29 CFR §4211.3

1. (a)Construction plans. A plan that primarily covers employees in the building and construction industry must use the presumptive method for allocating unfunded vested benefits, except as provided in §§ 4211.11(b) and 4211.21(b).
2. (b)Code section 404(c) plans. A plan described in section 404(c) of the Code or a continuation of such a plan must use the rolling-5 method for allocating unfunded vested benefits unless the plan sponsor, by amendment, adopts an alternative method or modification.