Roof Coverings, Underlayment and Roof Sheathing
Roof Covering: the roof covering is a critical part of the house’s first line of defense against hurricane winds and wind driven rain. It has historically been one of the weakest elements in that line of defense. In Hurricane Andrew, a study by the National Association of Home Builders Research Center showed that widespread damage to roofs seriously affected 77 percent of the homes surveyed. This damage to the roof covering led to severe water damage both during and after Hurricane Andrew. Damage surveys conducted following the hurricanes of 2004 and 2005 have also highlighted the frequency of roof covering damage. Of the homes that had enough damage to file an insurance claim, 95 percent had some level of roof covering damage. Studies have also highlighted the facts that damage to the roof covering during a hurricane can lead to substantial water damage in subsequent rainstorms. Both the initial water intrusion and the later leakage can lead to significant mold problems.
Extensive loss of roof covering and underlayment usually results in extensive water intrusion and internal damage including collapsing ceilings.
A Wind Resistant Roof
Building codes generally allow homeowners to install a second layer of asphalt shingles over an existing layer of old shingles. However, for a more secure roof during hurricanes and other high wind events, it is strongly recommended that the old shingles be removed before installing the new ones. This will help the shingles seal properly (wind rated shingles have only been tested when applied over a smooth roof deck, not over the bumpy surface of an existing layer of shingles) and provide a more wind resistant roof covering. When replacing the roof covering the opportunity exists to take several additional steps, all at relatively little extra cost (replacing damaged or decayed sheathing, re-nailing the roof sheathing to increase its resistance to being lifted off in a hurricane, and applying a high quality underlayment to make the house safer).
The house above was exposed to a direct hit from the eyewall winds of Hurricane Ivan and had been re-roofed with wind rated shingles, including re-nailing the deck, about 4 years before Hurricane Ivan struck. If the porch roof had not come off and knocked off the chimney, there would have been very little damage to the roof as no shingles came loose other than those next to the chimney. A couple of small sections of ridge vent suffered anchorage failures.
This house lost most of its shingles and some of the sheathing at the gable end.
If a high quality underlayment was properly installed, all roof edges should have a metal drip edge installed over the top of the edges of the underlayment and the drip edge at the eaves should have adhesive sealant under the edge to keep water from seeping under the drip edge. Complete the seal at the edges by applying a 4-inch wide layer of asphalt adhesive along the roof edges (eaves and rakes) to seal the edge and to adhere the perimeter shingles or starter strip (at the eaves) to the edge of the roof.
Choose shingles that match or exceed the wind rating for the design wind speeds in the area. There have been some dramatic improvements in the wind ratings of shingles in recent years. It wasn’t too long ago that the highest rated product was only rated for about 65 mph. Now, products are being evaluated with a new test method and rated with letters such as D, F, G, and H. The H rated shingles are rated for 150 mph while the G is rated for 120 mph, F for 110 mph and D for 90 mph. (See Table 1507.2.10 from the Florida Building Code Supplement.)
These rating are established at the time of manufacture based on the resistance of the sealed tab to uplift. The adhesive strip is critical to this rating. The uplift resistance has been correlated with the forces expected on that shingle for the indicated wind speed based on wind load research. It is best seen as a relative rating of the shingle products based on their ability to resist tab uplift at the time of manufacture, not a guarantee that it will not blow off in a storm with those winds after it has been installed on the roof for a number of years. Nevertheless, the ratings do indicate that the tabs of higher rated shingles are more likely to be well adhered to the top of shingles below.
The Florida building code requires that shingles be rated for the design wind speed where the home is located up to 110 mph (F-rated). F-rated shingles or those tested using the older ASTM D-3161 (modified for 110 MPH) and Miami-Dade County test protocol that requires testing of shingles by subjecting them to 110 mph winds for one hour are required anywhere the design wind speed is 110 or greater. Since the new higher rated shingles now exist, we recommend that you use one that matches or exceeds the design wind speed for the home’s location. The Asphalt Roofing Manufacturers Association, ARMA, has prepared a document describing the new rating system “ARMA Discussion of new high wind standards for shingles.” It states:
“When shopping for a new roof, ask you roofer to quote a roof that meets the minimum wind requirements for the area and the next two wind speeds higher. You might be pleased at the relatively modest cost increase to get additional protection.”
To maximize the high wind resistance of shingles they must be attached with nails and it is recommended that the nails be installed by hand instead of by a pneumatic nail gun. If a pneumatic nail gun is used, the settings need to be checked frequently to ensure that the nail heads are not being driven incorrectly. Both ARMA and FEMA have prepared documents that illustrate proper nailing of shingles (ARMA Nail Application of Asphalt Shingles – FEMA Asphalt Shingle Roofing for High Winds). The general recommendations for the nails are that they have a minimum head diameter of 3/8-inch and that they penetrate 3/4-inch into a wood deck or completely through plywood or OSB sheathing. In areas with design wind speeds greater than or equal to 120 mph, the recommended fasteners (required in the High Velocity Hurricane Zone) are ring shank nails with a minimum of 20 rings per inch and the penetration of the fasteners is 1-inch into the wood deck and clear through plywood or OSB sheathing. The nails are required to be corrosion resistant. Galvanized steel, stainless steel, aluminum or copper roofing nails are allowed in the Florida Building Code.
The FEMA technical guidance recommends the use of stainless steel nails for homes within 3000 feet of salt water. The location of the nails on the shingle is critical and the manufacturer’s recommendations need to be carefully followed. The nails should not be installed in the adhesive strips. Some manufacturers have added lines to show installers where to put the nails for their products. If there is one thing you want to check (and check several times during the course of installation, it is that the nails are being installed at the proper place on the shingles. You can learn where they should be nailed by looking at the instructions printed on wrappers/packages. Roofing experts examining why new shingles failed in recent hurricanes concluded that a primary cause was improper placement of nails.
A Watertight Underlayment
Once the roof sheathing anchorage has been checked and improved, if necessary, the next step is to add a high quality underlayment which forms a good water barrier. The roof covering is the “raincoat” that helps to shed water from the roof of the house. The underlayment provides the waterproof foundation upon which the roof covering (shingles, tile, metal, or other products) is applied.
There are a large number of options for installing an underlayment that will keep the roof dry during most normal rains and thunderstorms. The building code and reference documents from the various roofing manufacturers provide details on the options. You can get information on underlayments for tile roof installations from the Tile Roofing Institute (TRI), www.tileroofing.org, or the Florida Roofing, Sheet Metal and Air Conditioning Contractors Association, Inc (FRSA), www.floridaroof.com. For high wind installations, both the TRI and FRSA will direct you to the FRSA Concrete and Clay Roof Tile Installation Manual. In addition, the FRSA web site also provides the following:
• Glossary of roofing terms for the consumer
• Identifying parts of a roof
• What every property owner should know about hiring a roofing contractor (English and Spanish versions)
Other groups such as the National Roofing Contractors Association (NRCA), www.nrca.net also provide guidance. The NRCA maintains a roof installation manual that goes into a great amount of detail and while the four volumes are expensive to buy, they do have a copy that you can review online and even copy selected pages. Their guide is available at: http://www.nrca.net/consumer/technical/manual/manual.aspx
The particular fact sheet applicable to roofing underlayments is available by going to FEMA Underlayment Recommendations for Shingle Roofs. (FEMA_hgcc_fact19_Underlayment_for_asphalt_shingle_roofs) The main differences between the guidance given below and the FEMA guidance are the recommendations (below) that self adhesive membranes be attached directly to the roof deck (except in Dade and Broward Counties where fully adhered membranes are not allowed by Code. However, 4” to 6” wide strips of self adhesive are allowed throughout Florida), and that the drip edge at the eave be placed over the top of the underlayment with a bead of asphalt adhesive to seal the drip edge to the underlayment.
Since there is so much damage to roofs, typically 70 to 90 percent of homes that are damaged in hurricanes suffer roof damage, this document takes the approach of recommending systems that provide backup protection from water getting into the attic if the roof covering is damaged. These better methods provide underlayments that can also serve as the weather covering if they are called on to do that because of damage to the normal roof coverings. These top-of-the-line options are not the lowest cost alternatives, but they may make the difference between having water damage and trying to temporarily keep water out while waiting for repairs to be completed by a professional roofer.
The basic steps to achieve a safer, more wind resistant roof is the installation of an self-adhered underlayment applied directly to the wood deck or the use of a hot mop No. 30 or No. 43 / 90 lb organic or modified cap sheet.
When you have a storm resistant underlayment you are ready to move on to the weather roof. The options for underlayments outlined above should be acceptable for use with most roof coverings. If one of these upgraded roof underlayments is not installed, flashing tape should be applied over the joints between the sheathing to provide backup protection from water pouring through the gaps between the sheathing if the roof covering is blown off. This flashing tape comes in various widths. We recommend the use of 4” to 6” wide tape. This procedure is acceptable throughout Florida—including Miami-Dade and Broward Counties.
Installing self adhesive tape over seams between roof sheathing.