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Roofs are a critical component of the building envelope. Maintaining the integrity of the roof assembly during a storm is a key step to minimizing damage to the home and contents. Yet roof systems, especially in homes built prior to Building Code revisions enacted as a result of Hurricane Andrew, are a well-documented weak link in the average home’s structure.
The Hurricane Retrofit Guide prepared by the FL Division of Emergency Management, Bureau of Mitigation, reports that a survey by the National Association of Home Builders found that 77% of homes affected by Hurricane Andrew experienced widespread roof damage. This roof damage led in turn to extensive water-related damage to structures and contents both during the storm and in the days that followed. More recently, 95% of all homes filing insurance claims after the hurricanes of 2004 and 2005 devoted some portion of the claim to repairing roof damage.
The key point, is that when the roof fails, rain enters the structure saturating walls, ceilings and personal possessions. The days that follow a tropical storm are usually hot and without electricity. With no power to run HVAC systems, building components and personal possessions remain saturated and mold forms. In fact, it is this type of water-related damage rather than wind-related destruction that accounts for the majority of financial losses due to hurricanes.
To view the entire Hurricane Retrofit Guide series of recommendations for improvements that make homes more hurricane-resistant, write down the following web site and visit it after you complete this course:
www.floridadisaster.org/Mitigation/rcmp/HRG/index.asp
Retrofit activities to strengthen roofs should focus on the following four areas:
- Improving the condition of the roof structure, especially bracing gable ends and reinforcing the connection between the trusses or rafters and the supporting wall.
- Making sure the roof deck sheathing is sound and nailed according to the appropriate standard for the Basic Wind Speed zone in which the home is located.
- Making sure the underlayment beneath the roof covering is sound, fastened appropriately, and can function as a secondary water barrier if the roof covering fails.
- Making sure the roof covering is appropriate for the Basic Wind Speed zone, is sound, and is appropriately fastened to the roof deck.
The extent of possible improvements in each of these 4 areas increases significantly if the existing roof covering is ready for replacement. During the process of re-roofing, far more can be accomplished in all 4 of the above areas than if the retrofit work is carried out from the attic access below the sheathing.
source: FL Division of Emergency Management |
One final important point about retrofits aimed at strengthening the roof is that the 4 areas for improvement are inter-dependent. For example, installing a high-wind-resistant shingle on a poorly fastened roof deck will not prevent a roof failure.
Similarly, replacing damaged sheathing and re-nailing the entire roof deck to current codes, but failing to reinforce the connection of the trusses to the supporting walls is also a recipe for failure. In the first case, the sheathing below the wind-resistant roof covering may fail creating large holes in the roof plane.
source: Simpson StrongTie |
For the entire roof assembly to perform well during a storm, each of these 4 areas needs to be addressed.
The rest of this section will look at ways to improve the connection between the roof assembly and the supporting wall. The other critical improvement to roof structure, gable end bracing, is the subject of Course Module #4 in this series (Bracing Gable Ends). Retrofit improvements to sheathing, underlayment and roof coverings will be addressed in the remaining sections of this Course.
Reinforcing Roof to Wall Connections
source: IL Emergency Management Agency & FEMA |
Most wood frame homes are built using a construction technique known as “platform” framing. This technique creates a number of transition points that rely on gravity and fasteners to resist structural movement due to imposed loads.
Some examples of these transition points include:
• Mud sill to foundation wall
• First floor floor assembly to mud sill
• First floor walls to first floor floors
• Second floor floors to double top plate of first floor walls
• Second floor walls to second floor floors
• Roof assembly to double top plate of second floor walls.
source: IL Emergency Management Agency & FEMA |
Each of these transition points is a weak spot in a high wind event such as a hurricane. Resisting the wind-induced stresses of a hurricane requires the creation of a “Continuous Load Path” that reinforces all of these transition points and transfers extraordinary wind loads all the way to the foundation. The next drawing illustrates the concept of Continuous Load Path and identifies the transition points that need to be reinforced.
The roof to wall connection is one of the most critical of these transition points. As we learned in the Course Module #1 (How Hurricanes Damage Homes), wind flowing over a roof creates uplift forces that try to suck the roof off the building. The traditional method of attaching a roof assembly to the top plate of the supporting wall is to toenail each rafter or truss with 3 x 8d or 10d nails. This, plus gravity acting on the combined mass of the roof assembly components, is all that holds conventionally installed roofs in place. Uplift forces generated by hurricanes have shown how easily this traditional installation can be overcome.
source: IL Emergency Management Agency & FEMA |
One way to strengthen the roof to wall connection to resist these uplift forces is to install metal framing anchors wherever trusses or rafters are attached to the supporting wall below. There are several ways to go about this.
If the existing roof covering is being replaced, the first row of sheathing along the eave can be removed exposing the point at which each truss or rafter is attached to the wall below. This makes it possible to reinforce these connections with metal framing anchors. The next several pictures show examples of framing anchors that can be used to reinforce the connection between trusses or rafters and the supporting wall.
source: Simpson StrongTie |
source: Simpson StrongTie |
source: Simpson StrongTie |
source: IL Emergency Management Agency & FEMA |
source: IL Emergency Management Agency & FEMA |
WHENEVER USING METAL FRAMING ANCHORS, IT IS EXTREMELY IMPORTANT TO INSTALL THE TYPE AND NUMBER OF FASTENERS REQUIRED BY THE MANUFACTURER TO ACHIEVE MAXIMUM LOAD RESISTANCE.
Another important point is to pick an anchor that engages as much of the framing as possible. For example, if the trusses or rafters align with the studs in the supporting wall, pick an anchor that extends all the way from the stud to the truss or rafter and provides for fasteners in the stud, both of the double top plates and the truss or rafter as well. This engages the wall studs and sheathing of the lower wall in resisting the uplift forces.
source: FEMA Technical Fact Sheet no. 17 |
With the first row of sheathing removed, it should even be possible to install a strap type anchor such as the Simpson MTS12 (shown) or the USP MTW12 by Gibraltar Industries that wraps over the top of the truss. This type of anchor provides extraordinary uplift resistance.
source: Simpson StrongTie |
source: Simpson StrongTie |
source: Simpson StrongTie |
Anchoring the truss or rafter only to the top plates still leaves the possibility that the uplift forces would be great enough to withdraw the nails holding the double top plate assembly to the studs below. If the truss connector is only attached to the top plates, the connection of the plates to the studs beneath has to be reinforced with another connector as shown in the next picture.
source: Simpson StrongTie |
source: Simpson StrongTie |
If the existing roof covering is NOT ready to be replaced, it is still possible to reinforce the roof to wall connection. In most cases, lack of head room where the roof meets the supporting wall makes it impossible to access these connections from the attic crawl space. One alternative approach is to work from the exterior of the home, exposing the roof to wall connection by removing soffits below the eave overhangs. It is generally not possible to install an anchor that wraps over the top of the truss or rafter, but it is still possible to anchor the trusses or rafters to the top plates and to reinforce the connection of the top plates to the studs below. Another approach is to work from inside the house and remove drywall from the joint between the ceiling and the outside wall.
source: FL Division of Emergency Management |
Naturally, this results in the most disruption to the occupants, but it is an effective way to expose the roof to wall connection points and install metal framing anchors.
If the supporting walls are made of concrete or masonry CMU’s (“blocks”), the retrofit options are basically the same. Once again, the main question is whether or not the existing roof covering is being replaced. If so, the roof to wall connection points can be exposed by removing sheathing at the eaves and gable ends. Then the trusses or rafters can be secured directly to the supporting walls by the appropriate combination of metal anchors installed with approved masonry fasteners.
source: Simpson StrongTie |
source: Simpson StrongTie |
In some cases, trusses or rafters may already by anchored to the top of the concrete or masonry wall by metal straps or clips. If these anchors are embedded in the top of the concrete or masonry wall, it is important that they are properly aligned. If they are "offset" from the trusses or rafters, there is potential for wind-induced uplift before they engage.
source: FL Division of Emergency Management |
If the existing roof covering is not ready to be replaced, the top of the concrete or masonry wall can be accessed from the exterior through the soffits or from the interior by removing the wall finish at the ceiling to exterior wall joint.
source: FL Division of Emergency Management |
When the supporting wall is made from or CMU’s ("concrete blocks"), attaching the roof assembly to the supporting wall only makes sense if the masonry wall itself is adequately reinforced. Necessary points of reinforcement include:
- Horizontal joint reinforcement in the mortar joints
- Vertical reinforcement with steel rebar in grouted CMU cells
- Continuous “bond beam” or “tie beam” of steel reinforced concrete around the entire top of the wall.
These types of masonry wall reinforcement are extremely expensive to add after the home is built.
source: FL Division of Emergency Management |
Another critical roof to wall connection point is at the gable end overhang. At this point, the uplift forces developed by wind flowing over the roof are increased by the force of the wind pushing up on the overhang itself.
Gable end overhangs are traditionally framed in one of two ways:
• As a separately framed “ladder” attached to the face of the gable end wall and supported by attachment to the cantilevered roof deck sheathing.
source: FL Division of Emergency Management |
• Or as a series of “lookouts” or “outriggers” attached to the side of the first truss in from the gable end and extending out over the top of the gable end truss.
source: FL Division of Emergency Management |
The “lookout” method requires that the height of the gable end truss is shorter than the other roof trusses by the width (usually 2x4) of the cantilevered lookouts. This is actually the easier of the 2 systems to reinforce. With the sheathing at the gable end removed during the re-roofing process, the points where the lookouts are attached to the main roof truss, AND the points where the lookouts cross the supporting gable end truss can be reinforced by appropriate metal connectors. If the gable end wall is framed conventionally, the lookout to rafter connections and the lookout to gable end top plate and stud connections can be reinforced with the same type of connectors used at the eaves. This type of retrofit can also be carried out from the attic access if the existing roof covering is not being removed.
For “ladder” type gable end overhangs, though it is possible to add more fasteners through the framing member that attaches the ladder to the gable end wall, the primary form of reinforcement is to strengthen the connection of the roof sheathing to the gable end truss or rafter. The next section will review steps to accomplish this.
source: FL Division of Emergency Management |
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