Why does my roof have a sag (drop) in it?
It is very common to see undulation in a tiled roof on a older home. This can be caused by many reasons such as split rafters and strutting issues however the most likely is that at some point in the homes life the roof tiles have become aged and deteriorated.The sealant that is protecting the tile has deteriorated. When this happens the tiles can absorb water and become water logged which in turn doubles the weight of tiles to around 120kg per square meter. When the tiles become so heavy the timber cleats that they rest on (Barrup truss) have twisted or warped under pressure.
The roof however can be fixed.
If it is the rafters that are struggling under the load they can be reinforced with a tensioner thread(see pic below). This commonly used and quite effective. The wires are bolted to either end of the hanging beam and a thread in the centre of the hanging beam pushes the rafters back up into its original position.
Why is my ceiling in the garage sagging down?
The garage ceiling is a common spot to see large amounts of undulation in the ceiling linings. There are multiple reasons this can occur:
No insulation – No insulation above the garage is a very common in newer homes. It is a cost saving for the builder however it can cause major issues. During the cooler months of the year, moisture that is in the air that enters the roof void can easily settle on the reverse side of the plasterboard creating a softer product and weakening it at the stress points (fixings).
Ceiling paint – The paint used is the same as the interior ceiling white that is not moisture resistant. Therefore letting moisture penetrate the plasterboard.
The roller door top gap – The gap above the roller door can let wind blow freely into the garage causing the plasterboard ceiling linings to slap up and down on the ceiling joists slowly wiggling them loose at the screw points. Common signs of this are nail popping (when the nail head protrudes the plaster board) or alternatively the paint starts to flake directly above the roller door.