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Tips 7

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.

Underlay at the eaves/box gutter has formed a sag/trough under the first course of tiles, due to the lack of a support board. Note the lap in the underlay that has allowed water collected in the trough to drain through into the building below.
This needs to be checked before the first piece of underlay or counter batten is fitted. The use of an extruded or vacuum formed plastics support tray, either as part of an eaves ventilation system or as a direct replacement for the support board, will provide some support. Any plastics underlay support needs to be both deep and capable of spanning up to 600mm to be as good as a plywood board.
      The tell-tale signs of no underlay support, or a back fall away from the fascia, at the eaves are water coming through the soffit, especially under valleys, hips and any side laps in the underlay. Another is to look at the condition of the junction between the barge and fascia boards. If it is rotten or the paint is flaking unexpectedly, it is likely that water collected on the underlay has drained off the ends of the sag in the underlay and onto the barge-board, making more work for the UPVC replacement fascia board and barge board suppliers.
      To blame the carpenter for not installing the underlay support board is easy and in most cases correct, but it will be the roofer who will always get the blame and have to put it right. It surely makes more sense to get the carpenter to do his job properly, or claim for extra work on the quotation for doing the correct detail, and save your reputation. The problem may take months or years to show, but when it does the client will want immediate retribution. Rushing the construction of the eaves could be an expensive mistake!
Compiled by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove, Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774
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