Is your home Hurricane proof? (roofing)

In general, the streamlined aerodynamics of a four-sided hip roof provide much more protection than the two-sided gabled roof, and flat cement tiles offer greater protection than barrel tiles or shingles. You should not be able to see plywood seams on the gable ends (if the gable wall framing is wood).

The gabled ends should preferably be constructed on CBC instead of wood frame. Current research indicates that many of the roof failures (roof truss and shingle failure) could have been prevented if the building inspections during construction were properly performed. One weak link opens the door to disaster: if shingles fly off, the underlying roofing materials become soaked and weaken. Or, once you allow movement, then you’ve got cracks and you’ve got fatigue. Roof leaks can be caused by poor workmanship in the installation of tiles, tar paper or plywood or simply by the age of the roof.

_502187_georges_keywest300.jpgAttics guard homes from the sun and wind – they breathe, absorbing heat before it reaches the living area and exhale heat through vents under the eaves and vents on top of the roof. Turbines and chimneys should be covered with caps when a hurricane threatens. Attached residences should have fire walls and other dividers in the roof to prevent the roof from acting as a single unit during a hurricane.

Pitch Of The Roof:
Flat roofs and roofs that slant only slightly are more vulnerable. Flat tar-and-gravel roofs should have a pitch of at least 3/8″ per foot to allow for proper drainage. The pitch on all other roofs should ideally be greater than 20 or 30 degrees but not too steep. The reason for this is because when wind flows over a lowpitched roof, the same aerodynamics that enable an airplane to fly can help lift a roof from the foundation of the house: the low pressure of the high-speed winds as they pass over the house are offset by the high air pressure contained within the house resulting in an upward lift force against the roof as the wind tries to equalize the pressures.

The design of the early structures in the Keys with their moderately steeped sloped roofs stood up to sustained winds of 140 mph from Hurricane Donna in 1960. Lower sloped roofs in the area produced well over 90 percent of all the roof failures.

Roof Trusses:
A small, low attic is not suitable for storage – materials in the attic put an unwanted strain on the trusses. Trusses must be cleanly nailed together at the joints with nails near the center of the truss. If nails miss or are poking out (not driven in straight) of the side of the truss, the strength of the nail is reduced. Trusses with large knots, splits/cracks or bark on the wood do not meet building codes. Cracked trusses can be caused by improper installation, poor quality wood or added stress on the truss in addition to faulty manufacture.

You may find pieces of wood shimmied in beside a truss – when the truss was installed, it did not sit evenly on the walls of the house. These pieces of wood should be nailed to the truss to prevent movement if the shimmy pops out. Gable end trusses differ from conventional trusses – instead of diagonal weaving patterns designed to resist upward and downward forces, the members run uninterrupted straight up and down. The flat or weakest side of the wood on the gable ends should not face outward.

To repair trusses: Engineer places a 2-by-4 splint on both sides of the truss for additional support around the crack or break (cost $200 to $400). The 2- by-4 should be at least 8 feet long. Periodic maintenance: check periodically for insect damage as well as cracks in the trusses.

Hurricane Straps and Truss Plates

The South Florida Building Code requires straps or truss plates to be attached to each cross beam or roof truss (everywhere wood is joined together). Truss plates should be installed with two 6-penny nails on each beam. Hurricane straps are imbedded in and protrude from the concrete tie beam of the building along various points at the top of the exterior walls of the house. This is what holds the roof to the frame of the house. Every roof joist, rafter or truss must be hurricane strapped. They should emerge from the concrete tie beam flush with where the truss sits; a gap will enable the roof to lift up by that same amount. The straps should be at least 8 inches deep in the tie beam and anchored every six feet on the gabled ends. Straps should be one inch wide, 1/8 inch thick galvanized steel. They must wrap over the top of the truss, and should be nailed through every wood truss with three 16-penny nails on the far side of the truss with the point of the nails bent over.

There should be no slack after nailing. To repair missing or rusting straps: the contractor removes the edge of the roof to anchor new straps into the concrete and attach the straps to the trusses: an 18″ strip of drywall or plaster from the ceiling and wall is cut where the truss is set; the strap is secured to the truss and when the bottom of the strap is run vertically down at least 9″ and secured with power-actuated concrete nails or lag bolts at three points along the perimeter concrete tie beam; ceiling and wall board are replaced and re-painted (cost approximately $450 per strap).

Bracing of the Roof:
Proper bracing is inexpensive and easily installed by a general contractor or engineer ($200-$300).

Horizontal or lateral bracing of the gabled ends of roofs is not required by the South Florida Building Code but it is good business practice. When heavy winds hit a gable roof head on, the trusses will topple onto each other unless sufficiently braced (the domino effect). Lateral bracing is how you tie the gable end truss to the rest of the truss system. Since hip roofs deflect the wind better than gable roofs, their horizontal bracing is not as crucial. Proper lateral bracing means having the proper bracing along the bottom of the trusses and the center of the gable end truss.

Homes should have substantial horizontal bracing: a 2×4 or a 2×6 that stretches from one gable end of the roof to the other. Attach one about one-third of the way up from the bottom and one as close to the peak as you can get it. Attach this board to every single truss as you go along. Attach two more on the opposite side of the roof in the same manner.

1×4″ boards known as rat runs should be found between each truss and running every 10 feet to lend support against being pushed sideways. These boards were meant to be only temporary reinforcement during construction. Don’t discount them but instead, reinforce them with additional nails.

Walers are 2-by-4 horizontal braces that are attached to the vertical studs on the gabled ends. Both members should be joined with a metal bracket or a tightly wrapped metal strap attached with at least two nails at every point. They should run the length of a gable end because they help distribute wind forces among the wood studs (larger spanning walers should be 2 by 12’s). The walers channel the force through the braces which are attached to them and back to the trusses. Braces are only as good as the walers to which they are nailed.

Diagonal bracing (or “X” bracing) connects the top on one truss to the bottom of an adjacent truss. Framing of wooden gable ends should be 2-by-4’s braced back diagonally to the first four rows of trusses to prevent lateral movement (otherwise, one truss will bear too much pressure). Along the way, that board is nailed to the trusses at locations where diagonal web and vertical or sloped truss members join. The brace keeps the ends from being sucked off during strong winds. Start with the top peak of a gabled end, attach a 2-by-4 and extend it back to the bottom of the fourth truss. Do this at least every six feet across the gabled end and then repeat the process by going from the bottom of the gable end to the top of the fourth truss.

The floor of the attic can be reinforced to provide additional bracing by installing four footwide, 1/2 inch plywood with screws along the entire length of the attic along the middle.

The roof overhang houses the soffit panels which should be braced back to the walls. In Dade County, this is required only if the overhang is greater than 15 inches. Plywood should be 5/8″ thick (all lesser thicknesses as well as all particle boards failed during Andrew). Plywood must be nailed with a minimum of 8-penny rounded resin-coated nails to trusses at 6 inch intervals along the sides and every 12 inches in the middle of the plywood and preferably clipped together with aluminum clips at all joints. If the nail misses, it should be removed. Otherwise, the nails will work their way out of the roof, leaving a hole.

Check the underside of the plywood while in the attic for exposed nails. If an entire row of exposed nails is present, the plywood should be re-nailed. That portion of the roof will have to be removed and re-nailed. (Cost $450 per 100 square feet). Check also for gaps in between the sheets of plywood. Gaps greater than 1/8″, which is allowed under the Code, can cause wind from the attic vents to blow through the gaps and lift that portion of the roof.

With a shingle roof, the winds can blow through weak asphalt shingles and through the gaps. If you have a leak, look in the attic for gaps in the plywood and/or rotting wood. Particle or pressed board was allowed during the 1980’s and has since been outlawed.

Nails and Staples: Plastic-coated nails must be used to fasten plywood to the trusses to bond the nail to the wood and truss. Roofs that are attached by staples are not as strong because the staple itself may become crimped by the application of the staple gun (banned in Dade County since Andrew).

Also, the stapled shingles blow off more easily because the surface of the nail head is greater than that of staples. Hand Nailed vs. Nail Guns: nail gun nails are t-shaped and grab less decking than the rounded hand nails, are often driven too deeply into the plywood, and you cannot always tell when they hit wood. Recent tests show that a combination of gluing and nailing of sheathing to the trusses, doubles the holding power over nailing alone.

Tarpaper should only be attached with roofing nails and tin tabs, not staples. If a roof has a low slope, use double felt under shingles. In 1957, the South Florida Building Code required #30 felt on top of the roof decking. In 1980, this was changed to two layers of #15 felt. Since Andrew, #30 is again mandatory as is an inspection of the trusses, bracing and deck before the felt application. Roof Tiles should be consistently secured with evenly spread mortar and nailed down (nailing is not required by the Code unless the pitch of the roof is steep – otherwise you may use nails or cement or both). The mortar installation is more important than the nails and proper installation is a must. The mortar should be evenly applied in a continuous horizontal strip, not globbed on. The cement must be wet enough and the tile must not be too hot: moisture can be absorbed from the mortar resulting in the surface of the tile not adhering properly to the cement. During Andrew, the nailed tiles in the most damaged areas were blown off and more than half of the roofs with cemented tiles also blew off (due to improper cement application). Never install tiles on top of old shingles.

Flat cement tiles are better than barrel tiles because they are heavier and their shape makes them less likely to catch winds although the porous clay tiles adhere better with mortar than the cement tiles. Use a 90 pound hot mop membrane waterproofing, not a 43 pound as allowed by Code, and insist on copper flashing and drip edges, not galvanized. Regardless of the type of covering used, installation over a sound and tight roof system (properly nailed and spaced roof sheathing) is the most important factor. Cost to re-roof: $350-$400 per 100 square feet for cement shingles ($450-$500 for barrel tiles). Add $500 for installation with nails.

Shingles: Shingles should be nailed (not stapled) carefully to the roof: six roofing nails (two in each tab directly above the sealant strip) should be used in each asphalt shingle and should be long enough to pierce 3/4″ into the roofing deck. Since Andrew, in Dade County the number of nails required per shingle was raised from four to six. Each shingle has three tabs on one end that are not coated with granules with the adhesive strip approximately center on the shingle.

Plastic must be removed from the adhesive strips for the shingle to properly bond to the other shingles. What makes one shingle more wind resistant than another is the sealant used on the shingle. The tougher shingles are darker, less attractive and more expensive. Asphalt shingles with embedded zinc particles are made to withstand mildew, but minimum 250 pound weight shingles are preferred, and if the roof has a low slope, use double felt under the shingles. Cost to re-roof: $175 per 100 square feet for fiberglass shingles (add $500 for installation with nails instead of staples).

1 Response

  1. Paul says:

    ” Truss plates should be installed with two 6-penny nails on each beam. ”

    The latest wind mitigation inspections (4 page) for insurance companies now state three nails, not two. Many people will soon be losing their insurance discounts with this change.

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