The A, B, C & Ds Of Retrofit Roof Design
By Dale Nelson, Roof Hugger Inc.
When it comes to metal roofs there is no big difference between roof design for new construction and a re-roof design for an existing building, with one major exception. When we do an existing building we are stuck with the existing roof panels and the existing purlin spacing.
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 loads 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.
Typical roof zone plan. (click image to enlarge)
The new roof must withstand the full forces calculated for each of these “Zones”. This must often be higher wind speeds than before and currently, without the 33% reduction for wind load pressures allowed by the previous codes. (Yes, this means that even before you start, the EXITING BUILDING may no longer even meet the old code.)
Since no two buildings are exactly alike, the size and shape of these “Zones” varies from building to building and depends on several 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 A: Collect the basic information needed:
• Width and Length of Roof Area
• Mean Roof Height
• Roof Pitch
• Note Parapets 36” or Higher
• Building Configuration: Open, Partially Enclosed or Enclosed
• Exposure Category: B, C, or D
• Occupancy Category: I, II, III, or IV
• Basic Three Second Gust, Design Wind Speed
STEP B: 2 choices here, you or your customer can hire an engineer to calculate the loads and design the sub-framing system, or you can obtain a quick estimate of the design pressures from the NRCA Website at www.roofwinddesigner.com. This information is preliminary in nature and must eventually be reviewed by an engineer but it will give you a very good idea of the loads. If we assume a 100 x 150 x 20, 1:12, 100 mph, exposure “B”, occupancy category “II”, this program will calculate the loads by zone. It will then produce a print out with assumptions and details outlining the preliminary loads: Zone 1 (roof area field): 18.0 psf, Zone 2 (roof area perimeter): 30.2 psf, and Zone 3 (roof area corners): 45.4 psf. (This print out is also a nice sales tool) The corner zone/edge distance will also be calculated and listed and in this case it is 8’-0”. This means edge and corner loads must be met 8’ in from the eave and rake areas.
STEP C: If the existing purlin spacing is 5’ (typical in older buildings) you would compare the manufacturer’s published capacity for the new panel on 5’ purlin spacing to the values provided by an engineer or estimated by the NRCA site. (Check the panel values to make sure they have not been increased by 33% for wind loads) Let’s assume the new roof is to be a 24” o.c trapezoidal standing seam with a useable panel capacity of 42 psf on 5’ purlin spacing. This means 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. The other option is to consider a narrower rib spacing or heavier gauge panel that has a capacity in excess of 45.4 psf on 5’ purlin spacing in which case no additional framing is needed.
Notched "Z" over special fitted sub-rafter. (click image to enlarge)
STEP D: If additional framing is needed to reduce the purlin spacing, it must be determined what that framing consists of. We have several options here: Framing could be additional purlins installed from under the old roof (often very difficult if not impossible). Grid systems made from “C’s”, “Z’s”, Hats or a combination of all can be added on top of the existing roof or we could use Pre-Notched “Z’s” with fitted Sub-Rafters to achieve the correct panel support spacing at the mandated deflection limits. This corner reinforcement must of course mate to and be the same height as, the sub-framing system to be mounted on the main purlins.
Seem a bit more complicated than it used to be? Well, it is, and just because the existing roof is 26 ga. “R” panel on 5’ purlins doesn’t mean a new 26 ga. “R” panel roof on 5’ purlins will meet the current code. You have to go through the above process and check it out. After a couple of times you’ll get the hang of this process, it’s really not too bad.
How do you apply this newfound wisdom? Assume a prospect calls needing to re-roof their existing metal building -- don’t tell them your dog ate your estimating pad, or that they will have to tear down their old building and start over, just get the basic information and “wade into the mud” like you always have.
Assuming this is a budgetary quotation and you don’t want to hire an engineer just to do an estimate, simply go through the above process using the “preliminary design” information. Just be certain to “Qualify your Proposal” and make it “Subject to Final Engineering”. More complex roofs, higher roof pitches, steep terrain, insurance issues, etc. (which I have ignored for this article) can change the final design, sometimes significantly.
If your client is OK with the budget, have them agree to fund the engineering costs (typically very minimal) so that the preliminary estimates and design can be confirmed. Changes, if required, are much more easily handled when an engineer can help explain any unusual conditions. Now everything is complete so get your permit and GO!
Changing the way we do things is never easy but now that it is a bit more complicated, some competitors may just throw their hands up and walk away while others will ignore the new codes and do it the way they always have. This is good news because it means there will either be LESS COMPETITION or your competitor will appear less informed. This ultimately means a MORE PROFITABLE job for you and a better result for your customer. Use these changes to demonstrate your professionalism as contractor, capture a bit more of the metal roofing market and put a few extra bucks in your pocket.
For more information on Roof Hugger, go to www.roofhugger.com/helpful_documents.asp?ctid=901.
About Roof Hugger
Roof Hugger notched sub-purlins enable replacement of new metal roof panels over an old metal roof. They can be used over screw-down or standing seam roof systems and can accommodate new standing seam or screw-down panels. For more information on metal-over-metal retrofit re-roofing, contact Roof Hugger at 800-771-1711 or visit www.roofhugger.com/index.asp?ctid=908.