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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Solar blocking

Nope, I am not trying to block the sun, but this is a good reminder that the whole roofing project was happening because we were getting ready to install a photovoltaic array on our roof. And one question to resolve was: How should we attach the solar panels to the roof?

Some systems are weight based, meaning they are not physically attached to the roof structure, but instead weighed down by concrete blocks, for example. This method has the advantage that there is no hardware that penetrates the roofing membrane. The disadvantages are that the roof needs to be able to accommodate the extra weight, and more importantly that the City of Chicago would not permit weight based systems, according to my solar installer.

We needed to come up with a solution to anchor the solar array to the roof joists.

The solar panels are mounted onto rails in rows. There is one rail towards the bottom and one towards the top of the panels. The rails in turn are mounted onto posts, which are anchored to the roof structure. There is a short post towards the front of the panel and a longer post toward the back, so that the panel faces the sun at an angle. (If you struggle with some of the terminology, go to: Solar lingo)

If you look at the plan sheet above, you can see that almost none of the posts (red dots) line up with the roof joists (dotted line going from left to right). So, anchoring the posts directly into the roof joists was not an option. Instead, we planned to indirectly anchor them. We installed and anchored three rows of blocking that was running perpendicular to the roof joists (the dotted line running from top to bottom), to which we could anchor the posts.

The blocking consisted of two two-by-fours laid long side down, stacked on top of each other, and anchored into the roof joists.

This solution had to fit into our new roofing system. That is why we used two layers of 1 ½” polyiso boards, so that the insulation would be flush with the blocking for the solar posts and ease the installation of the roofing membrane.

I had marked the layout for the solar blocking right after the roof tear off and kept track of it during the insulation installation, including where to anchor it to the roof joists. I cut and removed insulation and laid out the two-by-fours. I made sure to stagger the joints between the bottom and top row. I also made sure that no joint was over a roof joist, or at an anchoring point for the posts.

After I had the two-by-fours anchored down I foamed around the edges to keep the insulation assembly sealed.

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Roof insulation

I only had two major snafus on this project, both self-inflicted. One had to do with the roof insulation – or – to be more precise, the attic insulation.

I did an excellent job insulating the attic, first with rock wool followed by foam board. And in that process I created a cold roof deck. A cold roof deck during winter runs the risk of getting wet over the years (for more information go to: Do-over dilemma).

With the old roofing torn off, I found to my delight a bone-dry roof deck. Maybe this was an indicator that the risk of a cold and wet roof deck was marginal, but nevertheless I’m glad I made the decision to sandwich the roof deck between layers of insulation (rock wool below and foam board on top) to keep it warm.

What type of insulation to use?

I used a fair amount of salvaged extruded polystyrene insulation (XPS) on the project and thought of using it on the roof too. But my roofer, Pablo, balked at that idea. I quickly realized that all the roof insulation I ever saw was polyisocyanurate insulation boards (or polyiso in short). And there is a good reason.

Although I didn’t find any independent publications, articles from the roofing industry and manufacturer associations indicate that polyiso is fairly fire resistant and does not melt and drip like polystyrene. That is a rather important factor, considering that we planned to install a modified bitumen roofing system, also known as torch-down roof. And as the name suggests, it involves a torch and heat. So having fire resistant insulation boards is – let’s say – imperative.

The downside of polyiso insulation is its cold weather performance. To quote Martin Holladay from Green Building Advisor:

“At temperatures below 50°F, polyiso performs worse than it does at a mean temperature of 75°F”

And what does that mean? In warm conditions, polyiso outperforms XPS insulation. Under cold conditions, polyiso is about on par with XPS (R-value of 5 per inch). And under very cold conditions, it may drop below an R-value of 5 per inch.

When I was sitting at the roofing material supplier to order the materials for our project, the price for the polyiso boards was less than I expected, which made me suspicious. After combing through the material specifications, I realized that the boards in questions had a cardboard based facer.

Unlike XPS or expanded polystyrene (EPS) insulation, polyiso always comes with a facer on both sides. The facers contain the foam core during the production.

Having a cardboard facer, which runs the risk of disintegrating or deforming when it comes into contact with moisture, was not acceptable in a roofing situation. And the potential flammability of a cardboard backing may negate the fire resistance the polyisocyanurate provides. A fiberglass facer would be the material of choice. Slightly more expensive and a special order item (because most roofers don’t want to spend the extra money), but moisture resistant, dimensionally stable and safer.


We installed two layers of 1 ½ inch polyiso boards across the roof. I did pick 1 ½ inch boards because they fit with the solar blocking, but more about that in a later post.

To maximize the thermal performance, we staggered the joints of each layer. The polyiso boards, like most materials, expand and contract with rising or falling temperatures. When contracting, the tight butt joints may morph into a slight gap, which would allow thermal energy to escape. By staggering the joints, I have at least another layer of foam board over that gap that would slow that escape.

We fastened the boards mechanically to the roof deck so that they don’t blow away and are a solid foundation to which we can adhere the roofing system. To do so, we used long insulation screws with insulation washers.

We have, what you would call, a typical low slope roof. What is not so typical is that the bottom of the slope is blocked for about 12 feet by our staircase extension. To prevent water from ponding up against the extension, we added some tapered insulation to add a slope that would allow for positive drainage.

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