Rebuilding a chimney crown....Chim-chiminy Chim chiminy...our Chimney Cap Project

This post is a perfect example of how projects can drag on for years, and yes I do mean years!

We had our flat roof torn off the summer of 2006 and some faithful readers may recall that at that time we discovered that our parapet wall was very deteriorated. We had to determine how to address that issue. Back story here.

We planned to undertake the chimney project ourselves to save money, but ultimately, we ran out of time and we ended up paying our contractor to rebuild the chimneys before winter. The project was not completed though because we still needed chimney crowns and caps. Due to other projects, this didn't happen until 2 years later..just this fall.

Although my blog posts have been sparse lately, we have actually been still working. Steve completed the cast-in-place chimney crown and installed the chimney caps last weekend just before winter's arrival. Hooray!

The first photo illustrates the starting point this fall- one of the two double flue chimneys exposed to the Chicago elements.

Before undertaking this project, Steve did internet research. He found various plans for pouring cast-in-place chimney crowns made of concrete. He called up the Masonry Advisory Council and the Portland Cement Association. Below is what he learned.

The chimney crown must be made of concrete and not mason mix because concrete is used for foundations etc... while mason mix is suitable for laying brick.

Secondly, the crown should be at least 3" thick and it needs to overhang the chimney by a few inches with a drip edge to prevent rainwater from traveling down the chimney causing rapid deterioration. He also found out that the concrete crown must have a bond break from the chimney, that is there needs to be a separation between the chimney(made of brick) and the crown (made of concrete)because both materials expand and contract at different rates and would compromise the chimney with the different movements during freezing, thawing etc...

The concrete crown must to be sloped so that water will drain off of it. The recommendation was that there should be a 2" slope to shed rainwater. Rebar was suggested to reinforce the crown also.

Steve built wooden forms for both chimneys. The concrete would then be poured into them. The form was screwed into the mortar of the chimney using masonry screws. And alternative method would be to use shims underneath the bottom of the frame, which would hold the form in place. With this approach, there is no masonry repairs to do from the screw holes afterwards.

In these photos you can see the completed form ready for pouring the concrete. Here are the steps he took to create this form:




First, he bolted 2x4s around the perimeter of the chimney flush with the top course of bricks.

Because Steve wanted more than a 1 1/2" overhang (width of a 2x4), he screwed a second layer of 2x4 over the first. This created a 3" overhang.

Then, he took 1x8 lumber and attached it around the 2x4s, which created the box you see here. The 1x8s were also flush with the bottom of the 2x4s. This "box" would contain the poured concrete, which would then become the cast-in-place chimney crown.

This close up photo shows more detail on the form he built. Steve nailed in place about 1"from the form edge 1/4" strips of wood. This creates the drip edge (indentation in the concrete). It was removed with a screw driver after the concrete was dry leaving a channel. This drip edge  channel prevents water from running down the side of the chimney bricks.

To create the "bond break" between the chimney and the concrete crown, Steve covered the entire chimney top with 10 ounce sheets of copper, which extended beyond the chimney base by 1/2". He used 10 ounce copper because that is what was available, but 7 or 8 ounce would be fine.

He tightly wrapped the flues in 2 sheets of corrugated cardboard. (Cardboard was removed after concrete dried and gap was then caulked.

Rebar was put along the length and width and between the two flues and was tied together with wire and then raised up to the mid-point of the crown using rocks.

Steve wasn't sure if the weight of the concrete might cause the form to blow out and break apart, so he took some wire and wrapped the box to be safe. Since the form was screwed together (for strength and easy disassembly), Steve thinks this reinforcement is not needed.

In this photo, he was ready to pour the concrete.

He mixed up the concrete, poured it and created the recommended slope. He took a hammer and pounded all sides of the form to eliminate air bubbles. According to Steve, it was about 75 blows. He is nothing if not detailed and is helping me write this if you can't tell!

He then tarped the form with plastic sheeting to slow the drying process, which makes for stronger concrete. He kept it tarped for a week.

**It's important not to tarp the flues for safety reasons. Chimneys need to operate at all times...CO2 etc...! We do not have fireplaces, but our boiler and water heaters are connected to the chimneys and need to be vented outside for safety.

After the concrete cured for a week, Steve unscrewed the form, removed the drip edge strip of wood underneath, removed the cardboard and installed foam backer rod into the cavity and then applied a generous amount of caulking around the flue.

In keeping with our new thriftiness, we purchased inexpensive galvanized painted steel chimney cap kits.

Of course, there are chimney crowns made of aluminum or copper, which will last forever but are much more costly. We opted to be budget conscious and these should last 10 years. Assembly is very easy. They get mounted to the cap as seen.
One thing Steve did to extend the life of the structure was to raise the cap using 1" chrome spacers, which will hopefully prevent moisture from corroding the bottom of the frame.

So, we have now stopped the rapid deterioration of our chimneys and hopefully we will be solid for some time to come.

Many people never go up on their roof until there is a problem. It's very important to inspect your roof once a year. We usually do this in the fall.

Steve is really becoming quite the amateur mason around here. I think I'll keep him for a while longer. :o)