the bottom shelf that holds the soil in, cut circular holes in them to hold whatever pot you want and drop them in the hole. The "outdoor" version of something like this: https://goo.gl/images/s493p9

I think that may be shallow, why don't you add another board to make it deeper? I think that 10" is minimum, and deeper possibly better. Lettuce may be OK in shallower; but kale, basil and others I grow have quite deep roots when I take them out at the end of season. I grew basil (only once) in 7" deep pot, and it was just so-so. Others grown in deep pots or in ground always grow much larger and healthier. Coriander has a long taproot, and if stressed (as it could be in shallow container), will rush to bloom and produce flowers and seeds - you want good foliage. Other could do the same. It is very good idea to elevate boxes, they will all need good sun.

Edenchild - I’ve seen this but they are quite expensive. I think building this would beyond my current skillset.

I’ve been shopping around and I found an elevated garden bed that is similar to what I want. It’s 3 feet long x 1.5 feet wide and 1 foot deep.

It should be deep enough, I think. I can plant the ones with deeper roots in a separate container.

Is it worth doing a 5:1:1 mix for this or shoud I just used normal potting mix?

What do you consider a "normal" potting mix?

@funkyhat - bagged potting mix from any garden store.

The high-end soilless mediums are better, the 'cheap' stuff is sometimes cut with actual soil. As long as it's a soilless mix it's probably just fine.If evaporation is a concern something with added vermiculite will help retain moisture and the coir/peat/vermiculite will provide the drainage and space for toots to grow. Sometimes the perlite floats to the top, so you can put mulch or something on top to weigh it down.

The answer to if the 5:1:1 will get too dry? I don't know. How hot does it get where you are and how much do you like watering? If you don't mind watering every day at high summer it probably won't be a problem. I have no experience. I defer to others (others: this is your queue).