- You carry out structural alterations
- The performance of the new covering will be significantly different to that of the existing covering in the event of a fire
- You are replacing/ repairing more than 25 per cent of the roof area, in which case, the roof thermal insulation would normally have to be improved.
The removal or alteration to any roof elements could affect how the roof works and cause movement to occur. Movement could cause cracks to occur in the walls and, possibly, the eventual collapse of the roof. When performing work on any roof, care should be taken to ensure the roof will continue to perform effectively and without any movement.
Existing Pitched Roofs
The existing roof structure that forms the loft space has a number of timber elements that make the overall pitch. Each element enables the roof to span across the building and support the tiles/covering on top as well as being able to transfer the loads (weight) created by any wind and snow down to the walls.
Listed below are the typical elements of a pitched roof:
- Ridge Board – This forms the apex of the roof and is where the rafters are fixed to both sides.
- Rafters – These are the timbers that form the main pitch to the roof and support the tiles and battens.
- Purlins – These are long pieces of timbers that are normally seen half way along the rafters and act like beams to reduce the span (unsupported length) of the rafters.
- Struts – These support the purlins. They are fixed at an angle with one end connected to the purlin and the other on to a load bearing wall or a timber spread across ceiling joists. These are the diagonal timbers seen in the roof.
- Ties – These are timbers which stop the roof from spreading and form an A-frame shape. They can either be the ceiling joists (as described below) or can be fixed half way up usually above the purlin and are fixed horizontally from front to back. (Common in terraced houses).
- Ceiling Joists – These can act as ties, but mainly support the ceiling below. Their sizes are usually relatively small and will not be able to take the load of any typical room used in a house.
Existing Flat Roofs
Flat roofs are more simple and generally consist of joists that span the gap between two walls. These are covered by panels which, in turn, are covered in felting or other such coatings as required.