Lean-To Installation | Metal Construction News (2022)

By Mark Robins Senior Editor Posted October 01, 2020

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A metal lean-to is a simple way to add another structure to a new or existing building. It gives additional usable space alongside another building, tying in at the other building’s eave or below the eave. Formed by “leaning” rafters against a wall to form an extra roof, lean-to’s have only one slope and depend on the adjoining structure for partial support. Creating a sheltered, enclosed area for a variety of uses— from a covered area to a completely enclosed addition—a lean-to extends the area of the original building, protecting whatever is stored underneath it from the elements.

“While a lean-to may generally be considered roof only, it is also possible to enclose the wall and may or may not be open to the other structure,” says Russell Bowen, senior sales representative, Quicken Steel, Claxton, Ga. “A lean-to roof can also wrap around the front and sides of a building.”

Planning a Lean-To

When planning a lean-to addition, efficient use of steel to achieve appropriate bay sizes balanced with good economy of scale and cost is a good indicator of whether a lean-to is applicable onto an existing building. An evaluation of the structural capacity of the existing outer wall to support the load of the new lean-to addition will indicate if additional structural members are to be included adjacent to the outer wall. When matching lean-to materials to an existing building, often it is more efficient to contrast than to match, or to re-skin the existing to match the new addition.
James S. Yu, AIA, NCARB, SyDesign, Port Huron, Mich.

How Far Out do You Want to Span with the Lean-To?

It has to be designed to maintain a decent roof pitch in order to maintain proper water flow and drainage. The further you span, the lower the eave height will be and bigger frames will be needed to support the extra weight.
Russell Bowen, senior sales representative, Quicken Steel, Claxton, Ga.

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Lean-To Types

“The most economical solution for adding a lean-to structure is to utilize a metal building system that can provide clear span properties at approximately one half the weight of a conventionally designed structure,” says Thomas M. Corry, division manager, CDMG Building Systems, Canonsburg, Pa. “[It] will provide the serviceability of a conventionally designed steel structure. Lean-to structures have a single-slope roof sloping away from a higher structure, but in certain instances lean-to structures can be constructed with gable roofs depending on the process-use requirements for which the lean-to is being constructed.”

There are different types of lean-tos. Jacob Dooley, vice president of operations, Simpson Steel Building Co., Vallejo, Calif., say three of the most common are a continuous lean-to, dropped lean-to and wrap-around lean-to. “A continuous lean-to is a lean-to that will have the same roof pitch as the main building’s roof. There will be no gap between the roof height and the lean-to roof height. A dropped lean-to will typically drop a few feet below the height of a building and then start. A wrap-around lean-to is a lean-to that will start on one side of the building and then continue and wrap-around a corner and go down the other side of the building.” Jeff Reese, PE, senior sales engineer,Nucor Building Systems, Waterloo, Ind., explains lean-to’s can be clear span (no interior columns) or multi-span (interior support columns) depending on its width.

Lean-To Installation

The procedures for installing a lean-to are generally similar to those for the initial installation of a metal building. Corry says at the project’s start, “Establish the location of the rafter tie in and drill the rafter connecting holes in the existing building column flanges. Place shim packs on the foundation under the base plates directly below where the column flanges will be. All the shim packs need to be shot to elevation so that when the columns are set, they are level and at the proper elevation. All column base plates should be Transform an open space alongside a building with metal grouted after installation to ensure that the entire column base plate is in bearing on the foundation.”

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Erection then can begin by starting in a braced bay by standing up columns and providing temporary bracing for stability until the rafters, bracing, girts and purlins are installed to maintain the frame's geometry. “As the lean-to structure is being detailed out in the field, this is when squaring of the structure occurs,” Corry says. “This involves surveying the top of the columns for plumb, installing temporary bracing as required, tightening anchor bolts and adjusting permanent bracing to maintain the geometry of the structure as designed.”

Reese offers a more concise lean-to installation explanation saying the first thing to do (when attaching to an existing structure) is prepare the building for the lean-to rafter attachment. “Following that, you must set the new column at the end of the lean-to. Finally, the lean-to is complete by attaching the lean-to rafter to the new column and main building.”

Some lean-tos are easier to attach than others. “A steel truss frame style building will simply use a plate that bolts between back-to-back structural columns, leaving a thin slot to come through the sheeting,” says Dooley. “Once this connection is made, all the remaining work can be done from the outside of the building.

Columns would be fastened to anchor bolts that would be anchored in footings—these can be just small piers. Rafters would be bolted to the tops of the columns. Purlins would be mounted to the rafters perpendicular and the sheeting would be screwed to the purlins on the roof. Then a fewpieces of rake trim, and gutter or eave trim will finish it out. Many will also build a decorative covering for the columns, which will typically be red iron: red primer coated steel.”

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Travis Barnes, rigid frame building sales at Quicken Steel, explains when a lean-to is being added to an existing building some additional steps may be necessary to ensure a structural sound connection that meets both engineering standards and is weatherproof. “If the existing structure is enclosed, the existing walls will most likely need to be cut into upper and lower sections to properly flash for weatherproofing. Flashing will need to be placed behind the wall sheeting and over the top of the lean-to roof.”

Corry explains once the building is square and detailing is complete, insulation and cladding can begin. “It does not matter if you start with the walls or the roof. Cladding the walls first sometimes makes installing the roof easier due to the slight roof overhang at the eave.”

Structural Issues

Do not ignore the structural issues involved with lean-to installation—one of the biggest being the stability of the structure the lean-to is being attached to. “The existing structure needs to be able to handle the increased load the lean-to will add,” Dooley cautions.

One structural issue, Mike Momb, technical director of Hansen Pole Buildings, Browns Valley, Minn., notes is in cases where the lean-to’s high end does not match up to sidewall eave heights of the main building. “In snow country, the engineer of record needs to consider the effects of snow slide-off and drift loads. If attaching structurally to the main building, main building footings must be sized to adequately support added weight of the lean-to.”

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Barnes agrees that an engineer can determine if the existing structure is designed to support the extra load and what types of connections are required to support both the main building and the lean-to. Reese concurs that a chief concern is making sure the existing structure is designed for the lean-to addition and if it is not, a structural engineer should be contacted to review the existing building to see if any reinforcement is required.

The ability to handle increased weight loads is not the only potential lean-to error. Dooley advises following the plans for installation that are prepared ahead of time. “Doing something without reviewing the plans is always the most likely way to cause errors. [For] sufficient support, footings are important and must be provided; a major error some make is to skimp on the lean-to footings. Another error that is sometimes made is to not be careful with the notch around the rafter coming through wall sheets; these will need to be sealed up, so the closer the notch fits the better.”

Bowen says some installers skip cutting wall panels to properly flash the connection from the lean-to roof to the main building wall. “Some additional framing might be needed to be able to properly attach the flashing and the upper wall panel sections once they are cut. Foam closures alone will not be adequate protection for stopping water leaks.”

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“Not doing your homework,” is the biggest mistake that Corry says is often made when installing a lean-to structure. “A survey of existing conditions can save you countless man hours in the field when it comes to installation,” he adds. “A survey is required to identify the conditions at the exact point where the rafters will connect to the existing building. The existing building will need to be opened to expose the face of the existing column flange. It is not uncommon to find existing columns are not where they are anticipated to be due to fabrication and installation tolerances. Special connections at the end of the rafters can be provided to account for geometrical misalignments found during the survey.”

Installing New Versus Retrofit

Most new buildings designed with a lean-to will come with all of the necessary parts to attach the components. All the additional loads and connection details are accounted for in the new building design and detailing. Bowen advises that when retrofitting an existing building with a new lean-to addition, care must be taken that proper connection brackets are used and that the bracket types are compatible for both materials. Reese explains that reinforcing the existing structure may be required and all of the connections (drilling of holes, welding, etc.) will have to be done on-site.

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Dooley believes installing a lean-to addition onto a new building is much easier because the building can be designed with the lean-to attachment ahead of time. “You will know exactly what components you need and how they will be attaching before you order your materials. All of your materials should match up nicely, making it a much easier install. Retrofitting one onto an existing building can be more difficult. You will need to know more about the existing structure’s load capabilities and where it will attach to. Odds are you also will not be able to find the same materials you used previously, which can make things more difficult.”

Corry explains there is custom color coil stock for cladding to match existing colors but it will come at a cost. “If it is important to identically match profile and color of an existing building, [a metal building] may not be the way to move forward. Hiring a structural engineering firm to design a conventional structure and having a third party provide cladding in a specific profile and color may be for you, but expect the cost to be two to three times that of a complete metal building.” Color matching may not necessarily be an issue. Momb says his clients have “never had a concern about matching materials; their interest has always been about covering the space affordably.”

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