Getting to grips with roofing terminology, especially if you have no experience in construction, can be a steep learning curve. Understanding the difference between various roofing elements, including the difference between joists and rafters, is a good place to start your education.
Clarifying the Difference – Joists are used on their own to form a low-slope or flat roof. Rafters are the angled structural elements used in sloping roofs. And within a sloping roof joists (horizontal beams) are often used as well to tie the lower ends of the roof rafters together and give the entire roof structural stability.
WHAT ARE RAFTERS?
Rafters are a popular option for many homeowners and builders to provide structure for the roof deck plywood or OSB and shingles.
Definition: A rafter is the sloped component of the roof framing that extends from the peak of a section of roofing down to the eave.
Installation: A collar tie, also called a rafter tie or tension tie, is used near the peak for structural integrity. A ceiling joist, aka a rafter tie, forms the lower side of the completed rafter.
Rafters are set parallel to one another, as trusses and beams are. And as with trusses, a ridge beam is needed at the peak, and the lower ends are secured to the wall plate. A fascia should be added at the perimeter to cover the ends of the rafters.
If you know what a truss is, which is the complete triangular structure often built in a factory and forming the shape and slope of the roof, then think of rafters as the sloped parts of a truss system. Rafters are typically the choice when a roof structure is stick built on the job site rather than using prefabricated trusses. Keep in mind that the roof load might be lower for a rafter roof than one built with trusses.
Rafters can be designed to handle many different angles of slope and also provides support for insulation and an attic space. They can be constructed using 2x8s, 2x10s, 2x12s and similar dimensional lumber based on the roof structure specifications.
WHAT ARE JOISTS?
Roofing joists are usually dimensional lumber, though engineered wood and steel are options. The discussion here is about timber joists. The material and its dimensions determine span possibilities. Joists are the horizontal components of a roof structure. When the roof is installed, the central joists form the ceiling of the open space beneath the roof.
Floor joists function in a similar way. The key factor to remember is that they are horizontally installed – or nearly horizontal.
Ceiling joists are able to hold the dead load of interior finishes, such as plumbing, lighting and fans, and also support insulation.
CONSTRUCTION/INSTALLATION
Between the two options for wooden structures, rafters or joists, the installation of joists takes a lot less time since there is little or no pitch involved and the joists are already manufactured. It will take an experienced crew of roofers around one day to fully install joists on a home of 1,500 to 2,000 square feet.
On the other hand, stick framing a roof structure of the same size using rafters and collar ties instead of prefab trusses take a lot longer. Installation for rafters can take up to 7 days, building site conditions and the difficulty of the rafter project.
Further Reading: Gable Roof Vs Hip Roof
COST
The average price to install a roof using rafters is between $6.50 and $15 per square foot for both materials and labor. The labor cost is typically the majority of the expense.
The cost for using rafters varies greatly based on the size of lumber – from as small as 2 by 4’s all the way up to 2 by 12’s for the creation of the roofing system. The overall cost ranges from around $11,000 to up to or over $25,000 for the installation of rafters on a 1,500 square foot home.
Joists are a bit cheaper and average around $4 to $5 per square foot and the overall cost for a 1,500 square foot home will range anywhere from $8,000 to up to $11,000 for using joists for the roofing project.
The labor for the installation of either joists or rafters is included in the prices above, but you can expect to pay anywhere from $25 to up to $75 per hour for labor costs when that part of the estimate is itemized.
Further reading: Cost of Rafters Vs Trusses
LIFESPAN/DURABILITY
Both rafters and joists have a very long lifespan and are considered extremely durable due to the fact that they are protected and covered from the environment. You can expect your rafters and joists to last as long as the house will last, which can be up to 200 years or more depending on environment and overall housing maintenance.
MAINTENANCE
Joists and rafters need little or no maintenance at all, as long as the completed roof deck is properly installed and protected from the environment. There should be no need for a regular maintenance schedule, but the plywood or OSB covering either should be inspected every time the structure is reroofed.
ENVIRONMENTAL IMPACT
Rafters and joists are both usually produced from softwoods like pine and fir. As such, they are considered green building materials, both sustainable and eco-friendly.
CLIMATE CONSIDERATIONS
Rafters and joists can both be used in warmer climates, such as states located in the South that do not get much snow at all. Rafter systems are a better choice in northern climates because they bear snow weight and shed the snow better than a joist-only flat low-slope roofing.
CONCLUSION
It’s not so much that there are advantages and disadvantages of one over the other – the point is that rafters and trusses simply refer to different parts of a roof structure. Rafters always have sloped components. Joists are always horizontal.
If you’re building a sloped roof, you will use rafters and joists, unless you choose roof trusses instead.
Rafter systems are built on the construction site vs trusses, which are prefabricated and shipped to the job site. Compared with joists/flat roofs, a roof built with rafter systems gives you much more flexibility for roof slope and shape, though span is limited by the dimensional lumber available. Plus, sloped rafter-built roofs are much preferred for protecting the structure from heavy rain and snow.
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Jamie Sandford is the Chief Editor at RenoCompare (