Winds Under 110 mph Simply Do Not Damage Concrete Tiles

With tile roofs popular across Florida, and with questionable claims about roof damage raising concerns in insurance and engineering circles, two Florida consulting engineers have offered this guide to aid engineers, attorneys, adjusters and property owners in examining wind damage to concrete tile roofs. This can be used to help understand the dynamics of wind and its effects. Key points have been highlighted in boldface type.

Introduction

Buildings are obstacles to wind currents and will cause changes in the direction of airflow. Redirection of the airflow results in varying magnitudes of negative and positive wind pressure over building surfaces. In general, inward-acting (positive) pressures are produced on windward surfaces, and outward-acting (negative) pressures are produced throughout the building surfaces (Dalgliesh, 1965). Further, since the flow of air cannot negotiate sharp discontinuities in building surfaces such as wall corners, eaves, roof ridges, and roof corners, the flow will separate from building surfaces, resulting in outward-acting pressures. Failure of cladding in these areas typically will occur first, followed by damage to unsupported components or loosely fastened structural elements. The evaluation of wind effects on structures is outlined in the American Society of Civil Engineers (ASCE) Minimum Design Loads for Buildings and Other Structures, with the 2010, 2016, and 2022 editions (referred to as ASCE 7-10, 7-16, or 7-22; respectively) being the ones most often utilized.

Evaluating concrete roof tiles in accordance with ASCE 7 and knowing the areas of the highest pressures is important. A proper analysis cannot be done without basic knowledge of wind dynamics. The formulas used for any calculations must be from approved sources and should be true and accurate.

The shielding effects of nearby buildings, trees, and other ground-level obstructions are well documented. It has been shown that in all but the most devastating windstorms (e.g., hurricanes and tornadoes), most conventionally built structures will not suffer major wind damage to the primary structural system. Broad differences in the performances of buildings exposed to wind forces often are observed during a storm event, even between structures in proximity to each other. Structural performance also depends on variations in location, orientation to the wind direction, and the quality of the original construction or subsequent repairs.

Even in relatively severe, but localized weather events like a tornado, the strongest winds are straight-line winds. During a hurricane, winds are usually straight-line at the site, while the larger wind patterns are circular. Therefore, evaluation of damage should follow the general principles outlined in the preceding paragraphs. Seemingly inexplicable wind behavior is uncommon and can be verified or discounted with close examination of the damaged structure. Further, if damage has occurred to the structural frame, then displacement will be evident in the attached finish materials. Substantial damage to exterior cladding typically will precede damage to interior finishes. Damage from wind forces is not “hidden” or concealed by the structure, only to appear at a later date.

Type of Damage to Tile Roofs from Storm Forces

Since the flow of air is redirected at sharp discontinuities in building surfaces, such as wall corners, roof eaves, and ridges, higher wind forces are typically experienced at these locations during a strong wind event. On a systemic level, initial roof tile failures (displacement) will normally occur first along the edges, corners, and ridges of the roof and can progress inward to the field tiles as pressure forces increase (Florida Roofing and Sheet Metal Contractors, December 31, 2020). This behavior of wind forces is recognized by applicable building codes that require these areas to be designed and constructed to resist higher forces for a given wind speed than other parts of the building. Greater resistance to uplift at the eaves and ridges/hips is usually achieved with a stronger method of attachment, e.g., via mechanical anchorage or additional fastenings. Testing of the ridge and hip concrete tiles has found the uplift forces need to be in excess of 230 pounds of force to dislodge an adhesive-set tile and in excess of 500 pounds for mortar-set tiles (Mirmiran, 2006).

Initial wind-related damage to a tile roof is usually the result of one of two mechanisms: wind-uplift forces of sufficient magnitude to overcome the securement of the tile to the roof substrate, or collateral impact-type damage to the tiles from wind-borne debris, such as loose tiles or tree limbs. The latter type of damage can occur randomly on the roof during the storm and will often show up as tiles that are “shattered” or have a “spider web” crack pattern.

In contrast, damage caused by footfall is generally characterized as individual cracks that extend horizontally, vertically, or diagonally across the tile. Cracks that extend across the lower corners of individual tiles are also typically related to footfall or thermal expansion and contraction between individual tiles or between the tiles and the substrate (Tile Roofing Institute, 1999) (Boral, 2000).

If the tiles are not properly aligned, there is the potential for point loading that puts irregular pressure onto the corner, causing it to fracture. This most often happens when the tiles are applied too tightly together. Most tiles are designed to be installed with a 1/16″ shunt or separation between the tile bodies. If this shunt is not maintained, damage from foot traffic or the expansion and contraction of the roof deck could result. Debris left in the channel during application could also result in point loading that may break the corners under foot traffic.

Fractured tiles can occur if the speed of the wind is able to lift a tile while not removing it from the roof assembly. This type of fracturing will usually not occur at speeds below 100 mph in the field.

One key component to evaluating wind damage to the tile roof system is the condition of hip and ridge tiles. If the hip and ridge tiles are loose or not bonded, they are at risk of being blown off the roof. The hips and ridges, as well as the edge tiles have the greatest wind pressures. If these tiles are loose but not moved or missing, then the roof has not had a wind event with enough force to remove, lift, displace, or damage the field tiles. Checking for loose hip, ridge, and rake tiles that can easily be lifted yet are not displaced will discount any claim of wind damage due to lifting of the tiles.

Wind-Speed Pressures

Pressure can vary during an event. It is important to perform the minimum calculations to determine the maximum and minimum values on a roof covering. The corners, hips, ridges, gables, and angle changes will result in elevated positive and negative pressures. The pressures at these key locations can be high for even low wind speeds but the area of impacts are minimal, typically less than 3% of the total roof area. The whole roof is immersed in air flow that is fairly uniform, moderate suction, and further changes in the slope or shape of the roof system will not greatly affect the pressures.

Design Wind Pressure

Often it is claimed that low wind speeds result in damage to a concrete tile roof. Tiles have been tested for years using wind tunnels and wind calculations. Testing standards for approval in Miami-Dade, for example, date back to 1994 and started with wind speeds of 110 mph. Generalized calculations indicate that wind velocities adequate to generate wind pressures sufficient to cause damage at speeds below 110 mph would be limited to low-slope roofs typically covered with modified bitumen or shingles. Damage to concrete tiles due to uplift or displacement at wind speeds below 110 mph does not happen, and any claims of such damage are false.

The design pressure is a pressure generated using calculations after adjusting for obstacles or structures. In short, the wind pressure is used to determine the actual pressure used for design and can vary by location. The design pressure is greater than site conditions—site conditions that typically will not be exceeded during a normal storm event. An extreme weather event that has a once-in-500-year occurrence is not accounted for.

Tile Movement

The biggest misconception for roof damage as a whole is that most engineers do not actually utilize the full calculation for a roof. Concrete tile roofs require a moment calculation to determine the lifting potential. In other words, even if the uplift is greater than the tile’s weight, it does not mean it will lift the tile.

Reviewing the standards, it is easy to determine that the uplift must be calculated at the components and cladding areas to obtain a reasonable value for the pressures. The uplift pressure is greater at the corners of the building and along hips and ridges. It is important to note that, to determine the pressure due to wind, you must also consider the uplift due to air migration under the roof tiles. The calculations require the uplift moment to be calculated and the restoring moment due to gravity. The difference of the two moments will determine the potential for the roof tile to move. If the moment difference is zero or less than zero, then the roof tile will not move.

The gravity moment will also be found in approval notices for each roof tile as well. Identifying a roof tile and knowing the year of installation can be used to find an approval sheet, giving the gravity moment and saving a lot of time versus calculating.

Rain Adds Weight to Tiles

Tiles are typically concrete and are constructed utilizing minimal amount of water to do a dry pack. The result is a tile with a density that is 130 pound/cubic foot (lb/cf) while normal concrete is 150 lb/cf. This means the concrete tiles have a mass that is 14% less than normal concrete. This results in voids that will fill with water during storm events. Testing has determined that the tiles can have an increase in weight of 9% or more due to water. This will increase the restoring moment due to gravity.

The bottom corner of the tiles can be chipped over time and the small broken piece will usually be found on the roof. This also is true for fractured tiles that have multiple sections. Small pieces of roof tiles have a minimal weight and if still on the roof, then the wind was not of such an elevated level to remove the light pieces. While small pieces can remain on the roof due to shielding effects of the roof or due to the low wind speeds not capable of displacing the light pieces. The chipped corners are not due to wind and are a typical occurrence for tiles that are placed tightly together, undergo thermal stresses, debris in the interlock, and point loads on the corners. The manufacturers have bulletins on the chipped corners and causes.

High Winds Will Likely Blow Tiles Off

When uplift forces are of sufficient magnitude to overcome both the weight of the roof tiles and the securement of the tiles to the substrate if anchored, it is likely that the affected tiles will be either blown off the roof or be displaced from their installed positions. Wind effects may cause some tiles that are already loose to slide or move out of alignment with the adjacent tiles or courses of tile. Wind uplift forces will not lift large numbers of well-attached ridge/hip caps and deposit them directly back into their originally installed positions. The fact that the tiles can be lifted is not an indication of damage.

Conclusion

Wind speeds below 100 mph will not damage roof tiles made of concrete at the fields, as noted above. Displacement of hip and ridge tiles is possible if the tiles are de-bonded. Claiming a roof tile can chatter or move is a false claim for speeds below testing speeds and which does not account for the rain that will increase the weight of a tile. Wind speeds well below the testing threshold of 100 mph will not damage the roof tiles. Calculating the pressures at each wind speed can be cumbersome but it is easy to determine the uplift and find if the basic speeds do not generate enough lift in psf to even lift a tile.

It is important to question professional claims of damage in low wind speeds if there are no missing or displaced roof tiles and no supporting documentation. It is important to note that opinions without backing by studies, codes, mathematical computation, or research are not considered professional opinions. Claimed concrete tile damage from low-speed winds is incorrect and not supported by testing standards, wind tunnel testing, ASCE 7-10 or 7-22, or other mathematical formulas noted above.

George Miles is an engineer with Alligator Consulting Engineers, based in Daytona Beach. He has testified and drafted reports for insurance claims litigation. Daniel Frates is senior principal engineer at SDII Global, an engineering firm with offices in Tampa.

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