Rafters transfer wind and gravity loads along a clear load path through bearing walls into the foundation. The pitch influences rainwater drainage and architectural styles. Rafter angle and length determines the roof slope or pitch. Rafter ties and ridge connections allow rafters to work together resisting these uplift forces. Resisting Wind Upliftĭuring strong winds, negative pressure tries lifting the roof upwards. The triangular layout gives rafters added rigidity. The angled shape allows rafters to efficiently resolve the roof’s weight into vertical and horizontal forces that can be carried down into the walls below. To understand how rafters work structurally, we must consider: Transfer of Gravity Loads Rafters provide a simpler framing solution well suited to smaller roof dimensions. Trusses allow roof systems to span greater distances, but require more complex design and fabrication. Transfer loads primarily along their lower edge.Though both are sloped structural elements, there are key differences: Roof rafters are sometimes confused with trusses. Understanding how to properly design and install roof rafters ensures structural integrity and weather-tightness for the entire building. Give the roof its sloped shape and architectural form.Tie the structure together at eave lines and ridge line.Transfer roof gravity and wind loads to walls below.Provide inclined support for roof decking.Roof rafters provide essential functions: Rafters are typically made of wood, steel, or other materials arranged in a triangular framing pattern rising from the walls to meet at the ridge line of the roof.

Test your knowledge with 2 full practice exams based on the Residential Plans Examiner Exam & 1 Plan Review Test.In structural engineering, a roof rafter is a sloped structural member that forms the roof framing system. Want to become a Residential Plans Examiner? * Reference Source – 2018 International Residential Code – So this sums up the basic code requirements for collar ties.įor more information regarding detailed requirements, be sure to check out Section R802.4.6 of the 2018 International Residential Code (IRC). Rafter ties are a completely separate topic of discussion but as for now this outlines some of the differences between a collar tie from a rafter tie. While collar ties are located in the upper one-third area of the attic, rafter ties are located in the lower one-third area of the attic space.Īlso while a collar ties needs to be no less than a nominal 1 inch x 4 inch member, a rafter tie must not be less than a 2 inch x 4 inch member. While collar ties are used to resist ridge uplift caused by wind loads, rafter ties resist rafter uplift (thrust) caused by wind pressure applied against the wall which moves up under the roof overhangs. Even though they sound like they have similarities, they are very different. Many people confuse collar ties with rather ties. To see how ridge straps must be fastened, we must refer back to Table R602.3(1) above which basically is the same as collar ties. These straps must also be space no more than 4 feet on center and be a minimum 20 gauge with a 1-1/4 inch width. In lieu of collar ties, the code allows you to use metal collar ties also know as ridge straps. What if you did not want to install collar ties in the attic area? Is there another option? By now you know that a collar ties is a wood member with a nominal size no less than 1 inch x 4 inch, spaced no more than 4 feet on center, and located within the upper one-third area of the attic space.

Rafter ties hip roof