How to Add a Lean To Onto a Shed

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25-09-2016, 04:50
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When your shed or other storage building no longer provides enough room, you can add additional storage if you add a lean-to onto a shed. If the existing shed is structurally sound and has an exterior wall to which you can attach your lean-to, adding a lean-to can be a fairly simple project.

Plan Your Project

  1. Know your local zoning laws. Consult the local building authorities to learn about code requirements, needed permits and property set backs for storage buildings.
  2. Draw a plan for your project. Consider the following:
    • Plan the length and the width of your lean-to.
    • Decide where and how you want to attach the lean-to to your shed. Also, decide how the roof of the lean-to will interface with the roof of the shed.
    • Choose materials for the foundation. You can use pressure-treated skids since the addition will be small.
    • Plan the roof angle. The roof only has to be pitched in one direction to shed water.
  3. Choose the materials you will use. Lumber is a fairly strong and inexpensive material for framing, and tin is a suitable roofing and siding material.
    • Other options include shingle roofing, rolled roofing, metal studs for framing, and cementitious board or composite sidings.
    • Choose the materials that are suitable for your location and climate as well as materials that will match your existing building.
    • Be sure to plan to use a sheath of CDX plywood on the wall that abuts against the structure. CDX plywood resists indirect moisture.
  4. Calculate the amount of each material you will need, price them, and purchase them. Some basic items that a lean-to addition for a tin shed would use include:
    • Poles for supporting the eave framing.
      • 4X4 pressure treated southern yellow pine will support a lightweight roof framed with 2X4 boards, spanning less than 15 feet (4.6 m) or so.
      • For a longer, heavier roof span, 6X6 timbers or even steel columns may be more suitable.
    • Rafters for framing the actual supporting structure of the roof will need to be strong enough to support the weight of the lathing, the decking and the workmen who will walk on the roof while installing it.
      • A somewhat typical span of less than 10 feet (3.0 m) may be framed with southern yellow pine if the rafters are free of large or loose knots and are otherwise structurally sound.
      • Lodgepole pine, spruce and other softer pine species are too knotty and not strong enough for roofing.
      • For roof spans 10 feet (3.0 m) or greater, 2X6 nominal framing or larger, should be used.
    • The rafter nailers spanning between the posts on the eave side of your lean-to must be strong enough to support the load of multiple rafters.
      • Use a minimum size of 2X6 nominal southern yellow pine.
      • Nailers attached directly to the wall of the building onto which the lean-to is being added can be the same size lumber as the rafters themselves as long as the nailer is attached securely to the wall of your building.
      • Make sure to use code-required anchors such as hurricane anchors depending on your local building code.
    • Lathing strips, or the framing members that lay across the rafters that the metal roofing is attached to should be sound southern yellow pine or a similar lumber.
      • 1X4 lathing lumber is sufficient to support a normal load on spans where the rafters are located at 24 inch center spacing or less.
      • 2X4 lumber is easier to fasten to (it bounces less when nails are driven into it), and may not be significantly more expensive than the 1X4s.
    • Nails or screws to act as fasteners.
      • Nails should be large enough to penetrate the attached member and the supporting member deeply enough to secure the two pieces.
      • Screws may be used to attach dissimilar materials, such as metal framing, roofing, or siding to wood framing, or even for joining two separate wood members.

Set the Posts

  1. Lay out the area within the footprint of your addition. Remove any debris or vegetation that will be a problem during construction or future use, and grade the soil to a suitable elevation. If you are going to leave an earth floor in the addition, compacting the soil probably will not be necessary.
  2. Dig your post holes for the supporting posts.
    • Measure the distance between the wall to which you will fasten the lean-to and the starting corner.
    • For an addition that will span the full length of the structure you are adding to, you can simply pull a string line across the end of your building, or even eyeball a starting point for your post.
  3. Set the opposite end post in the starting corner, using a sledgehammer to drive it securely into the hole.
    • Pull a line between the two ends to align the position of the intermediate posts. While you can use a measurement between the building and each individual post and between posts to set the intermediate posts, using a string line will make the task easier.
    • Dig holes and set your intermediate posts.
  4. Use a line level or builder's level to mark a bench elevation (a reference grade) on each post.
    • This line will determine the bearing height of the rafters.
    • Start by marking the end posts, and use either a plain line or a chalk line to mark the intermediate posts.
  5. Notch the top of your posts so the rafter nailer or rafter supporting joist will rest on the notch.
    • Draw a notch about 1-1/2" (about 4 cm) deep into your post.
    • Use a circular saw set to a 1-1/2" (about 4 cm) depth to cut into the post at the base of the notch.
    • Set the circular saw to maximum depth. Measure out 1-1/2" (about 4 cm) on the top side of your post and cut into the top of the post on the same side as you made the first cut. When your saw blade meets the first cut, the wood block should fall off, leaving the notch for your nailers.
    • Repeat the process with each post.
    • If you choose to, you may simply nail directly into the side of the post, but doing so places all the weight on your fasteners.
  6. Set the nailer on the notches.
    • If the nailer is not long enough to span the total length of your shed, make sure any joints are set against a post to ensure maximum bearing potential for the nailer.
    • Nail the nailer into place, making sure the spacing between your posts remains correct.
  7. Plumb the outboard wall and backfill the posts to hold them in place.
    • For areas where high winds or severe storms are likely, backfill the posts with prepackaged concrete mix.
    • Otherwise, simply shovel dirt into the holes to secure the posts.
  8. Attach any purlins you plan to use on the outboard side of your lean-to. Remember to nail everything solidly as you install it, so you will not overlook a critical connection before moving to the subsequent step in the project.

Construct the Roof Supports

  1. Fasten the nailer to the side of your existing building where you will attach the upper end of your rafters.
    • To attach a wooden nailer to a metal-sided building, use a self-drilling screw with sufficient threads to hold the nailer tightly. If possible, nail through the siding into the building's framing members.
    • Regardless of what method you choose to attach this member, make sure it is fastened securely. This is critical because the nailer it will support the weight of the roofing, the framing and the person performing the installation.
  2. Lay out the rafter spacing on the upper and lower nailers.
    • Starting at 1 end, measure the span you determined to use when you designed the building, and mark each space.
    • Marking the edge of the rafter rather than its center will make the mark more visible when you position the rafter.
  3. Determine the roof pitch by stringing a line from the top rafter attaching point to the bottom one.
    • Hold a speed square (also known as a rafter square) plumb against the top nailer where your string is fastened.
    • Read the angle on the square's angle scale.
  4. Cut 1 end of the rafters at the angle given to you by the speed square.
    • Test the cut by holding the rafter in position. If the cut does not fit tightly, adjust it. A good fit will increase the hold of your nails when you attach the rafters.
    • When you have established the best angle for the top cut of your rafters, cut each one using that angle.
    • Unless you are certain both nailers are exactly parallel, do not cut the lower end of the rafter. This can be done after the rafters are installed, if cutting them is necessary.
  5. Nail the rafters against the top nailers using a toenail nailing method.
    • Use a pencil to mark the position where you want your boards to join at a right angle.
    • Place the rafter about 1/4" (6 mm) in front of the nailer.
    • Place the nail about 1/2" from the connection point and nail it in straight into the rafter. The key to toenailing is to drive the nail in straight and then at an angle. Tap the nail into the wood about 1/4" (6 mm).
    • Pull the nail up at about a 50-degree angle. Drive the nail in until it protrudes slightly from the rafter.
    • Reposition the board to ensure that the angle is exact. Finish driving your nail through the rafter and into the nailer.
    • Avoid starting the nails too close to the end of the board, as this may split it, and make a strong connection impossible.
  6. Space the lower end of your rafters on the layout marks and toenail them into place. Use as many nails as possible without splitting the rafter, especially if you are not planning to use a structural anchor like a hurricane clip to supplement the nail's holding ability.

Add the Roofing Material

  1. Lay out the spacing of your lathing. These are the strips that span perpendicular to the rafters to which you will fasten the tin or other roofing material.
    • For 29-gauge metal roofing, spacing can be up to 30 inches between centers.
    • Nail the lathing securely, with a minimum of two nails at each rafter, being careful to keep them aligned.
  2. Cut back the roofing on the existing building if needed so that the new roof can fit correctly beneath it.
    • Typically, a lean-to roof will lay at a different pitch than your existing roof.
    • The lean-to's roofing will need to fit fairly snugly underneath the existing roof to prevent rain from blowing into your addition.
  3. Lay your metal on the lathing, beginning at 1 end. Some roof metal profiles have a direction of run," so that the laps fit correctly to guarantee a good, weatherproof roof system.
  4. Fasten your metal roofing with a suitable fastener. Wood screw threaded hex screws with neoprene gaskets are ideal.
  5. Install trim of your choosing to finish off your lean-to's roof.
    • The metal angle will close any gaps between the lathing strips and the roofing so that water will not blow in through them.
    • Use a piece of break metal configured in the right dimensions to provide a decent finished appearance to these areas. The trim will serve 2 useful purposes in one step.

Complete the Interior and Exterior

  1. Install any partitions you will use to divide the lean-to's floorspace into different usable areas.
    • The shed in the photos is 10 feet (3.0 m) wide and 21 feet (6.4 m) long, so a partition was installed to create a 7x10-foot space on one side, and a 14x10-foot space on the other.
    • This partition was created by installing steel stud purlins between one of the outboard support posts and a nailer fastened vertically to the existing shed wall.
  2. Add doors as needed. Choose a pre-framed exterior door to make the job simple.
  3. Add flooring. If you want an earth floor, then you can skip this step. If you prefer a wooden floor, then lay your flooring material over the skids that served as your foundation.
  4. Add the exterior walls. You can nail wide boards the studs to construct exterior walls. You can also add siding if you prefer.

Finish the Job

  1. Check all of the fasteners to make sure none were missed during construction.
    • Pay close attention to the screws or nails that attach the roofing components. Also check the siding, if used, and any trim installed on corners or edges.
    • Make sure any metal corners (if you side the lean-to with tin) are rolled or formed in such a way that no sharp edges are exposed.
  2. Clean up the work site and away your tools.


  • Use fasteners designed for the roofing materials you select. For metal roofing (tin), use neoprene washered screws or lead-capped nails designed for this purpose.
  • Use strong fasteners for all structural connections. When nailing 2X nominal members together where the nails support the load on them, use at least 16d cement-coated nails, and make sure the nails penetrate sufficiently.
  • Many building departments do not regulate small projects like lean-tos as long as the additions are under a minimum square footage area and you have no plans to install any utilities such as electrical and plumbing. Still, you should check with your local agency to confirm.
  • Square, level, and plumb all corners so that you can easily attach siding and roofing materials.
  • Metal roofs of steep enough pitch (as well as satellite dishes) can be sprayed with a quick coating of PAM or similar spray oil before snowfall, and the weight of the snow will cause it to slide right off and not stick. This step can save your project from collapsing under heavy snow.
  • If you draw a small floor plan and take it to your local lumberyard or home improvement store, they may be able to help with doing a take-off of the materials you will need.


  • Use caution and proper methods when working off ladders and scaffolding.
  • Use proper safety equipment for all work involved in this project. Safety glasses are recommended when using a hammer to drive nails. Also, gloves should be worn at all times when handling metal studs or metal roofing which has sharp edges.

Things You'll Need

  • Necessary permits
  • Framing and roofing material
  • CDX plywood
  • Poles
  • Rafters
  • Rafter nailers
  • Lathing strips
  • Fasteners suitable for each area of construction, including 16d cement-coated nails.
  • Ladder and/or scaffolding
  • Post-hole diggers
  • Hammer or nail gun
  • Square
  • Chalk line
  • Line level or builder's level
  • Circular saw
  • Prepackaged concrete mix
  • Shovel
  • Drill
  • Speed square
  • Siding, if needed
  • Pre-framed exterior door
  • Boards for flooring, if needed
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