Restoration Guide: Exterior Walls Design Engineering
Editor's Note: This is Article 2 in Chapter 2: Exterior Wall Guide of the Old House Web Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.
2. DESIGN AND ENGINEERING OF EXTERIOR WALLS
Wood is the most common framing material in the U.S., so this section of the guide focuses on the rehabilitation and restoration of wood framing systems. The design and engineering section of the Exterior Wall guide pays particular attention to structural and seismic retrofitting techniques.
Section 2--Improving Seismic Resistance
2.1: Design and Engineering Basics
Load-bearing walls transfer weight from the roof throughout the structure to the foundation, which disperses this weight into the ground--much the same as a copper grounding rod disperses an electrical charge. Seismic resistance isn't limited to ground movement caused by earthquakes, though; extremely high winds also can cause a home to shift and settle, particularly the roof. New homes have well-established methods of anchoring a home to its foundation using anchor bolts and heavy-duty hold-downs bolted or nailed to 4x4" or 4x6" posts. Older homes weren't constructed to these standards. However, there are several methods for anchoring load-bearing walls to foundations during home restoration to improve seismic performance.
Most home renovation techniques require exposing existing framing in order to access wall studs, wall plates and foundation framing members.
2.2: Using Anchor Bolts to Attach Sills to Old House Foundations
Older homes with brick or concrete block foundations often prove troublesome to anchor because these materials tend to crumble when drilled for anchor bolts. The same can be said for older concrete foundations. Even when anchor bolts are properly drilled and secured with epoxy, rotted foundations tend to shear near these anchor points during seismic activity. For this reason, decaying foundations in old houses should be replaced entirely.
Use a rotary drill to punch holes for anchor bolts. The hole should be wide enough to allow for the bolt and a good amount of epoxy. Distance guidelines are as follows:
- no more than six feet apart
- within 12 inches of joints in the sill
- no less than nine inches from ends of a sill board
2.3: Hold-Downs and other Anchoring Systems
Hold-downs are used to provide added seismic strength. Hold-downs secure a wall's bottom plate to the foundation via bolts into a wall post and an anchor bolt into the foundation. It is recommended to add hold-downs when you have sufficient room to drill for them. Construction hardware manufacturers such as Simpson Strong-Tie also make many straps and anchors to attach plates and floor joists to sills and foundations. These straps or anchors can be attached six or 12 inches apart on sills and at every joist to significantly increase an old house's seismic resistance.
2.3: Reinforcing Cripple Walls
Cripple walls are used to bring a foundation to level height across the footprint of a home. Reinforcing cripple walls prevents buckling during seismic activity. The easiest way to increase the shear strength of cripple walls is to sheath them in plywood--making sure you attach the studs to the sill plate. Nail with 8-penny framing nails every six inches at plywood edges, no more than 12 inches in the field.
2.4: Roof-to-Floor Connections
Use of exterior sheathing to provide a continuous load path is commonplace today, but it's not often found in old houses. When remodeling, if the exterior cladding has been removed, it is recommended to sheath walls prior to re-siding the home. In single-story homes, plywood sheets should run vertically and attach the mud sill at the foundation to the top plate below the roof line. In two-story homes, plywood should run from the mud sill to the second-floor rim joist. Continuous blocking between studs is required if plywood sheathing does not reach the top plate or second-story joist, but it's much easier to buy 9-foot or 10-foot sheets of plywood than to block wall studs.
Straps and hold-downs using continuous threaded rod also are used to provide a continuous load path in new construction, but these techniques can be extremely difficult to properly install during a home renovation.
Section 3: Wind Resistance
Home Renovation Basics
Damage from hurricanes over the last decade is well documented and proves the need for stronger design and engineering, issues that can be addressed when remodeling an older home. Code requirements for wind resistance are common in new construction but are lacking in old houses. These recommendations are suggestions for strengthening roofs in old houses; in some cases it may be better to consult an architect or engineer.
Since frame to foundation connections are covered in the previous section, this portion of the Exterior Wall guide focuses on ways to increase wind resistance in roofing connections.
Reinforcing Wall-to-Rafter Connections
Connecting exteriors walls to rafters and trusses is crucial in transferring wind loads to the building and preventing the roof from being torn off through wind uplift--wind hitting the walls and rising. The easiest connection is to use truss clips, which connect rafters to wall plates with joist hanger nails. However, to access the rafter and top plate you must either remove the top portion of wall cladding and soffit materials, or interior Sheetrock or plaster.
Strengthening roof overhangs in old houses usually requires removal of existing roofing materials and nearby sheathing to access the rafters. Old houses commonly were built with shallow overhangs nailed to the end rafter. The remedy requires removal of the existing overhang and providing continuous anchor framing to the rafter one back from the end using 2x4" or 2x6" "lookouts." More information on this topic can be found in the Old House Web roof guide.
Section 4: Ways to Reinforce Masonry Wall Construction
Reinforcing masonry buildings should be done in accordance to what the builder or renovation professional finds during remodel work, and solutions often require professional guidance. Reinforcement techniques are quite often the same as with wood-framed structures--anchor bolts to attach top plates to masonry walls, and truss clips to attach rafters to wall plates.
Section 5: Moisture Damage
Often during home renovation, decades of exposure to water has damaged integral structural components of the house. Repairing existing mud sills may require use of timbers and jacks to secure and lift the floor while new sills are installed. Small portions of rotted sill or support posts can be fixed with liquid epoxy, a method often used in historical preservation work.