I've been reading this for a couple of days and have a few thoughts. First. red cedar shrinks when it dries out, so shiplap is a possibility since the gaps will leak soil all over the roofing material. Furtilizers will leak some nasty acidic or base stuff as well, amplifying the problem
Laying the boxes down onto the roof sounds like a recipe for rot, particularly on a flat roof. Controlled watering will help, but rain will flood unless you work out a way to drain it quickly, or cover it when it rains. I don't think I'd like climbing up onto the roof every time it might rain.
Oh my aching back. Bending down to tend to plants, especially edible plants, will get old, in, say, 6 months. And have you thought about carrying the soil back down or transfereing it to replacement boxes in a few years when your spine is that much older? I'd consider building some sort of framework to elevate the boxes, which will allow you to suspend some sort of water collection and drainage underneath the boxes. Run some heavy plastic roofing material underneath and attach to some pipe to carry the water down and away from the roof and foundation.
Minimize the wet contact with the roof since the wet framework will wick moisture to the end pieces. Perhaps a heavy coat of some sort of waterproofing rubber on the ends would limit this source of mischief. What about wrapping a couple pieces around the sides of each box and connecting them to relieve outward pressure, which I bet will bow the sides and pull out the connectors at the ends, no matter what kind.
I've used cedar extensively for water valve control box covers in the back yard, and just the wet/dry cycle will destroy the tightest of joints quickly. Personally, I'd make narrower boxes, longer with reinforced sides every 18-24 inches.
If you raise the boxes, consider attaching taller 2x4 redwood pieces centered on the boxes with a wire strung between them with clear plastic on rings so you can simply pull them over the plants when you need to control moisture. If plants are edible, leave ends open for pollinating insects. Wasps will probably love this thing, although the cedar will help with that while it's still aromatic.
This structure, no matter how you set it up, is going to be fairly high maintenance. Will the architects and engineers be available to help with fixes? Will they help pay for those fixes?
Do you have a way to set up a stairway up from the inside. If you have edible plants that would be nice. Change out one of those windows maybe? Just speculating.
Or at least a fixed outside ladder and motorized/hand cranked lift next to it? You'll be grateful you did it for decades, so will the next buyer.
Not meaning to be a negative nellie, but having made stuff, including planters, out of red cedar meant for fences, I suggest you think this through a bit more. I've made a lot of my outdoor projects out of redwood. Very forgiving stuff that turns gray but just doesn't rot.
My solution would include a 2x2 or 2x3 redwood floor set across the roof, set fairly close with a rubber drainage set underneath. I'd set it on a framework perched on top of, but above the roof, next to the point where the flat roof is affixed to the wall. Start the drainage there and continue it to the other side. I'd attach the other side of the 2xs to the top edge of the outside wall so no weight was carried by the roof, and no water would settle on the the flat roof. Run one or two drain pipes or troughs off to the side into a drain that runs 4-6 feet away from the foundation on a downhill run into a french drain, if it isn't already there.
Setting the trough bottons width wise across the boxes sounds like a better idea to me than setting them lengthwise. I also found lining with shade cloth helped, but if the soil is left wet, will only add to the wet/dry cycle. Or rather, make that the wet/dry/crack and cry cycle. I'd line the bottom with fine galvanized 1/8th mesh, a layer of pebbles and a layer of rocks. Total about 2 inches. That's what lasts longest for me.
As usual, there's my $.250 worth.
The more I do, the less I accomplish.
Last edited by DesertRatTom; 05-05-2020 at 12:04 PM.