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THE WORLD CHAMPION RE-ROOFING SYSTEM
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Site Content (c) 2008 Roof Hugger, Inc.
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| ROOF HUGGER ENGINEERING AND TECHNICAL INFORMATION |
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ABC's of Retrofit Roof Design
In 2004 most states and municipalities adopted the new “International Building Code” (IBC). This code differs dramatically from the previous building codes in many ways. The most important to us is how it looks at the design load for roof wind pressures. The roof is now divided into three zones: The “Field” or center of the roof, the “Edge or Perimeter Zone” and the “Corner” zone of the roof. The loads for each “Zone” must now be calculated separately. ROOF ZONE PLAN
The new roof must withstand the full forces calculated for each of these “Zones”. This must be done often at higher wind speeds then before and without the 33% reduction for wind load pressures allowed by the previous codes.
Since no two buildings are exactly alike, the size and shape of these “Zones” varies from building to building and depends on numerous factors. Each building must be considered individually and engineered based on existing conditions and proposed changes. The basic steps in the process are as follows:
- STEP 1: Collect the basic information needed. PROJECT QUESTIONNAIRE
- STEP 2: An engineer should calculate the design pressures for each zone of the roof. A quick estimate of the design pressures however can be obtained from the NRCA Website at www.roofwinddesigner.com. This information is preliminary in nature and must be reviewed by an engineer but it will give you a good idea of the loads. If we assume a 100x150x18, 1:12, 120 mph, exposure “C”, occupancy category “II”, the program will calculate the loads by zone and will look something like: Zone 1 (roof area field): 33.4 psf, Zone 2 (roof area perimeter): 56.1 psf, and Zone 3 (roof area corners): 84.4 psf. The corner zone/edge distance will also be calculated and listed. (If the corner distance is calculated at 8’ and the purlin spacing is 5’o.c., then the corner dimension would be 8’ in from the rake and 10’ up from the eave – 10’ is used because the new grid framing must go to the next existing purlin line) SAMPLE REPORT.
- STEP 3: Compare the new panel capacities, as provided by the panel supplier with the existing purlin spacing. If the existing purlin spacing is 5’ (typical in older buildings) you would compare the capacity of the new panel on 5’ purlin spacing to the values provided by an engineer or estimated by the NRCA site. If we assume the panel capacity is 60 psf on 5’ purlin spacing, then the panel can meet the estimated loads in Zone 1 (field) and Zone 2 (perimeter) but not Zone 3 (corner). Therefore, additional framing must be added to reduce the purlin spacing in the corners. This needed framing can be achieved by adding additional purlins from under the old roof (usually very difficult if not impossible) or by adding additional framing on top of the existing roof.
- STEP 4: If additional framing is needed to reduce the purlin spacing, it must be determined what that framing consists of. Roof Hugger, Inc. has designed a number of options for reducing the purlin spacing in the corners and perimeters when needed. The specific design will depend on the existing panel type and rib spacing. If the existing roof is a 12” o.c., “PBR” type panel, Roof Hugger, Inc. has several FLORIDA PRODUCT APPROVED Systems that may work in this case. See SAMPLE PBR / PBR RETROFIT PLAN. Other existing panels may require special grid designs consisting of Hats, Cee’s or Zee’s or a combination of all. Consult Roof Hugger, Inc. with your project details.
- STEP 5: Once the reinforcement grid has been determined and the height of the grid established, the Roof Huggers can be estimated. The Huggers must match the height of the corner and perimeter grid systems. See SAMPLE VERT. SSR / TRAP. SSR RETROFIT PLAN.
If you need assistance with the above process please feel free to call Roof Hugger, Inc. (800-771-1711) to discuss you specific project. The Engineering firm we use, Force Engineering and Testing (281-540-6603 Terry Wolfe), is also available to provide engineering directly to you on a very reasonable fee basis for the sub-framing and panel design analysis for your retrofit project.
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Mailing Address: P.O. Box 1027, Odessa, FL 33556
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Shipping Address: 144-B Whitaker Road, Lutz, FL 33549
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