Last month, in the first of this two-part series on solving diffi­cult roof problems, a multitude of problems were outlined. These problems included:

• Bad roof decks – steel, concrete, gypsum, structural cement fiber, etc.

• Large quantities of penetrations and/or equipment, piping, etc.

• Multiple, different, adjacent roof ele­vations.

• Curved roofs. • Drainage issues that require chang­

ing the direction of slope. • Desire to change the look of the

building. • Building additions that conflict with

good roofing practice.

This conclusion of the series will pre­sent various solutions to the challenging roof problems outlined in Part 1.

PROLOGUE Before retrofitting any building, proper

consideration should be given to require­ments for adequate and properly placed insulation, location of the vapor retarder, ventilation, snow and ice control measures, and impact on adjacent and nearby build­

ings and property. All relevant code require- Wood or light gauge steel trusses can be ments (fire, life and safety, loading, etc.) used to span across bad deck areas and to should be fully followed. Design profession- eliminate large valley areas. Trusses typi­als may need to be consulted to assist with cally are spaced two to four feet on center various code re­quirements. A

Light-gauge steelregistered profes­trusses used over asional engineer conventional roof to should always eliminate a snow-review any exist-drift condition.ing structure for

the ability to carry all code-required loads before any retrofit is started.

SOLUTIONS Typically, a roof recover

cannot adequately correct the issues previously de­scribed. Tapered insulation can be used cost effectively to eliminate small valleys and roof steps. However, tapered insulation relies on an ade­quate supporting roof deck. The volume of tapered insu­lation required to create steep roof slopes or fill in val­leys of a curved roof can, in many cases, be cost prohibi- Traditional slope framing system creating a steep-sloped tive. retrofit over a conventional roof.

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and utilize a plywood or steel deck for truss bracing as well as wind diaphragm bracing. Trusses must have an adequate depth to span a particular distance. Typically, if a truss has a zero starting depth, the mini­mum slope must be two inches in 12 for a truss to span a distance without intermedi­ate supports.

Traditional sloped framing systems used to support stand­ing seam and through-fastened metal roof retrofit systems have limitations of height and span. Depending on the imposed loads, typical sloped framing components can span up to ten feet and can accommodate heights up to eight feet without too much of a problem. When

Right: The sloped framing system has eliminated the

10-foot step in elevation on this roof.

these parameters are exceeded, the com­monly used components might not work. Also, consider that installation costs can increase dramatically for system heights over eight feet tall.

However, using a combination of the sloped framing components and metal

building components can yield a system that can span greater distances and accom­modate taller heights. Utilizing cold-formed components that were developed to span 20 to 30 feet on metal buildings, these heavier gauge and larger sections can be used to frame these same metal building-like spans

The sloped framing system using cold-formed C-sections is spanning 20 feet and is over 16 feet tall.

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over an existing roof. Existing steps in ele­vation as well as valleys and other water and snow accumulation areas can be easily eliminated. Using these components, a stick-built, fully-braced framing system is constructed on the roof. Any variations in the existing roof structure can be adjusted for with a stick-built system. A pre-manu­factured truss system where all the trusses are built the same would follow the existing roof contours unless shimmed to the cor­rect elevation.

The new slope framing system transfers the dead, live, snow, and uplift loads to the existing structure through the vertical sup­port posts. The bracing members transfer horizontal wind loads to the existing build­ing’s wind bracing or diaphragm system. As previously noted, a registered professional engineer should be retained to verify the ability of the existing building structural components to carry the existing and any newly imposed loads. By eliminating eleva­tion steps that caused the snow accumula­tion, in some cases, the existing snowdrift load may be completely abolished.

In some cases, a retrofit slope framing system may be used in order to accomplish other goals such as an energy-efficient, mechanical system upgrade. Insulation is one component in the energy efficiency equation. If a mechanical system is changed from a few large older HVAC units to a new

ground source heatpump system, a sloped framing system could create an attic to accommodate the new roof heatpumps as well as the required ductwork. Other advantages can be realized, such as eliminat­ing roof penetrations, pro­tection of equipment and ductwork from weather, and less insu­lating and wa­terproofing of ductwork. Ener­gy paybacks on this type of mechanical sys­tem upgrade can be as low as three to five years, based on 2003 energy costs.

The place­ment of HVAC equipment in an “attic space” can create problems if issues are not properly ad­dressed. Heat, condensate, and

Left: The sloped framing system is used to allow the heat pumps and ducts to be run in the attic space.

Right: The depth of the 3" Tectum deck on this school, along with up to 14" of expanded polystyrene insulation, exceeded

the length of any available fasteners.

Below: Piggyback frames supported by the existing trusses eliminate the valleys in the side-by-side curved trusses.

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mine the structural capacity of all compo­nents. The engineer can determine the added dead load from a recover that the existing building can carry and the best location for placement of the new vertical supports.

From a loading standpoint, the piggy­back frame column locations typically cor­respond to the existing column locations. In this way the piggyback frame columns do not introduce any bending moment into the existing structural support members. The piggyback frame columns will impart down­ward and upward vertical loads and a small

horizontal load at their attachment points. A piggyback frame is typically not near­

ly as tall as the average pre-engineered metal building. Metal buildings can be any height, but the most common heights range from 14 feet to 30 feet. Piggyback frames ordinarily are not very tall. Usually, a piggy­back frame from two to 12 feet tall can han­dle the majority of existing roof conditions.

Another advantage of the piggyback frame is the elimination of as many as 90% of the typical fastening points through the old roof. This reduces the chances for leaks during installation and speeds the installa­

other by-products of equipment operation can be produced and must be eliminated. Make-up air is required for most HVAC equipment operation. A mechanical engi­neer should be consulted when placing equipment inside of an attic area or other enclosed space.

These retrofit systems closely resemble the common sloped framing systems avail­able from many manufacturers. The only change is typically the requirement for larg­er spans of up to 20 feet in some areas in order to span over the equipment and duct-work.

When the obstacles become overwhelm­ing, a new approach may be called for. A unique, little-known, cost-effective solution to challenging roof problems has its roots in pre-engineered metal buildings. A piggy­back frame can be designed to span any distance between desired support points, can be spaced to carry bar joists or Z-purlins, and can be configured for any desired slope or slopes.

A piggyback frame is a shorter version of the pre-engineered building rigid frame. This frame may be designed with similar rigid moment connections or may be braced to provide stability. The approach will de­pend on the particular job requirements.

Before any building is reroofed, a regis­tered professional engineer should perform an analysis of the existing building to deter-

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This store renovation included adding new pre-cast wall panels that were eight feet taller than the eave height. The existing Z-purlins could not support any added dead load, so piggyback frames eliminated the potential snowdrift area and were supported by the existing pre-engineered frames.

tion process. Once the piggyback frames are installed and the supports are made water­tight, the rest of the installation can progress with little risk of damage or disruption to the building interior.

While not the solution for every project, the piggyback frame can be utilized to solve unique and challenging roof problems in ways not available with other systems. Challenging roof problems are not unique to any particular type of existing construction. Any existing primary framing, secondary framing, or roof deck can deteriorate, become overloaded, or simply not accom­modate the desired new roof configuration.

In any roof application, the available roof systems and their support require­ments should be evaluated for suitability, cost effectiveness, and the ability to meet the project requirements. For the run-of­the-mill, everyday re-roof situation, there are a myriad of options available, but when the options are limited or nonexistent and the reroof becomes a real challenge, look to the non-typical approach.

The projects shown utilizing piggyback frames all have common threads. They were, of course, in need of a reroof, but some requirement or condition always elim­

inated the common, run-of-the-mill approach.

One project had adjacent curved truss­es with large valley areas. Many of the 2 x 12 wood purlins were rotting from severe moisture infiltration. The existing corrugat­ed metal roof fasteners were rusting and pulling out of the wood purlins.

One project had a Tectum™ deck plus expanded polystyrene insulation thickness of over 14 inches. This thickness exceeded any available fastener length. The Tectum™ deck and bulb tee sys­tem minimized the available attachment points for a standard sloped framing system.

One project had an existing pre-engineered building constructed in 1970 that could not handle the current code-required snow load. The code-required snow load had increased, causing the majority of existing building com­ponents to be under-designed. A newly-introduced snowdrift potential from new, pre­cast concrete wall pan­els added to the struc­

tural deficiency of the existing building. When a project calls for a unique

approach, remember that a custom-sloped framing system or piggyback frame can pro­vide an uncommon solution to challenging problems.

Raymond K. Heisey, Jr., PE, RRC

Raymond K. Heisey, Jr., PE, RRC, is senior sales manager of the Midwest Region for Butler Manufacturing Company. He earned his degree in civil and structural engineering from Lehigh University in 1978 and has worked for Butler Manufacturing Co. for over 26 years. Heisey has a wide vari­ety of experiences encompassing plant engineering, metal building design, computer system development, roof product development, regional sales, and sales management. He has been awarded two U.S. patents and numerous foreign patents on metal roof system components. Heisey is a registered professional engineer in the state of Missouri. He earned his RRC in 1993 and is listed in Who’s Who in Science and Engineering and Who’s Who in the United States. Heisey won six Butler sales awards in an eight-year period for exceeding goals as a sales manager. He has made various pre­sentations on roof-related subjects.

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