Thursday, January 26, 2017

Construction - Short Truss Wall

Since the concrete earth contact north wall is only 12' high instead of a full two stories, the difference must be made up by a short stick-built truss wall.* This post, the first of two on the short wall, focuses on solving the problem of the concrete wall below it being unacceptably out of level.  It describes the installation of the mudsill first, building the rest of the wall horizontally then standing it on the mudsill for nailing instead of attaching the mudsill, as is typical, before raising the wall.
After cutting the mud-sills to length, they were used as straight-
edges in the middle of the wall for determining the height and
spacing of the shims that would be necessary to keep the sills
 level when laid flat

The Unlevel Conundrum 
The rotary laser and a taut mason's line told me that the concrete wall was too low in several areas along the 58' span -- as much as 3/4" in one area. If the truss wall were to be built with standardized trusses already fastened to a mudsill, shims of various thicknesses would have to be used under the sill to keep it level.

There would be two disadvantages to this approach.  First, there would be a space under the sill that would have to be closed in some fashion to seal out bugs and critters and to eliminate air infiltration/exfiltration.  When I helped my step-son, Keith, build his house a few years ago, I saw how difficult, frustrating and shotgun-ish it was to try to force mortar into the gap under the mudsill after the wall
The shims were screwed down; the sill will
not sit directly over the concrete -- its outside
edge will be flush with the stucco and barely
extend past the bolts on the inside; additional
anchors will be added closer to the outside
edge of the concrete
was in place.  On the exterior, the termite shield was in the way of injecting the mortar so it had to be done from the interior with no guarantee that the space was thoroughly filled. Incomplete filling would be even more likely with my 2 x 12 mudsills than with Keith's 2 x 8 sills.  

The second disadvantage of filling the gap after the wall was raised would be that part of the wall and the roof above it would be supported by shims with whatever additional support a hit-and-miss mortar bed could offer.  A decent mortar bed would offer more support and having an uninterrupted mortar bed is even more of an issue our case. The outside edge of the 2 x 12 aligns flush with the stucco, which means it is cantilevered +/-4" over the insulation, cement board and stucco.  
Ideally, the anchor bolts should be located near the center of the sill.  In our case, with the inside edge of the sill being only an inch or two beyond the anchor bolts that the contractor installed in the middle of the 10" wall, the outside edge of the sill would not be sufficiently anchored.  Consequently, I will be adding many more anchors near the outside edge of the concrete, such that they are closer to the middle of the sill, after the wall has been raised and nailed to the sill.

Builders nowadays typically use a sill gasket between the mudsill and the concrete wall (actually between the sill and the termite shield) as an air seal and a moisture break.  With our 3/4" gaps, the typical 1/4" thick gasket would not suffice as either one. The DIY method detailed here provides air sealing but not a moisture break. However, the two layers of 6 mil plastic that sandwiches the insulation will keep the concrete bone dry then the house wrap and metal cladding will overhang the top third or so of the termite shield.  The contact between them will not be conducive to capillary attraction that is necessary for moisture wicking.

Holes for the bolts were drilled and
the sills were dry-fitted

The Plan 
After my experience with Keith's house and with the installation of the first truss wall, it made sense to install the mud sill first, get it level and well supported by mortar then stand the rest of the truss wall on top of it. Accordingly, I would use shims to keep the sill level while bedding and bolting it to the concrete wall. Then I would build the wall on the floor with trusses attached only to the top sill, not the mud sill, and stabilize the bottom with a temporary brace.  After the wall was raised, the individual trusses could be nailed to the mud sill.

Installing the Mud Sill 
For the rest of the exterior truss walls, I will be using 2 x 6s for tandem mudsills over the 13" wide insulated concrete walls.  When pouring the concrete walls, I intentionally placed the anchor bolts off-center in the concrete and protruding an extra 1 1/2" from the concrete so as to be situated midway between the mudsills and tall enough to receive a 2-by bridging across the sills.  However, the contractor for the north wall placed the bolts in the middle of the wall, making it awkward to use tandem mudsills. Consequently, I settled for a single sill using pressure-treated 2 x 12s.  Before attempting to
The near sill is still in the dry-fitting
position; the far sill has been
inverted and the termite shield
nailed to it
level the sills, I
 bored holes in them for the anchor bolts in such a way that their outside edges were flush with the surface of the stucco below them.  Four inches of the sill protruded out over the insulation and stucco which meant that less than 8" lay on the concrete.  With the bolts in the middle of the 10" concrete wall, the holes were located within a couple of inches of the inside edge of a sill which was an advantage for installing the sills but not an advantage for anchoring the wall long-term.

It would have been difficult if not impossible to have bedded the sills in mortar without shims to level them and support them at the correct height as the excess mortar was squeezed out.  The 2 x 12s were dead straight and 20' long so I used one of them as a straight edge leaning against the outside edge of the bolts to place shims about 8' apart and just outside the line of bolts. When the sill was settled into the mortar bed and bolted to place, the shims would dictate the correct placement of the inside edge of the sill while the outside edge could be beat with rubber mallets until it was level with the inside edge.  With the straight edge still resting on the shims, I used Tapcon screws to stabilize the shims so they would not move when laying down the mortar bed, reinstalling the sills and settling the sills into the mortar.
The near sill has already been bedded
level in mortar; the mortar is in place
for the far sill

Keeping the mudsills separate from the rest of the truss wall afforded the opportunity to attach termite shields before the sills were installed.  Instead of using roll flashing and bending it with a braker, I used drip-edge that is used for roofing.  It came already bent but the disadvantage was that, instead of one continuous piece, it took six 10' pieces to span the wall.

The sills were too heavy for one person to handle alone so step-son Keith came to help.  First, we dry-fitted the sills over the anchor bolts.  Then we used roofing nails to fasten the drip-edge to the underside and flush with the outer edge of each sill, overlapping adjacent drip-edges a few inches.  The overlaps will be pop-riveted eventually.
Sill covered by
plastic sheeting

After the shields were attached, we installed the sills one at a time so as to be sure it could be completed before the mortar began to set.  The mortar was tooled to place so that it stood proud the shims by at least 1/2". We dropped the sill over the bolts, added washers and nuts and began tightening while beating on the inside edge of the sill with rubber mallets to squeeze out excess mortar.  As soon as the inside edge was against the shims, one person stood on the outside edge of the sill while the other beat it with a mallet.  Eventually, the excess mortar exuded out and the sill was level crosswise as well as longitudinally.

Wet pressure treated lumber warps under direct sun due to uneven drying.  In order to preclude warping, the sills were stored under cover until they were installed.  The sun came out before the installation was complete so I kept the sills wet with a sprayer then covered them with plastic as soon as the installation was complete.  The object was to make them dry evenly, loosing moisture from all sides simultaneously.  

After the rest of the wall is raised and nailed to the mudsill and I know where the trusses will fall, I will install a concrete anchor between each of the original anchor bolts,  Where the drip edges overlap, I will join them with rivets and caulking.
View showing the termite shield; parging with stucco is
incomplete due to a cold spell before it was finished

A subsequent post, will detail the raising of the stick-built wall on top of the mudsill.

*  The reason for not extending the concrete to a two-story height was a matter of cost and ease of insulation.  The truss wall is cheaper and easier to insulate to an R-50.

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