Shrubs in Tubs - Part 1 - Load Panels

Shrubs in Tubs - Part 1 - Load Panels

The garage in our home extends beyond the edge of the house by about 5 ft (1.3m) in two dimensions, both south and west.  The builders built a parapet around this extension that is 24 in (.6m) high. The southerly trough is sunny and quite exposed. The westerly trough is "part sun" and is more sheltered from winds as it lies between us and the neighbouring home.

               

From the structural analysis we quickly learned that we could not fill the trough with growing medium otherwise that corner of the house would collapse. 

There was no real point putting sedums here especially when we had an opportunity to do something a little more creative and dramatic.  Like "shrubs in tubs".

However, the roof joists would not easily carry the concentrated loads from a tub (20" dia, 0.5m). So we ended up with a fleet of load distribution panels to spread the load across as many joists as possible. The loading maths works out with one shrub in a tub per panel, on average.

Each panel consisted of two sheets of 3/4" crezon plywood, glued and screwed and painted. 

Crezon is the commercial name for an exterior grade plywood, specially treated and prepared for the sign industry.. It is also described as MDO - Medium Density Outdoor -Plywood

MDO Plywood - The Paintable General Purpose Plywood Panel

 An EXTERIOR type plywood with a weather-resistant resin overlay bonded to the wood by heat and pressure. This process fuses the molecules of the overlay with the fibres of the wood to form a bond as strong as the wood itself. MDO has all the advantages of regular plywood as well as additional properties. The overlay, which has 28% resin content, resists water, weather, wear and degradation. It has texture that paint can grip with remarkable tenacity. Paint finishes on MDO are up to three times more durable than the same finish applied to ordinary plywood.

Crezon is not cheap ($70 for 4' by 8' by 3/4" sheet) but it is the right material for the purpose.

The load distribution panels were sized to give about 2" (50mm) clearance against the parapet and house walls.  They are all about 40" (1m) square in the south trough, and 48" (1.2m) square in the slightly wider west trough.

They are glued together and screwed. They are also painted with 4 coats of a premium outdoor latex paint. I don't plan to replace them in my tenure here!.

This next picture shows how they are laid out within the troughs.

West Trough

Within the troughs, the load distribution panels will be underlain by many (6) layers of the drainage layer material.  It is about 3/4" thick (12mm) (per layer) and is very springy.  The springiness helps with the even distribution of the load across the bearing surface, as well as providing superb drainage.

[[ These panels were actually hoisted into place after the main roof was completed.This drainage material was left over from the construction of the main rood living roof.  You have to buy whole rolls of it, When you use 1/10 of a roll for the original purpose, you then can get creative with the 9/10.]]

 The lasting benefit is that, while the panels themselves likely get hot in the sunshine, they actually shade the underlying roof membrane, with very little conduction via the drainage material.  Our garage is noticeably cooler this summer than last.  It still gets warm, but not to prior years blast furnace levels.

As it turns out we used almost all of the the scrap Crezon in the construction of the walking pads for the main roof.  Crezon is ideally suited to the damp  job at that location.  

Recover, reuse. Pity we couldn't get any more LEED points for it!