#Rafter span calculator code
Using either a rafter tie or the ceiling joists themselves can vary slightly in the size of the roof rafter used which is why the code creates two types of span tables. This connection is important because it resists the horizontal thrust from gravity loads and rafter thrust caused by the uplift from wind loads. When they run in the opposite direction creating a scenario where they are not attached to the rafters, then rafter ties can be used to achieve the tie requirement. When the ceiling joists are attached, they act as the tie.
We touched on the loads earlier but let us go over why it is important to know if the ceiling joists are attached to the roof rafters or not.Īccording to Section R802.5.2 each roof rafter must be tied across the structure via ceiling joists or rafter ties. Now in order to select the correct table, we must understand 2 things: Which Live Load is to be use and whether or not the ceiling joists are attached to the roof rafters. It is very important to select the correct table. Roof Rafter Span TablesĪs mentioned above, the span tables within the code list the allowable roof rafter spans for common lumber sizes based on what design load scenario is applied.Īccording to Section R802.4.1, roof rafters are to be sized based on Tables R802.4.1(1) through R802.4.1(8). The weight of the materials of construction incorporated into the building, including but not limited to walls, floors, roofs, ceilings, stairways, built-in partitions, finishes, cladding, and other similarly incorporated architectural and structural items, and fixed service equipment. This does not include construction or environmental loads.ĭead loads are defined by the code as follows: Loads produced by the use and occupancy of the building or other structure. Live loads are defined by the code as follows: The code recognizes 2 types of loads that are applied to roof rafters: Live Loads and Dead Loads.
Before we look at these tables, we need to make sure the correct design loads are determined prior to selecting the correct table. These span tables list allowable roof rafter spans for common lumber sizes based on what design load scenario is applied. Wood roof rafters are sized based on one of eight tables found in the code.
#Rafter span calculator how to
In this post we will learn how to correctly size a roof rafter based on its span, spacing and species of wood used.įun Tip: To learn the difference between the International Residential Code (IRC) verses the International Building Code (IBC), be sure to check out this POST. Section R802.4 outlines the wood roof rafter framing criteria which also includes prescriptive span tables. Please be aware of which version of the IRC you are using since the allowable spans can vary.) For the purpose of consistency, this post will reference the 2018 IRC. The allowable roof rafter span tables in the 2018 version are found in section R802.4 while the tables in the 2015 version are found in section R802.5. (Note: the table references have changed in the 2018 version of the IRC. The design and construction of wood roof rafters are found in Section R802.4 of the International Residential Code (IRC). In this post we will cover wood roof framing members, specifically roof rafters. The International Residential Code (IRC) recognizes two types of conventional roof and ceiling framing systems for buildings that are subject to the International Residential Code (IRC): wood framing and cold-formed steel framing. Roof/ceiling members must be properly designed to ensure they provide adequate support as required by the loads that are imposed on them as well as making sure these loads are correctly transferred to the supporting walls and/or supporting structural elements. Want to learn how to easily size roof rafters?Ĭhapter 8 of the 2018 International Residential Code (IRC) regulates the design and construction of roof and ceiling systems. How to Size Ceiling Joists Per the IRC | Easily Explained with Examples.How to Size Floor Joists Per the IRC | Easily Explained with Examples.
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