Flue and Masonry Chimney Installation
From NEPA Crossroads Knowledge Base
Installing a flue lined masonry chimney can be a real cost saver. It takes some planning and a lot of hard work but it can be done at a fraction of the cost of hiring a contractor.
Things to consider when planning is furnace location, obstacles such as windows, doors etc and offset from foundation. The first two are self explanatory. The offset is determined by how much your siding overhangs your foundation. The chimney can either go over the siding or over the house skin. In my instance I have vinyl siding so I opted to cut the siding out, put a J channel on either side of the cutout and mount the chimney ties directly to the plywood skin. If my house was wood sided I would have mounted the chimney to the siding.
In my case I offset the chimney from the foundation by 3/4 of an inch. This way I can remove the siding, leave the foam board that is underneath and be real close to the chimney lining up with the foam board vertically. The next consideration is keeping the chimney plumb. I used a 4' level to strike two plumb lines where the block will go coming off of the footer. Once it's out of the ground and back filled I'll put up a ladder and drop two plumb lines from the roof eave. This should keep the chimney straight.
The first step is digging the hole for the footer. This needs to be below the frost line. Locally in Northeast PA the frost line is about 3½ feet. Check to see what the frost line is in your area. If the footer is not below the frost line it may heave. The hole for the footer was dug 4½ feet. Also dig the hole wide enough to fit yourself in alongside the block. Chimney block is approx 16” x 16”. I only left myself a foot or so in the front of the block. It made setting the block below grade difficult.
Once the hole is dug, put about 4 – 6 inches of gravel as a base. I built forms for a 2’ X 2’ pad. It’s 10” deep. I purchased bagged, 5000 PSI, concrete. It’s made to support a structure. Once the concrete is set I chopped an 8” hole in the foundation for the clean out. This was done at the base of the chimney (first block) I drilled a series of holes about 1” apart with a masonry bit around the perimeter. Then pound out the center. If you have a concrete foundation you can rent or buy a hammer drill.
There are two types of clean out doors, cast iron and stamped steel. I used the steel because it was cheaper. In hindsight I should have spent the extra few dollars and purchased the cast iron version. The steel door is thin but it serves the purpose. I used construction adhesive to hold the door in place and caulked around the edges.
Install brick ties. These are small strips of corrugated metal that nail to the house and are mortared into the joint. I’m using two per block and installing them between every block. The purpose of these is to help keep the chimney from shifting and pulling away from the house.
The next step was to install the flue through the foundation. I kept it two feet from the ceiling joists. I'm not sure if there is a safety code but two feet seems reasonable to me. It’s the same procedure as the clean out. Put a slight pitch on the flue going up to the chimney. This will help the gases get out of the flue and into the chimney. Make sure the flue is sealed solidly to the flue liner with mortar. Any open spots could cause exhaust gas to leak into the house.
Some block manufacturers make precast block with a flue opening. The company I purchased the material from didn’t carry them. I used a 7” diamond saw blade to cut the back of the block out from one block and removed 1” from the block below and above the flue. The outside diameter of the 8” flue is 9 ½ “ so removal of some material from the block above and below was necessary. After I cut all three blocks I realized I could have cut the hole in the foundation a little lower and would have only needed to cut two blocks. It would have saved some work but that's why I'm an amateur.
The last thing to do for the chimney base is to apply a coat of roofing and foundation sealer (tar) to everything that is below grade. Wait for it to dry and back fill.
Installing the chimney block up to the eave is just repetition. I used brick ties on every block. I only installed 10 feet of block a day. I did that to give the mortar between the block a chance to set. After every third block I inserted a flue liner. I made sure the ends were sealed with mortar and placed a bit of mortar on each of the four corners of the flue to keep it centered in the block. This left an airspace between most of the flue and the block.
The next area that needed attention was getting through the eave. Carefully cut through the soffit material, eave structure and roofing. Once everything was removed double flashing was installed. The first layer of flashing was about 3 inches on either side of the 90 degree bend. It was inserted under the roof shingles and nailed. Take your time inserting it under the shingles so the shingles don't tear. Then the chimney block was laid to a height above the roof line. The second layer of flashing was built and installed. This layer was inserted into a joint between blocks to make sure no water got behind the flashing. It sits into the mortar joint about an inch. This layer ran down the side of the block, over the first layer of flashing and nailed onto the roof above the roof shingles. After the mortar was dry the part of the flashing that was nailed to the roof was coated with roofing tar.
The flashing was made by my brother in law who is a sheet metal mechanic. He purchased a roll of aluminum flashing and bent it at work. I'm not sure if you can buy flashing pre-bent or not. The aluminum sheet could be bent in a vice or over a length of lumber. The bends probably won't be as crisp as being bent on a brake but it would be functional.
We installed the last of the block, precast concrete drip edge and stainless steel cap. I made sure the top of the chimney was above the peak of the house by using a string line and level. Once all of the mortar was dry I put two coats of Drylock paint on the block and caulked everywhere between the chimney and the eaves and between the chimney and the vinyl siding's J channel.
It took about a month of working weekends and evenings from start to finish. The total cost of materials was approximately 600.00. I got a price from several contractors that ranged from 2,800.00 to 3,500.00. For me the work was worth the cost savings.
Here are the last pictures I have of this project. The chimney is complete but unpainted and not caulked.
I hope some of the information is helpful to the next person who tackles this DIY project.