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ASPHALT SHINGLE ROOFING AND THE2020 MINNESOTA RESIDENTIAL CODEMinnesota Department of Labor and IndustryGeneral details and building permitsBefore undertaking any re-roofing project there are several questions that should be considered to ensure a successfulproject. You should familiarize yourself with all aspects of the re-roofing process before you begin. There are variousconditions about your roof that may limit your product choices or affect the cost of your roofing job.The 2020 Minnesota Residential Code adopts the 2018 International Residential Code (IRC). Roofing materials mustbe installed in accordance with Chapter 9 of the 2020 Minnesota Residential Code and the manufacturer’s installationinstructions (R905.1). The Minnesota State Building is the standard of construction for the entire state (MinnesotaStatute 326B.121). The 2020 Minnesota Residential Code can be viewed Building permits are required for re-roofing existing dwellings. Building permit fees are established by the municipality.Building inspections are performed at various stages to verify code compliance. Permit costs and required inspectionscan be obtained by contacting your local department of building safety.Frequently asked questionsDo I need a new roof? How old is it? A roof that hasbeen properly installed, ventilated, and has not beendamaged can last 20 years or more. An inspection of theroof should be done periodically. Look for cracks, curledor cupped shingles, worn mineral coatings, exposed nails,previous patches, holes and exposed underlayment orsheathing.Does the roof leak? If the answer is yes, it is necessaryto determine why. If you have inspected the roof and itlooks sound, the problem could be roof flashing. Manyroof leaks are result of bad or misapplied flashing. Youshould spend time in the attic looking for water stains,particularly around vents, chimneys and vertical wallelements above the roof. Flashing can sometimes bereplaced or repaired without installing a whole new roof.Do it myself or hire a professional? This is a question onlyyou can answer based on your skill level and time. Anasphalt roofing project can be successfully accomplishedby the homeowner if you take the time to become familiarwith the roofing procedures. Be sure to plan your projectaround the weather and allow enough time to get aproper cover on the roof before it rains. Steep-slopedasphalt roofs and those with multiple valleys can presentspecial problems, so be sure you have the right equipmentand skills before undertaking this type of roofing project.Other types of roofs such as wood shingles, shakes andclay tile normally require special skills for installation. Ifyou decide to hire a professional, be sure the company is astate-licensed contractor or roofer.Should I overlay the existing roof or tear off the existingshingles? There are two options available for re-roofinginstallations. One would be to tear off the old roof beforeapplying the new one. The second would be to lay newshingles over the existing roof. Roofing materials areheavy, so multiple layers can affect the ability of a roof tohold the weight of winter snow.An overlay can be the less expensive option. However,it is not necessarily always the best choice. There areadvantages to tearing off the old roof before installing anew one. For example:

If there are any defects in the roof deck, they will3. The application of new protective coating over existingspray polyurethane foam roofing systems is permittedwithout tear-off of existing roof coverings.be revealed when the roof is torn off. These defectsshould be repaired before applying the new roof. Ifcondensation problems exist in the attic, they too willbe revealed when the roof is torn off. Properly designedattic ventilation can then be installed in order to helpeliminate such problems.4. Where the existing roof assembly includes an icebarrier membrane that is adhered to the roof deck, theexisting ice barrier membrane is permitted to remainin place and be covered with an additional layer of icebarrier membrane in accordance with Section R905and the manufactures installation instructions. When the old roof is torn off, an ice-protectionunderlayment must be installed before applying thenew roof. This will help prevent ice damage. Tearing off the old roof and starting with a clean deckbefore re-roofing may result in a smoother finished-roofsystem. Tearing off the old roof will typically result in a longerroof life than when the roof has been laid over. This isbecause they are installed smooth over sound materialand have new underlayment installed.Roof slopedeterminationAn overlay of asphalt shingles must also comply withthe shingle manufacturer's installation instructions foroverlays.What is roof slope and does it limit the choice ofshingles? Asphalt shingles can only be used on roofslopes of two units vertical in 12 units horizontal (2:12) orgreater. The slope of the roof is measured by the verticalrise of the roof to the horizontal run and is expressed asa fraction. A "4:12 roof slope" means the roof rises 4 feetfor every 12 feet of horizontal roof span.Recovering versus replacement? Do not install new roofcoverings without first removing all existing layers of roofcoverings where any of the following conditions exist: Where the existing roof or roof covering is water-soakedor has deteriorated to the point that the existing roofor roof covering is not adequate as a base for additionalroofing.Roof slopes between 2:12 and 4:12 can use shingles, butrequire roof application techniques to take into accounta greater potential for ice dam water backup. Roof slopesbetween 2:12 and 4:12 require a double underlaymentapplication in accordance with Section R905.2.7. Slopesof 4:12 and above can use standard asphalt roofingapplications (R905.2.2). Where the existing roof covering is wood shake, slate,clay, cement or asbestos-cement tile. Where the existing roof has two or more applications ofany type of roof covering.What are the ventilation requirements? Roof ventilationis required and must be installed in accordance withSection R806 of the 2020 Minnesota Residential Code andthe manufacturer’s installation instructions.Exceptions:1. Complete and separate roofing systems, such asstanding-seam metal roof systems, that are designedto transmit the roof loads directly to the building’sstructural system and that do not rely on existing roofsand roof coverings for support, do not require theremoval of existing roof coverings.Enclosed attics and enclosed rafter spaces formed whereceilings are applied directly to the underside of the roofrafters must have cross ventilation for each separate spaceby ventilating openings protected against the entrance ofrain or snow. Ventilating openings must be provided withcorrosion-resistant wire cloth screening, hardware clothmesh, or similar material with 1/16 inch minimum to 1/4inch maximum openings.2. Installation of metal panel, metal shingle and concreteand clay tile roof coverings over existing wood shakeroofs are not permitted when the application is inaccordance with Section R908.4.2

Minimum vent area: The minimum net free ventilatingarea is 1/150 of the area of the vented space. Exception:The minimum net free ventilation area is 1/300 of thevented space provided both of the following conditionsare met:lower edges of the roof, in addition to installing adequateventilation and proper insulation in the attic. The codein Minnesota requires this special waterproof shingleunderlayment at the eaves or lower edges of the roof.How can you determine if the roof is properly ventilated?An attic needs to breathe. An effective ventilation systemwill help prevent attic heat build-up, attic moisture andcondensation, and ice dam build-up.1. A Class I or II vapor retarder is installed on the warm-inwinter side of the ceiling.2. At least 40 percent and not more than 50 percent ofthe required ventilating area is provided by ventilatorslocated in the upper portion of the attic or rafterspace. Upper ventilators can be located no more than3 feet below the ridge or highest point of the space,measured vertically. The balance of the requiredventilation must be located in the bottom one-thirdof the attic space. Where the location of wall or roofframing members conflict with the installation of upperventilators, installation more than 3 feet below theridge or highest point of the space are not permitted.Research has shown that proper ventilation is necessary ifthe shingles are to last their design life.Code requirements for asphalt shinglesA typical installation of asphalt shingles is illustrated onpage 5 for use on roofs 4:12 and greater. However, thecode also permits application on a roof that has a slopeof less than 4:12 if the low-slope-roofing procedures areused.Roof deck sheathingRoof covering materials such as asphalt shingles must beinstalled on solidly sheathed roofs. Solidly sheathed roofsinclude structural panels such as plywood and orientedstrand board (OSB) with 1/8” gaps between panels asrequired by panel manufacturers.It might be necessary to add ventilation with your newroof to meet these standards.What function does shingle underlayment serve? Anunderlayment, commonly known as roofing felt, will: Wood boards are also considered as solidly sheathed,provided the spacing between boards complies with theshingle manufacturer’s installation requirements. If thespacing between boards is too large, a structural panelapplied over the deck boards is required to be considereda solidly sheathed roof.Protect the roof deck from moisture prior to shingleapplication.Provide a degree of back-up protection in the eventwater gets under roofing shingles.Protection against ice dams can be obtained by using aspecial waterproof shingle underlayment at the eaves orFastenersFasteners for asphalt shingles must be galvanizedsteel, stainless steel, aluminum or copper roofing nails,minimum 12 gage shank with a minimum 3/8 inch head,of a length to penetrate through the roofing materials anda minimum of 3/4 inch into the roof sheathing. Where theroof sheathing is less than 3/4 inch thick, the fastenersmust penetrate through the sheathing. Fasteners mustcomply with ASTM F 1667. Staples are not permittedfor shingle application unless specifically noted in themanufacturer's installation instructions on the shinglepackage.Ice barrierdeterminationUnderlaymentThe code requires that underlayment of one layer of nonperforated Type 15 felt lapped two inches horizontallyand four inches vertically to shed water. In addition, an icebarrier that consists of at least two layers of underlaymentcemented together or of a self-adhering polymer modified3

Kickout flashingInstall kickout flashing where the lower portion of a slopedroof stops within the plane of an intersecting wall claddingto divert or kick out water away from the assembly (seeillustration, page 5).bitumen sheet, be used in lieu of normal underlaymentand extend from the eave's edge to a point at least 24inches inside the exterior wall line of the building. Icebarrier protection at eaves is required in Minnesota.Valley liningsInstall valley linings in accordance with manufacturer'sinstallation instructions before applying shingles.Permitted valley linings include:Other flashingApply other flashing, such as flashing against a verticalfront wall and the soil stack, vent pipe and chimneyflashing, according to the asphalt shingle manufacturer'sinstructions. Replace existing flashings that are part ofthe assembly where rusted, damaged or deteriorated.Flashing must be reconstructed to comply with approvedmanufacturer’s installation instructions.1. For open valley (valley lining exposed) lined with metal:24 inches wide and of any of the corrosion-resistantmetals in Table R905.2.8.2.2. For open valleys: Two plies of mineral surface rollroofing, complying with ASTM D 3909 or ASTM D 6380Class M. The bottom must be 18 inches and the toplayer a minimum of 36 inches wide.Drip edgeDrip edge is not required unless specifically required bythe manufacturer’s installation instructions.Repairs or replacement of asphalt shinglesRepair or replacement of asphalt shingles damaged bywind or other events are not addressed in the 2020Minnesota Residential Code. Repair or replacementof asphalt shingles is only permitted when complyingwith the shingle manufacturer’s specified installationinstructions for such work.3. For closed valleys (valley covered with shingles): oneply of smooth roll roofing complying with ASTM D 6380and at least 36 inches wide or valley lining as describedin items 1 and 2. Self-adhering polymer modifiedbitumen underlayment complying with ASTM D 1970can be used in lieu of the lining material.Cricket and saddleInstall a cricket or saddle (see illustration, page 5) on theridge side of any chimney greater than 30 inches wide.Cricket or saddle coverings must be sheet metal or thesame material as the roof covering.Wind speed design for asphalt shinglesThe 2020 Minnesota Residential Code requires shinglesto be designed for a 115 mph maximum ultimate designwind speed (R301.2.1). This wind speed equates to amaximum basic wind speed of 90 mph, the same asrequired in previous codes. Asphalt shingle productlabeling must indicate compliance with ASTM D3161classifications A, D, F or ASTM D7158 classifications D, G, Hand identify a maximum ultimate design wind speed of atleast 115 mph (R905.2.4.1).Sidewall flashingSidewall flashing against a vertical sidewall (seeillustration, page 5) must be continuous or step flashingand a minimum of 4 inches and 4 inches in width anddirect water away from the vertical sidewall onto the roofor gutter.4

Cricket or saddle at chimney.Typical roof-covering material example.Underlayment runs 3" to4" up onto sidewallNail flashing to roof2" overlap7"5"5"Step flashing at sloped roof and vertical wall.Kickout flashing directs water away fromthe wall.Note: Illustrations are only examples. Verify andfollow code requirements and manufacturer'sinstallation instructions.Continuous sidewall flashing at vertical wall andsloped roof.Construction Codes and Licensing Divisionwww.dli.mn.govPhone: 651-284-5012 or 1-800-657-39445

2. Installation of metal panel, metal shingle and concrete and clay tile roof coverings over existing wood shake roofs are not permitted when the application is in accordance with Section R908.4. 3. The application of new protective coating over existing spray polyurethane foam roofing s