Ties are essential in cavity walls to ensure that the wall is structurally sound and stable. A cavity wall is as strong as a solid wall only if the two slender leaves are securely tied together. Lack of skill or care in installing ties can lead to distortion, cracking, or - in extreme cases - collapse of the outer leaf. The outside leaf is also the vital rain shield for the building. If ties are badly installed, this can lead to rain penetration and dampness in the inner leaf. This Good Building Guide gives good practice advice on installing wall ties in new building. 6 pages.