What Causes a Roof to Sag?

Connections

• The fastening between critical members of the roof's wood framing, such as rafters and ceiling joists, must remain tight; any loosening can weaken the connection between the members and lead to roof sag. Sometimes ceiling joists have more than one member, with metal fasteners used to connect the individual boards. If the fasteners are not tight, the boards can start to separate, causing the roof to sag. Also, improper nailing at the point where the ceiling joist connects to the bottom of the rafter can cause sagging.

Weight

• Excess weight on the roof can, over time, cause sagging. When roof rafters bear excess weight for a short period of time, they typically bounce back into place, but a lengthy period of supporting excess weight can reduce their ability to straighten out. For example, if rafters have to support a heavy load of snow for many months, the roof might sag permanently, even when the weight is removed.

Hipped Roof

• A hipped roof might exhibit sagging due to the ceiling joist not running parallel to the rafters. The joists do not connect with the rafters and therefore do not help support them. The thrust of the rafters does not go into the joist but into the roof decking instead. The decking, or sheathing, might not have the capability to support this extra stress and begins to sag as a result.

Collar Ties

• One potential solution to a roof sagging due to rafters that have insufficient support involves placing wooden boards, called collar ties, between two rafters on opposite sides of the roof to bear some of the thrust. Sometimes do-it-yourself homeowners attempt this type roof repair for drooping roofs, but it's not always successful. If you place the collar ties too high up on rafters, the roof will probably still sag. Have a professional do any work on a sagging roof.