Although we had the solar posts installed, we were not moving ahead with the solar array right away. There were a few other items that we needed to address first, namely the drainage layer and the insulated roof pavers, which will be the subject of the next posts [LINK].
I have designed and engineered enough green roofs to understand that materials on top of a roofing system should be separated by an appropriate drainage layer. In our case, we opted for a 1/4 inch dimple mat, with a geotextile on top. It is the very same material we used for the vent strip installation.
The 1/4 inch mat gave us enough flow rate to effectively drain precipitation off the roof and prevent the forthcoming insulated pavers from sitting in water. The geotextile that is attached to the top of the dimples helps to keep debris out of the 1/4 gap to maintain the needed flow rate.
The material selection came naturally: While scouting our regional reuse stores, I came across several rolls of the dimple mat, which must have been surplus from another project. I was short a couple of rolls, but was able to purchase those new to have enough square footage for our roof.
The mat came in rolls measuring 4 feet by 50 feet and should be installed perpendicular to the roof slope, starting at the bottom of the roof, similar to the base sheet and torch down membrane. For logistical reasons, we started laying down the mat at the top, but made sure that all the overlaps were pointing downstream.
We very carefully swept the roof first to make sure there was no debris under the drainage mat. While rolling it out, we cut small openings into the mat to fit it over the solar posts. We also made sure to weigh it down with pavers to prevent it from blowing off.
Along the parapet, we extended the drainage mat over the cant strips, similar to the base sheet and torch down installation. We can always cut the drainage mat back later, if needed, but we can’t add to it.