The simplest things seem to slip people's memory and yet are often the cause of major problems at a later date. For more years than I care to remember, all the best trade literature, specifications and roof tile recommendations have stated, or shown, that a support for the underlay at the eaves is essential. But I still see lots of new roofs with no adequate eaves tilt fillets.
      But why do we need an underlay support board or fillet at the eaves?
      Some of the many reasons are to collect condensation that forms on the underside of roof tiles and prevent it dripping back into the roof; prevent wind driven snow from being blown into the roof and to keep the building dry until the last ridge tile is fixed during construction. In each case the water that collects on the underlay will want to drain down to the lowest point, which should be the gutters, unless something prevents its progress, like a lack of an eaves tilt fillet.
      Underlay, being flexible, will follow the line of least resistance and sag under its own weight unless it is supported. Whilst pulling the underlay tight may sound reasonable, in practice there is little to keep it tight. Besides, underlay stretches and shrinks with temperature changes, creating slack.
     The only way to ensure that water does not collect in the sag of the underlay, and does run into the gutter, is to support the underlay on a rigid timber or plywood board between the top of the fascia board and the first tile batten for the full length of the fascia board. On steep rafter pitches it is simple to ensure the support board has a fall into the gutter, but on shallow rafter pitches or with plain tiles it is more difficult and critical, especially if the underlay is positioned below the counter batten or the fascia board height is incorrect. If a counter batten is proposed it may be essential to place the underlay over the counter batten rather than under it.