Category Archives: general construction

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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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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Roof tear off

When we bought our building in 2009, our roof got a clean bill of health by the inspector. It had a fairly new membrane that was estimated to be good for another five to 10 years. And it was.

But the most recent membrane was not the only one that had been installed on the roof. In fact, there were seemingly endless layers of membranes and other roofing systems that had been installed on our roof over the last century, probably going all the way back to the very first roofing system in 1902.

It amounted to a roofing system sandwich of about two and a half to three inches. The weight of all this roofing material was considerable. Rather than adding another layer to it, it was time to tear off all those layers, take off the weight, and have a fresh start with a new roofing system.

Removing the two and a half to three inches of roofing amounts to about 12 to 14 cubic yards of waste. In preparation, our roofer, Pablo, organized two 10 yard dumpsters and a trash chute.

The trash chute was important to me, because it prevents the waste from flying all over the place on its way down from the roof.

The tear off had to happen rather fast. You don’t want to be in the middle of a roofing project and have rain storms moving through. Ideally you get the tear off started and the new system finished while the dry weather lasts.

Pablo arrived at 7 am with his crew and a selection of brute force tools. By noon, all the layers of the old system were gone and in the dumpsters. We were down to the original roof deck, which was old 7/8 inch pine boards.

To my great relief, the boards were in excellent condition, except for one spot that had water infiltration at one point. I noticed the damaged boards when I installed the insulation under the roof deck. So we were prepared, and removed and replaced the damaged boards.

Related posts:

  • Roofing materials – delivery
  • Parapet extension
  • Front parapet re-building – 2nd step
  • Cornice repair
  • Front parapet re-built – 1st step
  • Front parapet demo
  • Bracing for the cornice issue
  • The roof project

Roofing materials – delivery

During our roof project I had lugged a lot of material up the back porch stairs onto the roof. Countless pieces of brick, and rather heavy but beautiful pieces of limestone, among other things. In the process, my thighs grew big and my arms long.

Our roofer, Pablo, told me that they did the same in the old days: Shouldering the roofing materials and lugging it up stairs and ladders.

But not any more. Today, access permitting, many roofing material suppliers use a truck with a crane.

This was a rather relaxing and refreshing experience after what felt like weeks on a stair climbing machine. Not only did we have all the roofing material hoisted onto the roof in minutes, but Pablo got his tools and equipment lifted too.

What made me a little nervous was the sheer weight of the palettes that were now sitting on the roof. I had to remind myself that it should be OK, given that they are sitting there temporarily, and that we had the roof structure reinforced.

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Parapet extension

We had emergency repairs done to the east and west parapet back in 2009. Back then I had the hunch that additional work may be needed once we began redoing the roof. That hunch became a prophecy.

Don’t get me wrong. The parapet repairs from 2009 were perfectly fine. The issue that I faced  had to do with roof insulation.

We insulated the roof between the rafters. And I learned (the hard way) that we needed to continue with the roof insulation above the roof deck. The insulation above keeps the roof deck on the warm side so stays drier during the winter months.

Adding layers of insulation onto the roof deck shortens the height of our current parapet. To maintain the code required height, I needed to raise the parapet. I called our mason back and purchased a few pallets of additional salvaged common brick.

I mentioned a couple of posts back that working with common brick requires certain steps and a certain type of mortar to produce a lasting masonry system. And I have described the process and materials in a previous post (Mending more masonry + Bricks and mortar). I recommend that you look up that post if you plan a project that would use Chicago common brick.

One key aspect is to make sure the common brick is properly soaked in water prior to installation. This prevents the brick from pulling water out of the mortar. And that in turn provides the desired bond between the common brick and mortar.

It is also a good idea to keep wetting the brick during and sometime after the installation for the same reason. So I kept busy spraying it down. This can be a full time job during a sunny and hot day. The common brick dries out rather quickly. Cooler and overcast days are easier.

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