Now that we’ve settled on the general layout of each floor it’s time to bring in the structural engineer. The structural engineer is necessary because of the extension we’re putting on the house. The engineer will review the architect’s designs and design a foundation and the structural components of walls capable of holding everything up over time without settlement or deflection.
Our plan is to add a 10 foot extension to the ground floor and a 6 foot extension to the parlor floor. The 4 remaining feet above the ground floor extension will serve as a deck for the parlor floor. The top floor will remain unchanged.
Currently, the rear wall of the building runs straight from the top floor to the rear basement foundation wall. This means that the weight of 3 floors of wall is held in place entirely by the rear foundation wall. Under our new design (shown in the North Elevation drawing above) each wall is staggered and thus we must place structural members under each wall capable of supporting the weight above it. We don’t have the designs yet, but mostly likely this will mean 2 large steel beams running the width of the building. One to hold up the existing top floor wall and one to hold up the parlor floor wall. The ground floor’s rear wall will be supported by the extensions foundation.
This past Saturday the engineer met us at the site with our architect to review the plans. We started by looking at the backyard and basement to begin designing the foundation. The engineer needs to design the foundation so that it can not only support our extension but so that it doesn’t cause any damage to our new neighbor’s foundations. If the foundation isn’t properly designed then over time there will be settlement, cracking and most likely an expensive repair project.
Luckily, one neighbor has no extension and the other only has a very small porch sitting on a concrete slab. We’ll need to underpin the porch’s foundation but according to our engineer this is pretty minimal effort compared to what would be required if either had a true extension.
We also need to determine what is under the concrete slab foundation of our home’s existing extension. It could be simply dirt, stones or more interestingly it could be some sort of landfill garbage. The engineer and architect told us several entertaining stories about digging up refrigerators and other sorts of debris on other jobs.
While they are both fairly confident that we’ll just find dirt, the DOB requires us to do a probe. This will involve contracting a team to bring out a large drill and digging a hole about 8 feet down to confirm the contents. Given that we haven’t closed and don’t own the home yet, we won’t be probing for a couple of weeks. I will post pics once this is done.
While we focused on the backyard and foundation design, the engineer also reviewed the rest of the building to confirm that we could move walls around as we desire. The good news is that because our brownstone is only 16 feet wide, we have no load bearing interior walls. Each floor is completely supported by the floor joists which run between the party walls. On wider brownstones this was not common because the larger the run the larger the floor joists. If the home is 20 or 25 feet wide the floor joist would need to be impractically large. On these wider buildings often the wall separating the stairs from the rooms is load bearing.
Although I’m a software engineer by trade, I did take some civil engineering classes in classes and understand the basics of these structural engineering concepts. Aside from designing the room layouts this has been my favorite part of the process.