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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Base sheet installation

It was time to waterproof our roof. Several days had passed since we tore off the old roof. The dry weather did hold, but we didn’t want to push our luck.

We opted for a modified bitumen roofing system, which I mentioned in my roof insulation discussion. This system consists of a base sheet and a torch down membrane (or cap sheet).

The base sheet comes in the form of 39 inch wide rolls with an adhesive backing. When I say adhesive, I mean aggressively adhesive. I sometimes felt like a fly that got a little too close to a glue trap while working with the base sheet.

To install the base sheet, Pablo, our roofer, carefully lined up the sheet on the roof. The adhesive backing is protected with a light blue plastic cover that is split along the middle of the sheet. This way we could lift one half of the sheet, tear off the protective cover, glue it down, and repeat the process for the other half of the sheet.

The sheets were installed 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. The base sheets came with markings to facilitate lining up the overlap for each row. This made it easy to keep everything neat and square.

Along the parapet, Pablo pulled the base sheets over the cant strips and terminated them a couple of inches above them.

To assure full contact and proper adhesion, the base sheet gets rolled a couple of times. That should assure a fully adhered roofing system that does not lift off during a storm.

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Roof end blocking

I talked about installing the insulation on the roof deck in a previous post. There was one small but significant trailing task that I could not ignore.

Our two layers of 1 ½ inch of polyiso were contained around the edges by the parapet. The exception was the lower roof end, where the main roof transitions to the back porch roof. Not only was there nothing to contain the insulation, but I had a small drop between the two roofs.

Bringing the insulation all the way up to the main roof edge did not seem a good idea. That edge acts like a step each time someone accesses the roof. Although the polyiso boards were pretty ridgid, they are not a material that’s suitable for steps.

The solution: Take two 2 x 8 joists, lay them flat on the roof edge so that they come up to the same height as the insulation, just as we did with the solar blocking installation. And we anchored the two 2 x 8’s into the masonry wall below. We referred to this assembly as the roof end blocking.

Hard 90 degree turns on roofing membranes are not recommended. They are subject to material fatigue and failure. That is why our roofer Pablo used a cant strip all the way around the parapet edges. Rather than folding the membrane up once at 90 degrees, we were folding it up twice at 45 degrees over the cant strip.

We used the same principle for the edge of the roof end blocking. I took the circular saw to the upper 2 x 8 and cut a chamfer to it. This way we can carefully apply the roofing membrane over the edge with two 45 degree folds.

This is all we could do at this point. But we needed to install another roof end blocking over this first one. More about that later.

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Vent strip installation

If you are a nerd like I am, you may have noticed that parapets are often the first masonry feature on a Chicago building to deteriorate. This could be explained by the roofing membrane (waterproofing) that is often lapped up and over the parapet.

To give you an example, here is a picture of our original parapet from 2009.

Lapping the roofing membrane up and over the parapet may make sense in terms of waterproofing the roof. But it also creates a vapor barrier on the parapet side facing the roof. The parapet can now only dry into one direction – the side facing away from the roof. And this increased vapor pressure could be the cause for an accelerated parapet deterioration. Something I recently ran into head on with our front parapet.

If I could eliminate the vapor barrier, the parapet would dry in both directions. And that was my goal.

The solution was to install a dimple mat along the inside of the parapet, and then install the roofing membrane flashing up against the dimple mat. This way I created an air gap along the inside of the masonry wall – a vent strip.

All that was left was to cut the mat flush with the parapet, after we had the dimple mat attached to the parapet and the cant strips placed at the parapet base. We were now ready to install the roofing membranes, starting with the base.

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