Walls lean for a variety of causes, due for example to failure below ground caused by tree roots, a cracked drain, frost damage to the foundations or inadequate foundations.
Mae walydd yn camu am resymau gwahanol, er enghraifft oherwydd effaith gwreiddiau coed ar y pridd, draen wedi hollti, difrod gan rew i'r sylfeini neu dim digon o sylfeini.