Photo: Kelly

How much fall should a shed roof have?

As well, seams are often elevated at least 2” off the surface of the roof and, thus, away from moisture. A quarter-inch slope is the lowest you would go with a metal roof – experts recommend at least a half-inch minimum to ensure a weatherproof interior for your shed.

plasticinehouse.com - Shed Roof Pitch: A Practical Guide with Examples and Pictures

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Years ago, when I was building my first shed, I thought roof pitch was of little importance. Since I live in a cold area, I figured I’d keep it steeper to cope with snow loads. For everyone else, I assumed any pitch of roof that repelled water would work. After working on many shed projects, however, I’ve come to find just how critical shed roof pitch can be to a structure. It’s true a steeper pitch is better at repelling moisture, but it also can offer more interior space and aesthetic appeal. A lower pitch also has advantages, primarily in that it uses less material. Below we’ll take a closer look at the benefits of various pitches and why they might work or not for your next set of shed plans, whether your building a tiny home or a garden shed.

What Is the Roof Pitch?

Roof pitch is the measurement of roof steepness. The way pitch, also known as roof slope, is measured is rise over run. So, if your roof rises 4 inches for every 12 inches of horizontal length, then the pitch of your roof is expressed as 4:12. A ratio is the most common way of expressing roof pitch or roof slope, but degrees are also possible. A 4:12 pitch is around 18.5 degrees, but not exactly. You can also divide the rise over run, multiply by 100 and get a percentage. Thus, a 4:12 pitch would be a 33% slope. However, rise over run is more efficient and easier to understand for the average DIYer. If you are following a plan for your shed construction, more often than not it will be listed as a ratio. Roof pitch calculator is great for calculating the pitch of your shed roof. Remember, roof pitch isn’t just for aesthetics. The slope of your roof is vital as a moisture barrier, among other things, and having a roof that is too flat can potentially cause your shed to cave in. As well, the shallower the pitch, the more you’ll need to consider more moisture-resistant roofing materials and underlayment. Another issue is that the steeper the slope of the roof, the more roofing material you’ll need, from roof framing to sheathing. The surface area of the roof increases with steeper pitches, and you’ll have to budget accordingly when planning your shed roof construction.

Types of Roof Pitches

Let’s take a closer look at the advantages and disadvantages of the different roof pitches. Have a look below to see the advantages and disadvantages of the various roof pitch types. It’s important to remember that roof pitch doesn’t necessarily have to refer to a gable roof – it can also be a lean-to, saltbox style or another type of roof with a flat surface.

Low Pitched Roofs

A low pitched roof is anything that is any roof with a pitch of 3:12 or less. Many roofing shingles do not allow applications onto roofs at this pitch. More commonly, roll on membranes such that use heat to bond seams together is used as a roofing material.

Pros

Uses the least amount of roofing material

Easy to access and install

Cons

Cannot handle heavy loads

Shingles often not rated for shallow roofs

Mid Pitched Roofs

Any roof from 4:12 to 9:12 is considered an ordinary angle for a roof. These are also the most common pitches for shed roofs. Again, the most important consideration is the weather. Use a handy tool like snow load calculator to help you determine if your roof can handle the weight of snow-based on its pitch. Also, if you are using pre-fabricated trusses, then you won’t be able to decide the pitch of your roof. Store-bought trusses like these are often 4:12. Shed trusses are no different and pre-fab trusses of smaller dimensions are likely going to be in the neighborhood of 4:12.

Pros

Pre-fab trusses are an option

Can use any type of roofing material

Cons

Still too shallow for some climates

Working on the roof more difficult than shallow roofs

Steep Pitched Roofs

You might ask yourself, why not just have the steepest roof possible? It would repel any type of weather with ease and snow buildup wouldn’t be an issue. The problem with a very steep roof is that if anything goes wrong, whether a leak or a missing shingle, then it is very difficult to reach and replace. As well, you will need much more roofing material on a roof pitch of over 9:12 than you would on a 3:12 roof. Roofing material costs can add up in a hurry, and the difference in surface area, and roof framing, between a shallow and steep roof, is significant. But don’t be fooled, a steep roof is an excellent defense against weather. Just be sure you have a way to repair the roof safely. You’ll also give yourself tons of extra interior space to use for storage space or another workspace.

Pros

Extra interior space

Great weatherproof option

Cons

More material needed = more expensive

Difficult to repair steep roofs

Factors Influencing Shed Roof Pitch

There are tons of reasons to consider when building a shed roof, and the first has to take into consideration the safety of whatever is going to be in your shed. Convenience and interior space are great, but if everything in your interior is wet from leaks, then it doesn’t matter how nice your roof looks.

Weather

Live in the desert? A flat or low pitch roof might be just what you need. Or is your climate more tropical? In this case, you’ll probably want a traditional 4:12 or so pitched roof. Live in Canada? Better raise that roof pitch a bit more to account for the buckets of snow you get every winter. Shallow roofs don’t really like moisture. They are easy to build and the cheapest option, but water can sit on top and seep into cracks between the roofing material. If you get lots of precipitation, then a standard mid-sized roof is best. It can repel water quickly but is still shallow enough to repair any sort of wind or debris damage that might occur during a storm. Finally, consider the wind in your area. Are big gusts common? Consider the wind exposure of your roof – a flat roof performs well as it is not exposed to any wind. A steep roof is very susceptible to large wind storms and the entire structure, including the roof, could falter if the steep roof is exposed to heavy winds frequently.

Let’s take a look at what to consider regarding snowy environments, below.

Snow Load

Using the snow load calculator I linked above, type in the data regarding your shed and roof size. This is a great starting point towards understanding if your roof type can handle the weather in your area. Understand that the snow in one area is very different from snow in another area. Some climates, such as Southwest Colorado, get heavy, damp snow. On the other hand, sections of South Dakota get lots of snow, but it tends to be very light. The psi of the damp snow can be up to 8 times greater than the light snow for the same area.

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Another consideration is ice. Ice can have double the psi as damp snow for the same area. If you intend to heat your shed, then ice buildup on the roof will be an issue. If you choose to heat your shed, start by insulating your ceiling. If you don’t, then you could use a roof de-icing cable to heat your eaves and slowly melt the ice away. Finally, if you have a steeper shed roof and lots of snowfall, consider snow stops on your roof. These prevent snow from falling off in great big sheets when it gets warmer outside. Snow falling off a roof can present hazards to people and the property immediately around the shed. Called snow guards, these can either be plastic shields screwed directly into the roof or a series of metal bars that run the length of the roof near the eave.

Roofing Material

The type of roofing material depends on the pitch of your roof. One of the largest shingle manufacturers suggests that their asphalt shingles shouldn’t be used on any pitch 2:12 or less. It should be obvious that shingles won’t work on a flat roof. Even very shallow roofs will experience moisture going up and under shingles thanks to strong wind or heavy rain. Shingles work on steep roofs, but you better have a long ladder when it comes time to fix or replace a missing shingle. Metal roofs can handle lower pitches – anywhere from ¼”:12 on up. Metal roofs also have seams, but many manufacturers recommend connecting their panels with screws and sealant. This provides a weathertight seal between all the gaps. As well, seams are often elevated at least 2” off the surface of the roof and, thus, away from moisture. A quarter-inch slope is the lowest you would go with a metal roof – experts recommend at least a half-inch minimum to ensure a weatherproof interior for your shed. Remember, metal roofing is more expensive than shingles. If you want the look of a metal roof, but with a steep slope, be prepared to shell out at least triple – or more – the cost of shingles for the same surface area.

Local Regulations

As with any shed construction project, I highly recommend consulting your local building code or a building inspector before choosing a roof pitch. Chances are your local code will stipulate a pitch minimum for outbuildings, such as a shed, and you want to be sure your shed fits the criteria to avoid fines or null and void insurance on the structure. Also, local codes account for the use of membrane or tar paper to use beneath the outer roofing layer. If you are making a loft or tiny home, then code may dictate the use of underlayment beneath a living space.

Ventilation

Proper ventilation in your shed is critical for keeping air moving and avoiding any type of mold or mildew issues. Also, if you plan on using your shed as a workshop or living space, then you’ll need an outlet to the outside to disperse fumes. Any pitched roof with an overhang – eaves – makes it easy to ventilate your shed. Simply installing soffit under your shed roof overhang allows air to flow in and out while providing a physical barrier. Alternatively, a steep roof can utilize vents in either gable to allow air to move through the upper part of your shed interior. A flat roof is more difficult to ventilate, as there is no overhang from a roof nor are there gables to insert wall vents. If you plan to have a flat roof, then consider having several windows to act in lieu of vents. There are some other shed roof features to consider when designing your shed. Let’s take a look.

Shed Loft

If you want to make a shop and a living space in your new shed, then consider a steep pitch and putting your loft in the attic. The steeper the pitch, the more headroom you will have. The gable ends can allow you to put windows, and using rafter construction can open up the interior of the roof space nicely.

Dormer Window

These are the windows that seem to “stick out” from the face of a roof. A shallow pitch cannot sustain a dormer window simply because the interior space makes a dormer window impossible. You need to have a roof pitch that gives you at least the height of a window and then some. A dormer window, therefore, is only used with steeper pitches that have a useable interior space in the ceiling area. These windows can act to open up the attic space of the shed further and allow light for what would otherwise be a very dim area.

Roof Design

Consider different roof designs when building a shed, but understand that pitch is pitch – whether you have a 3:12 gable roof or lean-to roof, the physics of moisture penetration won’t change even though the style is different. A gable roof has the same pitch on each side, as well as the same length. A gable roof is probably the most versatile type of shed roof and the most common as it can handle any type of pitch. A saltbox shed is a gable that extends further, incorporating a covered porch on the front or back. A lean-to is also versatile but features only one roof face. You would not use a lean-to for a very steep pitched roof because it would require you to build the wall on one side of your storage shed to a significant height. Gambrel roofs are made up of four rafters, two on each side. Each one is cut at a 45-degree angle, creating four faces of the same pitch. There is little variation in gable roofs, which makes it simple to construct. A gable roof repels weather, and particularly snow, well. However, they use a significant amount of roofing material. Lastly, a hip roof is a gable roof that has a flat front on one, or both, ends. These are more aesthetic features than practical because a hip roof reduces the interior space of your shed while increasing the complexity of the roof construction. A hip roof requires a medium pitch to be practical, as a hip on a shallow roof would barely be noticeable.

How to Determine Roof Pitch for a Shed

Determining shed roof pitch, for most people, is as simple as finding a roof pitch calculator online. However, we’ll take a look at how to do it yourself without requiring you to be a math whiz.

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How to Calculate Shed Roof Pitch Using a Formula

Calculating the roof pitch is straightforward. Remember that pitch is calculated as the height – or rise – of a roof pitch over 12 inches of horizontal run. Therefore, if your roof rises 4 inches over the course of 12 inches of horizontal run, then your roof pitch is 4:12. Constructing a new shed roof at the desired pitch is as easy as knowing how to use a speed square. As stated in the video, speed squares have useful markings on them from 0 to 12. Each number represents a pitch, so 1 would indicate a pitch of 1:12, 2 would be 2:12, and so on. Line up the pivot on one end of your rafter, at the corner. Then rotate the square until the desired pitch number is lined up with the top edge of the rafter. Make your line. Do this with the other end. Then you use the speed square and same pitch number to cut the birdsmouth joints – learn how to do that from my shed roof framing guide. Once you are done with your cuts, you’ve not only calculated roof pitch but also constructed rafters to fit your shed at the pitch you want. Now, let’s say you want to build an 8×10 shed with a gable roof. Since the ridge of our gambrel roof will run the length of the shed, down the center, that leaves us with about 4 horizontal feet from the ridge to the outside edge of the shed. You want the ridge of the gable to be 2 feet. That gives us rise over run of 2’:4’. The problem is that pitch is not calculated in feet, but inches. So here’s what you do. Take the larger number and divide both numbers by it. So we would get .5:1. Next, multiply both numbers by 12. That gives us 6:12. And there is your pitch. While your speed square can tell you roughly what the degree of your roof pitch is, it won’t be perfectly accurate. To find the degree of your pitch, divide the rise by the run. So for us, it is 2/4, which equals .50. Then use a scientific calculator to find the arctangent of .5, which would look like atan(.5) on your calculator. Finding the percent of your slope is easy if you know the height of your roof. Using our previous dimensions, the equation is (rise/run)x100 = the percent of your roof pitch. For our example, it would be 2/4 x 100 = 50%.

How to Calculate Roof Pitch Height

If you need to figure out how high your ridge board is going to be, then you’ll need to know the width of your shed. If it is 8’, then you’ll have a run of 4’ per side. Now, you’ll need to know the roof pitch factor to find the height of the ridge board – this requires some fancy math or, I prefer, I an easy chart like this. Multiply the roof pitch and the run factor and voila, you’ve got the height of your roof pitch, from wall plates to the bottom of the ridge board.

How to Find the Rafter Length

How to Find Existing Roof Pitch

You’ll also need to find the length of your roof from the eave to the ridge, also known as the rafter length. How to do this? Remember your middle school math! Use the Pythagorean theorem – a^2 + b^2 = c^2. So for our 8×10 shed, you’ll do 4^2 + 2^2 = x. The answer is 20. But you have to take the square root of 20 to find the length, and it ends up being around 4.5’. Sometimes we need to know our existing roof pitch to install new roofing material or for building inspection. Here are a few methods to easily find the roof slope on your existing shed.

Using 48” Carpenters Level and Measuring Tape

Go on your roof and put one corner of the level on your roof, then bring it up, so it is level with the earth. Lower the measuring tape to the surface of the roof 12” away from where the level meets the roof. The measurement from the roof to the bottom of the level is the rise, over a run of 12”.

How to Measure Roof Pitch From Ground

Measure the width of your shed from ground level and divide by two – that’s your run. For the rise, know that vinyl and clapboard siding typically have 4” “exposures” before the next course starts above and below it. Find the exposure that is perpendicular to where the roof begins at the side walls of the gable end of the shed. Follow that exposure to the center of your shed and count the number of exposures until you get to the ridge. Multiply by 4, then divide by 12. You have your rise over run in feet.

Using a Framing Square

On a finished shed, get a ladder and prop it against the slope of one end of roof line. Using a small level atop the flat end of your framing square, put the pivot point against the edge of the roof and bring the square and level up to horizontal with the ground. Use the markings on the square to see where the roof edge lines up with the pitch notations. If the shed is unfinished, then simply prop the speed square against the roof framing, such as the rafter, and repeat the same method.

Shed Roof Pitch Calculator

Knowing how to calculate roof pitch, rise, run, and ridge board height is essential to any shed construction. A roof pitch calculator can make things much easier. Simply plug in the values of your roof and it will show you slope percentage, degree, and even length. Even seasoned DIYers use this tool to check their measurements against their own, and I would advise having this tool nearby the next time you build a shed roof.

Conclusion

I hope this guide cleared up any questions you had about how to calculate roof pitch. Remember, if you aren’t sure about your calculations, there are tons of tools online to help you verify your pitch measurements. Please feel free to leave me a comment if you have any other questions about the article or if you have any suggestions for other tips and tricks for finding a roof pitch that I haven’t mentioned here.

plasticinehouse.com - Shed Roof Pitch: A Practical Guide with Examples and Pictures