Thursday, December 1, 2016

Construction - Second Story South Wall

The previous post concerned the construction of a permanent and temporary second story floor on which to work on the second story walls and roof.  Here we are dealing with the 10+ foot south window wall for the second story, the top of which is a little over 20 feet above the first story floor.

Change in Window Configuration
Click on the drawing

Originally I planned to use seven double-window sets as clerestories in the south wall, as shown in the drawing. When I laid out side-by-side the seven pre-made window sections containing two windows each, they clearly were too crowded. Consequently, I reduced the number to five double-sections, stood them up and viewed them from the street. The proportions were definitely better.  Then it made sense to add back a couple of single windows as a way of softening the monotony of the facade and to stay in compliance with the minimum code requirement of 4% glass-to-floor area.  So I dismantled the two extra sections and reused the components to make wall sections for single windows that will have the same 3' x 5' dimensions as the individual windows in the double sets.

Adding Height to the South Wall
An end of the truss jig was cordoned off with
 2 x 4s for making 24" x 15" inserts; the plywood,
  nailed to a 2 x 4 frame, serves as a gusset 
The plans call for 12" thick cathedral ceiling for the first story shed roof that extends southward from the second floor wall.  Accordingly, the height of the wall was designed to be high enough to accomodate windows above the roof that will be pitched at 12/2.5. However, I will be using ceilings that are +/-20" thick. If the wall height isn't increased by at least 8", the pitch of the roof would have to be lowered to keep the roof from overlapping the bottom of the windows.  This is not an option because a slope of less than 12/2.5 is not recommended for metal roofing. The pre-made window sections that will comprise most of the wall were designed for an 8' wall and will therefore have to be heightened by a total of 24"  -- 16" to satisfy the original drawing plus another 8" to accomodate the thicker roof. Unfortunately, the additional 8" will place the sills above the 44" maximum height for egress windows but, fortunately, only the single window in the bedroom at the east end of the house will be affected.  For it to meet code, there will have to be a permanent riser below it on which an occupant could stand while exiting in an emergency.

I temporarily modified the jig that I used for wall trusses to make 24" high inserts to go between the window sections and the floor.  Admittedly, the inserts looked a little goofy but, when tied in with bolts, nails and strapping, they did the job.  

Raising the Wall
The four wall sections before raising; the section with the saw
laying on it and without its top and bottom plates is to be
raised last (click on the photos to enlarge them for more detail)
The window sections comprised most of the second story south wall but a couple of wall trusses were needed at both ends of the wall to connect later with the east and west rake walls. As soon as the extra trusses were ready, I cut the four 2 x 6 top and bottom plates to length, laid them side-by-side and marked them for attaching to the wall sections and the new trusses.  Then it was a matter of nailing them to the trusses and wall sections while keeping the outer edges of the tandem 2 x 6s exactly 15" apart, the same as the wall components.  I divided the 58' wall into four units in order to keep the weight within reason for raising.  I left the top and bottom plates off of the shortest unit since it would be the last to be raised and might need narrowing in order to fit into the space between the adjacent units that were already up.
Unsuccessful attempt at using wall jacks

Step-son, Keith, helped raise the walls. We assumed that the sections would be too heavy for raising without wall jacks but, after we were half through with using them for the first unit, we realized that they were not properly adjusted for a 10' wall and had to lower the wall back to the floor.  Instead of adjusting the jacks, we removed them, under the assumption that we had under-estimated our ability to raise the wall without them. So, in order to keep the wall from sliding over the edge of the floor, we used rope to tie the bottom and the top of the wall to the existing wall to the north.  Then two of us raised the first unit surprisingly effortlessly and continued to do so for the other three units.

As soon as the first three units were upright and the bottom plates were temporarily fastened to the floor with duplex nails and the top plates fastened together with clamps, we could dry-fit the unattached 2 x 6 plates for the last unit then attach them to the unit.  When we raised the unit, it fit nicely between the two adjacent units.

Aligning and Securing the Wall
As the units were raised they were stabilized with braces between the wall and the floor.
Second story wall housing a total of twelve 3' x 5' windows
Fortunately, the raised wall was perfectly plumb in an east-west direction, which was to be expected because the individual window units were shop-made in a jig. And each of the four wall units was checked for square while still laying on the floor before nailing on the top and bottom plates. All that remained was to get the wall (a) straight at both the top and bottom, (b) level on top and (c) plumb in a north-south direction. The braces were attached to the wall with drywall screws so the screws could be backed out and refastened as the wall was tweaked.  The duplex (double-headed) nails between the bottom plates and the floor provided the same flexibility.
The wall from a different perspective; notice the nailers over
the window openings for safety reasons; the height is just
over ten feet; the window sills are slightly too high to meet
 code for egress

We started the alignment by straightening the bottom and the top using taut mason's lines while loosening and refastening the tops of the braces and the duplex nails as needed. The clamps sufficed for the top temporarily.  As with conventional 2 x 4 or 2 x 6 walls, we used a double top plate, i.e., a second layer of 2 x 6's stacked on top of the ones that were nailed to the wall sections before raising the wall.  We installed the ones on the interior side of the wall first and used them to pull the wall perfectly straight and hold it straight, assisted by a mason's line.  Of course, as is standard practice, we staggered the locations of the ends of the boards relative to the first tier of top plates so as to stiffen the wall.

The floor under the wall was not perfectly level so the top of the wall also was not perfectly level, primarily in one area.  In order to correct the problem, we used shims under the bottom plates to level to a mason's line the top of the wall.

All that remained now for proper alignment was to plumb the wall in a north-south direction and secure it definitively to the floor.  For plumbing, we used shims under the bottom plates as necessary while loosening and reattaching the braces.  Then we used 6" construction screws and nails to fasten the bottom of the wall to the floor joists and band-joists.  

Our location places us at risk for two catastrophes.  Two hundred feet below us is an abandoned coal mine and subsidence is not uncommon in ours and the surrounding counties.  Also the New Madrid fault in southeast Missouri near its border with Tennessee has a 25 - 40% chance of a magnitude 6 earthquake within 50 years and a 7 - 10% chance of a 7.7 magnitude.  Either would cause serious damage in the St Louis region. Consequently, the final job to secure the wall was heavy hurricane strapping to tie it to the lower floor framing and to the major beam under the catwalk.

Installing Cripples
The purpose of cripple studs under the window openings is mostly for fastening sheathing
Cripples anchored at the bottom by 2 x 4s
and drywall but a secondary function is to support the sills under the windows which, in turn, supports the window. The latter function is more important for conventional 2 x 4 or 2 x 6 walls whereby windows are supported usually by one 2 x 4 or 2 x 6 installed flatways.  With our truss walls, each window is supported by two 2 x 6 sills installed on edge thereby making the secondary role of the cripples largely moot.  

I spent a few minutes making a temporary jig in which to nail the double cripples to a short 2 x 4s before nailing them in place.  I opted for the 2 x 4s cross-ways of the bottom plates rather than toenailing the cripples directly to the plates to make the cripples easier
Quickey jig for nailing cripples to 2 x 4s
to install and for more secure fastening at the bottoms. The tops were notched around the sills so they were well-nailed on top.  I might have opted for toenailing if the bottom of the wall was to be exposed to the exterior in order to avoid
 thermal bridging by the 2 x 4s. But not to worry, the bottom of the wall will serve as the top part of the wall for the first story and the bottom third of the cripples will receive drywall instead of sheathing.

All exterior walls for the house are designed for trusses 24"oc due to their stiffness. The cripples, however, are 16"oc just like the typical 2 x 4 wall framing of most houses.

1 comment:

  1. Thanks for the great post on your blog, it really gives me an insight on this topic.

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As a do-it-selfer-in-training, I welcome your comments.