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How to Build Roof Over Bay Window | Helpful Tips & Insight

Building a roof over your bay window can be tricky. If you are not careful, the finished product may end up looking like an old-fashioned shed with nothing but a shingled roof and no walls.

That is why we have created this comprehensive guide to help you design and build the perfect roof for your bay window without any of that nonsense!

What Is A Bay Window?

Bay windows are a variation of the standard window that protrudes out from your home with three or more panes.

Bay windows have an L-shaped design and offer a unique look to any room.

This guide will show you how to build a roof over bay windows as well as other elements, such as stone veneer, window trims, and roofing.

The first step is to make sure you have the proper tools for building a bay window roof.

You’ll need several carpenter’s levels, framing squares, hammers/drills/bits, saws of all shapes and sizes (circular saws are best), pry bars of various sizes, and a few other hand tools.

You don’t need to go out and buy every single power tool on the market, but you will need some of them.

How to Build a Roof on top of Your Bay Window

Building The Roof Frame

First, we start with building the roof frame. This is going to be what holds up all your shingles so it needs to be sturdy enough for the job.

The size of the rafters is determined by the pitch (or slope) of your roof, which in layman’s terms means how steep it will be.

Use a simple formula for determining what size you should make your framing members:

Width x Height = Diagonal measurement

For example, if you are building a roof over bay windows that are three feet wide by four feet high, the diagonal measurement is twelve.

12 x 12 = 144

The rafters in this example need to be sixteen inches on center for an eight-inch pitch in order to support the weight of your shingles.

Once you have determined the size of the rafters, you need to build them.

Rafters can be made from wood or metal and both work well for constructing roofs over bay windows. Metal is stronger but may not hold up as long in certain climates (rust).

There are three basic types: purlins, girts, and trusses. The main difference between these is the pitch.

Purlins are far less steep than girts and trusses, which makes them great for roofs over bay windows because they offer much more support at a lower cost.

You should build purlin rafters first before building the other types of rafters in order to provide maximum stability during construction. Girts can be used as purlins but it’s not recommended, especially if you are planning on building the roof over bay windows yourself.

The next step is to build truss rafters using either wood or metal depending on your preference and available materials.

If you go with wood, there should be two pieces of lumber per rafter (it can be one piece of two-by-four because the rafter will be cut in half).

The length should always measure three feet longer than your bay window so it can fit inside. If you want to maximize headroom, you should make them four inches shorter instead.

The last step is attaching both types of rafters to each other and securely fastening them to the roof frame.

You should also attach them to the wall for added support.

The easiest way is using screws and you can choose between either metal or wood depending on your preference (metal typically works better because it doesn’t warp over time).

Creating A Roof Over Bay Windows 

Once all of your rafters are in place, you need to attach them to each other in order to form a roof.

There are several ways this can be accomplished, depending on how you want it done and what materials you have available.

The easiest way is using either metal or wood beams that span the width of the bay windows (or whatever length your bay window is).

This process may take some time because you need to measure and cut each piece of lumber or metal beam based on the measurements you took earlier.

You also need nails, screws, bolts, etc., in order to attach them together.

Once your roof frame is built for bay window roofs over it’s now time to start building the actual structure that will hold up your shingles.

What to Do When Building a Bay Window

  • Decide On Your Material Choice – Whether you are building a bay window or another element, this is the first thing to decide on.
    • You can choose from many different materials for your stone veneer including brick, natural stone products like flagstone and slate, ceramic tile borders or other artificial stones made of vinyl or fiberglass.
  • Prepare Your Walls and Trim – Before you begin building your roof over bay windows, it is important to remember that the end product will be attached directly to your home’s exterior walls with nails or screws rather than connected via brackets like most sheds are built.
    • This means that the integrity of your wall must be sturdy enough for years of pressure and weather.
    • If you are not sure how strong your walls are or if they can handle the weight of a roof, we highly recommend hiring a licensed contractor to assist with the building process.
  • Build The Rafters – The rafters will serve as the base of your bay window roof and must be built on either side of the bay window opening in the wall of your home.
    • You can build these either by hand or with assistance from power tools, but they must be sturdy enough to support the weight of the roof and any snowfall that may occur in your area (if applicable).
  • Install Shingles – Your shingling material will also need to withstand pressure over time, so make sure you are buying the correct materials for your roof.
    • Your shingles should be cut at an angle to run against each other and overlap.
    • This will prevent water from getting into any cracks or crevices in between them, which can cause leakage over time if not attended to properly.
  • Cut Overhang Material if Needed – If you need to cut veneer or lumber for your roof over bay windows, you can use either a circular saw with the appropriate blade attachment or another type of cutting tool.
    • Make sure to always wear protective gear when using power tools and know which direction is “up” so that you do not accidentally cause damage to other parts of your home’s exterior.
  • Cut Trim Pieces if Needed – Trim pieces are the outermost layer of your bay window roof.
    • These can be made from either lumber or veneer, depending on how natural you want the finished product to look.
    • It is important that these pieces overlap each other slightly so water cannot get in between them and into your home’s interior.
  • Apply Trim – Once you have cut your material, it’s time to apply the trim pieces on top of your rafters and underlayment.
    • Simply nail or screw them into place using a drill with a Phillips head bit attached for ease.
    • You can also use more powerful tools if they are available to make this process faster, but make sure that you are aware of how your roof is built and if it can handle the pressure.
  • Prime And Paint The Exterior Of Your Home – You should make this step a priority after each element of your bay window roof has been completed, not only for aesthetic reasons but to prevent any water damage or other problems from occurring over time.
    • You can choose any paint color you like, but make sure it is one that will stand up to the elements.
  • Paint or Stain Your Trim Pieces As Well – Your trim pieces should be painted or stained with your exterior paint for best results.
    • This may prolong their life expectancy and prevent weathering over time as well as give everything a uniform look.
  • Prepare Your Walls and Trim – Before you begin building your roof over bay windows, it is important that you clean the area thoroughly to remove all debris and dust.
    • It would be best if this task was handled by a professional cleaning service or with an extremely powerful shop vacuum (if applicable).
  • Prepare Your Floor – If your bay window opens onto wooden floors like ours does, make sure they are level so no water can seep in through them.
    • If they are not, you can use shims to bring them up or down as necessary so that the bay window roof sits flush against the floor when it is finished.
  • Build The Rafters – You will need three rafters for your bay window opening; two on either side of where you plan to install the window and one right in the center of them.
    • You can use either lumber or wood rafters depending on your needs, but be sure to paint it before installing if you are using veneer instead of natural stone products like we did.
  • Install The Rafters – Simply screw each rafter into place at the top of your bay window opening.
    • You can also use nails if you prefer, but make sure they are not too close to the edge of each rafter so you do not puncture your roofing material.
  • Install The Underlayment – As with all roofs, it is important that you install underlayment in between every layer before moving on to the next one for best results.
    • You can use the existing rafters to attach your underlayment material by screwing them into place on either side of every layer with a drill and Phillips bit attached for ease.
  • Install The Stone Veneer – To install stone veneers over our roof, you will need some basic tools like pry bars or flat head screwdrivers to remove the existing shingles or asphalt roofing material.
    • The easiest way to do this is by chipping away at it with a hammer until you can get underneath and pry off enough of it in one piece for each veneer installation.
  • Apply Adhesive – You will need adhesive that sticks well while also having some flexibility, so we recommend using a fiberglass roof adhesive.
    • You can apply it to the underlayment with either a paint brush or roller and then work quickly before laying each veneer piece in place for maximum adhesion and stability.
  • Install The Remaining Veneers – Once you have applied all of your stone veneer layers, make sure to let them dry overnight before moving on to the next step.
  • Build The Roof – To build your roof, simply attach four rafters evenly spaced across each bay window opening.
    • Be sure that you are attaching these directly onto top of the underlayment and veneer layers rather than into or through any existing framing in order to avoid potential problems.
  • Install The Roofing Material – To complete your roof, you will need to install a sheet of shingle or asphalt roofing material across the top.
    • You can attach it with nails and then use caulk all around each edge where they meet for maximum coverage and protection against water damage over time.

In most cases, people will attach a flat piece of stone to the top of their bay window roof or simply leave it open as an architectural detail.

In some cases, you may even want to use both options again for added flexibility depending on what you need in your home.

Working With A Professional…

Even though these instructions will get you through the roof installation process, it may be best to work with a professional for this particular project if you are not familiar with how to build roofs or have never worked on one before.

They can also help make sure that your structure is strong enough over time, so it does not collapse under the weight of snow or other environmental conditions.

With The Right Tools…

Finally, you will need to make sure that you have all of the right tools on hand before beginning this project in order to get everything done quickly and easily.

Some of these include a tape measure for sizing your bay window roof openings and a drill for screwing your rafters into place.

These and any other tools will vary by project, so be sure to check with professionals if you are not certain about what is needed before getting started on yours.


On Your Own – If you prefer to do things yourself, then learning how to build a roof over a bay window is an excellent option that will not cost you very much money.

You can find detailed instructions online or see if any of your friends have done it before so they can help show you the process step by step.

How to Build a Roof on top of a Bay Window