© 2009 by Raymond Alexander Kukkee
The Gable: The Most Popular Roof Design
Living in today’s urban environment, if your home doesn’t have one, you can probably look out the window in any direction and spot a standard gable roof close at hand. The gable roof is the most common style of roof construction in North America and is used for homes, garages, garden sheds, and even dog houses.
A standard gable structure has two surfaces or planes rising to a common peak and is actually a ‘double pitched’ roof.
Variations in gable roofs technically may include different pitches, or extra surface planes. Small dormers, which are usually small versions of the main roof may be included. Secondary roof structures on a standard gable roof are usually built to the same slope or pitch for aesthetic appearance. Other variations such as unequal planes on opposing sides rising to a common peak are built for some uses, but the standard gable is the basic roof with two matching planes rising to a single peak.
The slope of a gable roof can be very gentle, or quite steep, depending upon the roof design.
The slope, or steepness of the roof is expressed as rise over run, how many inches the plane rises for each foot of horizontal distance, or as pitch, as in a one-third pitch roof, for example, where the roof plane rises 4′ over a twelve foot width.
You Can Frame a Standard Gable Roof.
If you have basic carpentry tools and have average hands-on skills, you can build a gable roof for a home addition, a car port, or a garden shed. Even a dog house!
For a simple example, let us frame a standard gable roof for that handy 10′ x12′ garden shed you want to build in the back yard.
Perhaps you have already framed the walls using standard framing techniques.
If so, your new shed walls are standing plumb, the corners are square, and the top plates have been installed.. The shed is taking shape, so congratulations, you are now ready to frame the roof!
Considerations in Planning A Standard Gable Roof
To plan a gable roof, several basic decisions must be made:
- How steep is the roof to be?
- Will your roof be rafter construction or the more expensive prefabricated truss option?
- What is the eave width, or overhang ?
- Will the roof be sheeted with plywood for standard shingles, or strapped for alternative roofing, such as metal?
- What is the span, or the dimension from the outside of one exterior wall to the other?
The Truss Option
The truss option for a roof, increasingly common in home construction because of convenience, is a set of triangular frame structures specifically built to fit the span or width specified. The major benefit offered by trusses is that a building can be clear span with no interior load- supporting walls necessary, allowing total freedom of interior design.
Frame the Standard Gable
For our project purposes, we will frame a standard gable roof using the original rafter system called stick framing, and installing individual components that include :
- Rafters : the top, main framing member, also the heaviest framing member. For smaller structures, 2×4 is commonly used. For for wider roofs, use 2×6″ free of knots or defects.
- Ridge board: A centre board installed on edge horizontally at the peak that rafters lean against. Use 1×6″ or 1×8″ boards, free of knots.
- Collar ties : Short pieces of board installed to cross-tie one rafter to the rafter opposite, Collar ties are usually installed level. Collar ties may even be placed to function as ceiling joists in smaller buildings. Collar ties provide tension, preventing the rafters from spreading the walls when weight is applied to the roof, much as the bottom tensile members do in truss construction.
- Fascia boards: The facing boards along the edge of the roof that tie the ends of the rafters together.
- Ladder A pre-constructed assembly designed to extend the roof past, or outside of the wall to create eaves.
Let’s Lay out and Cut the Rafters
To lay out and pre-cut rafters, the pitch, or slope, is marked out in inches,such as 4:12, meaning 4 inches of rise for each 12″ of horizontal distance from the same starting point.
In our example, the shed is 10′ wide, so the distance to the center of the roof is 5 feet. The rise to the peak of the roof will therefore be 4″ for each foot of horizontal width to the peak, or a total of 4″ x5 = 20 inches.
Lay out a matching pair of rafters, one for each side of the roof. You will require three special angle cuts for each rafter; the peak cut, the bird’s mouth’ notch, and the tail cut.
To obtain the correct angle for the peak cut, set the corner of a framing square at 4″ and 12″ respectively along one side of the rafter lumber close to the end of the board. The resulting angle cut you mark across the board will be a 4:12 cut. * Use and read the numbers on either the inside of the square, the outside of the square, but not both!
Cut both rafters. For an equal pitch roof the two rafters must match. The next step is to calculate, mark and cut the bird’s mouth.
The birds mouth is a small notch about an or more inch deep located on the underside of the rafter which allows the rafter to sit squarely and flat upon the top plate of the wall.
Locate and mark the bird’s mouth on the rafter using the same technique with the square. If unsure where to mark the vertical cut, lay the rafters on a flat surface with the precut peak angles set correctly to help you visualize the process. Allow for a 3/4″ ridge board between them.
With simple logic and geometry, in our example, the rise, or height = 20″ and the distance from the center line of the wall to the outside of the plate is 5′ feet, or 60″. Using Pythagorean mathematics, the longest side of the triangle formed must be the square root of the sum of the square of the other two sides, calculated as 63.24″, therefore the distance from the peak to the birds mouth vertical cut is 63.24″ which can be rounded to 63-1/4″.
Measure and mark the required distance down from the peak on both rafters on the top side of the rafters. It is simple to verify the total distance along the plate line. The distance between the vertical marks for the opposing bird’s mouth cuts must clearly be 120″, or equal to the outside dimension, the width of the building. The distance from the peak to the vertical cut for the bird’s mouth notch must match on both rafters.
Alternatively, on a very small building a trial by error system could be used, with two pattern, or trial rafters tried in place but the demonstrated mathematical system is more accurate and offers the advantage and safety of working on the ground. Cut all of the birds mouths on the rafters to the same depth for uniformity.
Making the third and final cuts, the tail cuts on the rafters may be done prior to construction of the roof or delayed until after the rafters have been installed to ensure the line of tail cuts as installed are absolutely straight, resulting in a perfectly straight fascia board .
The horizontal distance from the outside edge of the top wall plate to the tail cut, allowing for the thickness of the fascia board, is the horizontal overhang or eave width. For our example, let us allow 12″ for overhang, less 1-1/2″ for the thickness of the fascia board measured horizontally from the vertical element of the birds mouth cut. Always remember, the tail cut is parallel to the peak cut on the pattern rafter and must end up be perfectly plumb when installed.
Let’s Put up the Rafters!
To install the rafters, first mark out their location on the top wall plates, on both sides of the building. Use 16″ centers or 24″ centers marking the plates in the same fashion as stud walls are marked on layout. Note that the position of the first rafter depends on the width of the roof overhang, and in some instances establishes the basis for framing the gable ends, as noted below.
*Hint: Lay out both wall top plates and your ridge board starting from the same end to avoid errors.
Installing the Ridge Board
Measure and mark the exact center line on the two end walls.
Install a vertical temporary support (2×4) securely along the center line on both ends, and place the ridge board in position on the center line, and at the correct height. Ensure it is level. In our example, the slope is 4:12, and being 5′ to the center of the wall, the TOP of the ridge board should be at exactly 20″ above the top plate of the wall to ensure the correct peak height. Brace the ridge board securely. Important: Leave extra ridge board extended past the wall to allow for the overhang.
On longer buildings, the ridge board may consist of more than one length of board, so either cut and butt-join the ends exactly where two rafters will rest to support the joint, or locate the joints elsewhere with short scab boards for support. On long ridge boards, do install a carpenter’s line to ensure the installed ridge board is absolutely straight. If a rafter does fall on the scab joint, adjust the vertical peak cut of the rafter to allow for the thickness of the scab board.
Place a pair of rafters in position on the wall plate and up against the ridge board, checking the cuts for an accurate fit. It is handy to have a helper and work safely from scaffolding if possible.
Ensure the vertical cut at the bird’s mouth is firmly against the wall plate and toe nail the rafter to the plate. Secure the rafter at the ridge by nailing the rafter through the ridge board. Install the matching rafter on the opposite side. Install all remaining pairs of rafters.
Snap a chalk line on the rafter tails if you have not pre-cut them. Cut the rafter tails vertically and squarely. To mark the tail cuts on a long roof, it can be helpful to build a template set at 4:12 or simply use a bevel square set accurately to mark all tail cuts at once. Verify the markings are plumb prior to cutting the rafter tails off. Install the fascia board ensuring it is as straight as possible. Remember the fascia board must be long enough to extend out as far as required to enable installation of the fly rafters that overhang at the end of the wall.
- After installing the first pair of rafters, install the matching pair on the other end of the roof and set all remaining matching pairs between to a line between the two rafter pairs to ensure the roof is straight.
- Never force a rafter into position if it is cut long, as it will force the ridge board out of line. Always install opposing pairs of rafters simultaneously to ensure the ridge board remains straight.
- The fascia boards on the longest sloped planes always run full length through the corner for the best overall appearance.
- To correct a fascia board that may not be perfectly straight, install it progressively from one end to the other, securing one end first and prying it up or down as required when nailing it to the tails of subsequent rafters .
- Worth installing: In Geographical localities having severe winds, install metal hurricane anchors or storm brackets on each rafter, attaching them securely to both the wall plate and the rafter. The inexpensive, specialized metal brackets provide substantial additional resistance to severe wind damage.
Framing Gable Ends and the Overhang
In framing the gable ends of a standard gable roof, several possibilities exist, depending upon the design of the roof.
- On a very small roof, for instance on a dog house, the rigidity of plywood sheeting will provide enough strength for the short gable end and overhang without extra framing.
- If the roof is to be strapped for metal roofing rather than sheeted, the last rafter on each end of the roof can be installed flush with the outside end walls and the gable end framed in with appropriately notched vertical studs. In that instance, the strapping, normally 2×4″ would be allowed to extend as required to provide the desired overhang. The fascia board is nailed directly to the end of the strapping.
- If a roof is large but the overhang is to be very narrow, the overhang may be achieved simply by adding short lookouts nailed directly onto the last rafter pair which has been installed flush on the outside of the end wall. The fascia board is then attached to the lookouts, or nailed on the face of added fly rafters. With a very narrow overhang, roof sheathing provides adequate additional support for the overhang.
Alternatively, a pair of ladder structures could be pre-assembled using standard framing methods and 1×4 or 1×6″ boards. The narrow overhang assembly will be nailed into place directly on the last pair of rafters, again flush with the wall plate .
Where a 2′ or wider overhang and therefore greater strength is essential, the first rafter pair on each end of the roof must be set back from the end wall at the time of layout, and a lower gable end wall build of a dropped rafters and vertical studs is first framed in place flush with the outside of the wall to provide support for the overhang ladder assembly.
For the end of each roof plane, the ladder structure consists of one matched rafter, a board that functions only to hold the assembly together, and several lookout studs. The inner 1×4 or 1×6 board is necessary only to align the structure as an integral unit while it is assembled, lifted into place and installed. The completed unit is then nailed securely upon the low gable end wall and directly to the face of the back-set first rafter.
With some awkwardness and difficulty, both gable end and ladder structures can also be framed entirely in place piece by piece. It is necessary that the ridge board and the lower-edge fascia boards be extended to attach the fly rafters at the end of the overhang. The lookouts may be subsequently installed for rigidity after the end rafters have been installed.
Following the completion of framing, collar ties are installed at the height desired, and appropriate sheeting and roofing is applied. Alternatively, strapping may be installed as required in anticipation of the installation of cedar shake or metal roof.
Now you know how to frame a standard gable roof. Happy Building!
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Photo credit: Jade (Morguefile) http://mrg.bz/ZUceYs