This is not a good plan. The boards will end up bulging from the pressure of the moist earth if they are attached only at 8' intervals. Figure on tripling the post supports. Much (most) of the 2" x lumber is not treated for ground contact and therefore will not last long as the walls of a planter. If the bottom is open, you don't need extra drainage but the soil in the box, and below the box, should drain well. Unless it's adorned with some remarkable details or construction methods, I can't conceive of a long, low wooden box spanning all the way across the property as looking good. It sounds like something that most people could not wait to get rid of if they purchased a property with this already on it. I'm pondering why you would do this. If it's just to add another 2' of height to the arborvitaes I'd think finding a taller plant or waiting a little longer would be a better way to go. I do not get it. But if someone held a gun to my head and forced me to design a long lasting raised planter of such dimensions, I'd look into doing it out of some other material first. In Colorado, I'd think acquiring stone would not be as much of a problem as it is in other locales. Honestly, do you really need a raised planter like this?

When my husband put our deck on about 15yrs ago, he also built planters. They are still wonderful! Just line them with black plastic and use potting mix. This year I used the moisture control by Miracle Grow and really liked it. My boxes are about 6ft by 1 1/2 ft. If I was doing it again, I would have them a bit wider. I have 2 planters next to each other. You will love it. The pressure treated wood is not a problem at all.

I've worked on my share of roof top gardens as well, but never in as severe a climate as yours. I would beg to differ with Nandina on the suitability of a Japanese maple in the location as you've described it, I doubt it will ever look good. You don't say how deep your planters are, if you don't have sufficient depth and width of soil in these planters, anything that can't tolerate frozen soil at the roots will not survive the winter. That greatly limits the selection of woody or evergreen plants. In general, whatever you end up using, I'd suggest that your automatic irrigation system provide up to twice daily watering for a few minutes each cycle for best plant/root health. In general the deeper the soil depth you can provide, the greater possibilities for larger sized healthier growth. You might also consider using portable pots that can be set inside the permanent planters which would allow switching things out seasonally more easily If you're interested in red foliaged small trees, you might do better with Berberis thunbergeri cultivars or Cotinus coggygria cultivars as being far more sun/wind tolerant. Another cool looking accent shrub/small tree for you might include Poncirus trifoliata 'Flying Dragon' I'd suggest you use some hardy succulents in the mix such as Serum spectabile or S. rupestre Angelina, Sempervivums, grasses hardy to your zone. If you'd be willing to consider duoble potting your containers and bringing them inside in the winter, you could plant some truly sun/heat/wind tolerant plantings using architectural succulents such as Hesperaloes, Aloes,Agaves, Dasylirions, Echeverias, etc. These could thrive in the more temperate months outdoors and be brou__ght into a greenhouse or sun porch for the winter, and you could plunk into fall/winter seasonal accents in their place if you still needed something out there in winter, or maybe just park some cut greens and cut colorful branches such as Cornus stolonifera or willows for winter. I would stay away from any plants up there that can't stand up to the wind and hot sun and some drought stress. So no big or soft leaved plants that will wind burn.

Kat, I think you have the right idea, but before I can say which way to go, can you get a shot of the territory? After seeing all those roofs collapse this winter from their snow loads, I understand the issues involved with flat roofs. When you say the deck is flat with the front door, you mean there is no height difference? Do you have gutters running along that roofline? And how much of a "pitch" does your roof have? Is the roof a simple design which is like two end gables and a flat roof on each side? And, do you want to do something that covers the entire length of the deck along the front of your house, or would a simple covered entry space be adequate? Also, how old is your current roof? Is it in need of work anyway? I'll try to find a picture of MoccasinLanding. It was basically a shallow roofline, but they had built a gable across the front which created a narrow cement pad stoop, held up with 4 wrought iron posts. (each end and then in between), so the front door was centered on the peak of the shallow gable. Altogether, the stoop and the gable were about 15 feet wide I suppose. What I'm thinking is maybe a smaller gable right over the doorway which can be enclosed against all weather and a small bench out there, and then not attached to the house, but mounted on the deck, a covered freestanding pergola or arbor running from the driveway/carport/garage/walkway/whatever, that can have an angled top to encourage rain or snow to fall off the side away from your house (or deck too if it is a narrow one) yet from the street will look like a flat surfaced pergola/walkway up to the front door. You could top this pergola with acrylic or Lexan panels, or even heavy duty canvas/Sunbrella fabric, which would allow light to penetrate, and not cut off the light from the house windows. If you want to move your front door, I think there is a project that our (absent) buddy Loretta did to HER tiny front porch to make it a really sweet AWAY room just inside the door. Your use would be different, but it would be sort of the same construction of a gable etc.