Roof drainage layer

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.

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Solar posts

With the roofing membranes and flashing installed, we were finally able to get to the first component of our solar PV array: The solar posts.

PV modules are installed in rows, and mounted on rails, which are in turn attached to the solar posts. A shorter solar post is used along the south side of the row and a longer post on the north side, to give the module a tilt towards the sun.

The solar post assembly consisted of four components:

  1. A base
  2. The post
  3. A curb
  4. Sealant

The base was seated in a layer of sealant and anchored into the solar blocking, which we had previously installed. The post was subsequently screwed tight into the base. The curb was also seated in a layer of sealant over the base and post, and then filled with a sealant composite. This way the roof penetration for the base anchor should be completely waterproof.

I had taken scrupulous measurements for the location of the solar blocking, which was concealed since the installation of the roofing membranes. The big question was, would we be able to accurately trace the location of the blocking with the measurements?

It turned out that we did not need the measurements at all. The morning dew pattern that collected on the roof showed us exactly where the solar blocking was, due to the thermal difference between the lumber used in the blocking and the roof insulation on either side. If you look carefully at the time lapse, you may be able to spot that pattern.

Sometimes a little luck goes a long way.

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Parapet flashing

It took us a day to install the torch down membrane and seal the two vent stacks. We now had a day left to take care of the flashing along the parapet, which is a much more tedious task than laying down a roofing membrane. Why only a day? Because our luck with rain-free days was about to run out.

Whereas we installed two membranes on the roof (base sheet followed by the torch down membrane), the parapet flashing only received the torch down membrane.

Our roofer Pablo had extended the torch down membrane by around four inches above the cant strip and up the parapet. He now measured and precut torch down membrane pieces to extend from the parapet edge down to about four inches past the cant strip onto the roof. And as usual, installation started from the bottom of the roof to the top to have the overlap in the direction of the water flow, in the same manner as regular shingles.

Pablo lined up each piece, folded back the top half, heated the back of the membrane with the torch, folded it back up, and pressed until it adhered firmly to the parapet. Next he lifted the bottom half of the membrane and repeated the process. This way no direct heat was applied to the dimple mat vent strip. The exception was the downstream edge of each piece, which Pablo very diligently warmed up and then sealed.

I can’t adequately express how much I appreciated Pablo’s diligence. Safety meant everything to him. At no time was he rushing anything but made sure he didn’t melt the dimple mat and that each seam was impeccable.

Also, his timing was perfect! It literally started drizzling while he was finishing up the last two pieces on the front parapet. What a ride!

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Vent stacks

As usual, the devil lies in the details! And those readers who carefully watched the torch down installation video may have noticed that we had two roof penetrations that were not addressed yet: the two bathroom vent stacks.

To integrate the vent stacks into the roofing system, they were fitted with a lead sleeve.

Our roofer Pablo began by applying roofing cement around the collar of the vent stack and painting a square base that was the same size as the square base of the lead sleeve.

I had cut the vent stack down so that it would fit the lead sleeve. Pablo slid the lead sleeve over the vent stack and set the base onto the roofing cement square. He continued by folding the top of the sleeve over the top of the vent stack.

This does seal the top of the vent stack, but more work was needed to seal the collar, or the square base of the sleeve. To start, Pablo placed a square piece of base sheet over the square sleeve base. The base sheet piece was about twice the size of the square sleeve base.

He also had cut a piece of torch down membrane that was about four inches wider than the base sheet. He heated up the bottom of the torch down membrane piece, slid it over the vent stack and firmly pressed it onto the base sheet.

Did you notice the rounded corners on the torch down membrane? Pablo explained to me that it is best practice to round off corners, because square corners tend to come loose over time. He continued to go around the edges of the torch down membrane and made sure it was fully adhered and sealed.

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Torch down installation

With the base sheet installed, we moved right on to the torch down membrane, while the dry weather lasted.

This process resembled the installation of the base sheet. The torch down membrane came in 36 inch wide rolls and also were rolled out across the roof, starting from the low end. The difference was that the torch down membrane needed to be heated with a torch (thus the name). The heat was applied to the bottom side of the membrane to soften the asphalt, which acted like a glue, and immediately rolled across the roof. Once rolled out, the melted membrane bottom cooled down, firmly adhering it to the base sheet.

Let me state that I appreciated the skills and confidence of our seasoned roofer, Pablo. My mind was racing with thoughts such as, “how do we properly line up each layer?”, “how far up the parapet do you go?”, ”how do you manage the joints at the end of one and beginning of another role?”, “how do you know when the roll is heated enough?”,etc.

While my mind was racing, Pablo had already installed the first couple of rolls. It was time for me to stop thinking and start watching, and eventually grab a tool and give him a hand. And the answers all arrived automatically:

How do we properly line up each layer?

The overlap between the torch down rows should not be on top of the overlaps of the base sheet rows. Pablo started with half a sheet so that the overlaps were perfectly staggered. Like with the base sheet, we started at the bottom of the roof working our way to the top. This way the overlap was in the direction of the slope and waterflow. Like with the base sheet, the torch down membrane has markings that tell you how much overlap is needed, and allows you to overlap perfectly straight.

How far up the parapet do you go?

Pablo went up the same distance as he did with the base sheet, basically over the cant strip and another four inches up. He stopped with the torch down just a notch below the end of the base sheet.

How do you manage the joints at the end of one and beginning of another roll?

When one roll ended, we slipped the new roll under it to get the same overlap as between each layer, which was four inches. It is basically the same overlap, just perpendicular to the layers.

How do you know when the roll is heated enough?

I still have no idea, and I am still not interested in finding out. Pablo pointed out that you need to be “very careful.” And knowing that Pablo is a master of understatements, I had no intention to ever use that torch and heat up the roll.

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