Thursday, January 26, 2012

4: Rebuilding the Structure of the Existing Building

As I mentioned in the previous post on building the new addition, it's helpful to think of the finished building as two separate "cee" shaped--as viewed from above--wall pieces that are being covered by one common roof. They are: The old building, now missing it's back wall, and the new addition, being "open" to the old building, having only three sides.

(Click on any image for a larger view.)

The "original", or existing, building was structurally inadequate and needed changing for several reasons--
  1. We didn't want to keep a bearing wall in the middle of the space.  The existing 2x8 joists could not cross the entire 21 ft. span without sagging.
  2. The height of the existing floor joists only allowed for 8 ft. ceilings, we wanted more headroom.
  3. The rafters were made from 2x4, rather than the 2x8 called for by modern building codes.
  4. The existing walls were made from 2x4s. We wanted thicker exterior walls to allow space for more insulation.
  5. The exterior bearing walls were balloon* framed, and so had a ledger board notched in them to support the floor joists.  This substantially weakened the walls.

*Diagram of Balloon Frame.The wall studs run all the way from sill to rafter, typically two stories.  The more "modern" platform framing has wall studs that are only one story tall.

The building itself is located about 7" inside the property line.  Were we to completely tear it to the ground, in order to comply with modern zoning codes we would have to relocate any new structure 3 feet inside that same property line, a serious drawback on a small city lot.  By keeping the existing building's footprint, or it's position within our property lines, we would be grandfathered in, or exempt, from those regulations.  That meant keeping some of the old frame and foundation rather than building everything new.

It also meant a lot of extra work.

The old building wasn't plumb, nothing was level.  Getting a twisted, wracked frame to merge with the newly built, and plumb, addition was a challenge.

I decided that the way to solve this problem was to build a new structure inside of the old frame.  With the new concrete grade beam in place, we had a wide enough foundation to accommodate a double exterior wall: the outer wall being the existing 2x4 balloon frame and a new inner 2x4 wall that would carry the weight of new floor trusses and rafters.

Here's a view of the second floor looking at, from what is now my kitchen, the existing building. Behind all the new framing you'll see the darker wood of the old balloon frame.  The actual, weight bearing, floors and walls are all new material.

The view of the third floor, looking in the same direction as the photo above.  I'm standing in what will later become the hallway next to the staircase opening.  The new framing on the right edge of the picture is the wall of the addition.  I worked my way forward, dismantling the old roof and building the new walls at the same time.

To remove the old roof, you had to climb out on the planks we set out on the old ceiling joists and use a sawzall to cut away one rafter at a time.

Easier said than done.  There were about 3 inches of material on top of each rafter.  Every chunk you cut out had to be carefully tossed down into the dumpster.

Virginia did her share of the demolition.  It wasn't easy.

After we removed the old roof, I continued building the new bearing walls and setting the ceiling joists on top of those walls.

This picture was taken just after the one above.  It shows the new bearing wall nestled just inside the old balloon frame.

The existing building, almost fully rebuilt.  I just needed to make three new window openings on the end wall.  Look closely at the center bottom of the picture and you'll see a pile of rope.  By running the rope through a pulley attached to the rafters above the skylight opening (in the upper right hand corner of the photo) we hauled all our materials to the top floor and/or roof.  Sheets of plywood were the hardest to move this way.

The exterior view at the same stage as the above interior photo.

No comments:

Post a Comment