Features

Putting a Lid on It . . . and keeping it on

By Peter Kerrigan, Director, Engineering Design Services

As a young boy, my mother took me and my siblings to see the musical “Fiddler on the Roof.” My younger brother nearly had a heart attack when the graveyard scene was shown. Myself, well I was simply horrified at the condition of the roof and very concerned for the safety of the fiddler. Clearly I was destined to be a structural engineer.
Roofs are like people, they come in all shapes and sizes and some are healthier than others. The negative effects of Hurricanes Irma and Maria in the Turks & Caicos Islands this autumn clearly demonstrated the importance of having a strong and fit roof when living in a hurricane-prone area.

About 80% of TCI residences received some level of damage from the hurricanes, most commonly to roofs.

Hurricane Irma, which struck the Turks & Caicos Islands on September 7–8, 2017, was the closest approach of a Category 5 hurricane to the country on record, with all of the country’s islands being affected. Accumulated Cyclone Energy (ACE) is a measure used by the National Oceanic and Atmospheric Administration (NOAA) to approximate the wind energy used by a tropical system over its lifetime. Hurricane Irma generated the most ACE on record in the tropical Atlantic, and even more than 18 entire Atlantic hurricane seasons in the satellite era (since 1966). Irma’s peak sustained winds were estimated at 185 MPH, with gusts even higher. That’s what the roofs of dwellings across the Turks & Caicos had to contend with.
How did they fare? In its rapid assessment report post-Hurricane Irma, the Caribbean Disaster Emergency Management Agency (CDEMA) reported that the Turks & Caicos Islands suffered nearly 80% damage to its housing stock and critical infrastructure during the hurricane’s passage. The report concluded that damage across the TCI was similar in nature and related largely to roof damage, especially level one damage. This was categorized as: loss of roof tiles, small sections of roof damage or destroyed windows or door damage, guttering and eave damage or loss, water damage to sections of ceilings and varying degrees of water intrusion.
Anyone who viewed the TCI “live” or via photos on the Internet in the weeks and months following the storm would bear witness to the magnitude or severity of roof damage. Blue tarp and peel and seal was too common a sight. Sadly, it was an especially wet “rainy season” in October and November, and those who did not have the means to quickly repair their roofs suffered greatly with additional water damage and resultant mold.
The geometry of a roof plays a significant role in how it performs in high winds. Steep pitched roofs react differently than shallow pitched or flat roofs and the direction of the wind relative to a particular roof often determines if the wind pressures are positive (pushing) or negative (suction). There is a misconception that the predominant pressure exerted on a building is a positive pressure. Depending on roof type and wind direction, there is just as much chance of a suction wind pressure being applied as a positive pressure.
It goes without saying that all roofs should be designed and built to withstand these pressures. Engineers determine these pressures from loading codes such as ASCE-7, an integral part of building codes in the United States, published by the American Society of Civil Engineers. The code describes the means for determining dead, live, soil, flood, tsunami, snow, rain, atmospheric ice, earthquake and wind loads, and their many combinations are then used as the basis for structural analysis and design.
The resultant design pressures are used to analyze the roof frame and cladding (covering) and the building as a whole. The negative pressures are used to determine uplift loads at rafter or truss-bearing points and those loads are used to determine what strap or mechanical fixing is chosen to restrain/hold down the rafter or truss. Simpson or USP are the most common straps or fixings. The too-often installed “hurricane clip” (See Figure D above) should not be used on residential projects. They are too small for most, if not all, residential projects and do not have sufficient capacity to cope with the uplift loads resulting from a hurricane. Instead, hurricane straps of adequate size should be used.
All roofs should be sheathed (decked) with either plywood or Tongue & Groove boards. It provides the substrate to which the peel and seal and roof finish is laid. The sheathing should be at least 5/8″ thick but some roof finishes require a fixing embedment of 3/4″ to achieve compliance, and in such cases the sheathing installed should be 3/4” thick.
The sheathing should be secured to the rafters at a minimum of 6″ centers throughout the roof surface. Leading edges along the ridge, eaves and hips should be secured at 4″ centers. Adequate fixings with appropriate embedment into the rafters should be installed. The hot- dipped galvanized “Ring Shank” nail is a good example of a recommended fixing.
The sheathing is not only utilized as a substrate on which to fix the peel and seal or roof finish. A critical part of its job is to provide diaphragm action to the roof. This diaphragm action essentially stiffens the roof plane and helps to transmit lateral (horizontal) loads to the main walls and downwards to the foundations. The sheathing and how it is connected to a roof in a hurricane zone is all-important.
The roof or roof framing can be constructed in many different ways and some roofs are by their nature more stable than others. A “stick” or “cut” roof is a perfectly acceptable roof if the rafter size is appropriate for the span, however if a collar tie is included the roof becomes more stable by default. A full truss is yet more stable and probably the most stable of all roofs by virtue of its triangulation. Each roof type and its associated member sizes will have their limitations, and depending on what roof or combinations of roof styles are chosen by a homeowner, the engineer must design accordingly.
The structure onto which the roof will be supported must also be well designed and fit for purpose. Reinforced walls should incorporate stiffener columns and belt or ring beams. Timber frame buildings should be braced to prevent racking or lateral sway. (This can also be achieved by sheathing the inside or outside of the timber stud walls with plywood.) The roof on a timber frame building needs to be connected to the wall, which in turn needs to be connected to the floor, and the floor to the foundations. And very importantly, the weight combination of all elements needs to be greater than the total uplift load resulting from a hurricane storm.
Any roof should be well maintained over its lifespan. All enclosed timber roofs should be well ventilated, but not too much so that it weakens the roof. Timbers and plywood should be pressure-treated.
We structural engineers are often accused of over- designing. My typical response is, “We design for the worst case scenario.” Now, many of us know what that worst case scenario looks and feels like. We all hope and pray we won’t have to experience one again. But in all likelihood, we will. So we should at least be prepared.
The finished roof is more than a roof. It is one of the most important components of a home. It protects the building, your family and your possessions. It should be well designed and well constructed—well fit to host a fiddler like Irma, should she turn up to play a tune.



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