Home Pitched Roofing Consultant Case Study: Eaves detailing

Consultant Case Study: Eaves detailing

by Jennie Ward

Technical Roofing Consultant John Mercer teamed up with roofing expert Chris Thomas for this survey on a traditional handmade clay plain tile roof, which has suffered from water ingress through the fascia and bargeboards. As John explains, as is often the case, the issue was with the detailing at the eaves and subsequent troughing in the underlay…

I recently inspected a traditional handmade clay plain tile roof where water has been running through the fascia and bargeboards, causing damage to the timberwork.  

I was accompanied during the inspection by Chris Thomas, whom I persuaded to come out of retirement temporarily to assist me. Chris has a vast wealth of knowledge on roofing, and he wrote an article many years ago on eaves detailing, so it was great to have him along.

Chris Thomas checking the roof pitch at eaves.

One of the most common problems I come across when carrying out roof surveys is the detailing at eaves, and in particular the lack of support for the underlay behind the fascia board or eaves tilt fillet. This causes a trough in the underlay directly behind the fascia, which can trap and collect water. The water invariably leaks through the underlay as it finds its way through any cuts and nail holes.

The importance of the underlay’s function as a watertight layer is often overlooked. The underlay is the last line of defence if water is driven through the tiling in extreme weather conditions, and with the use of vapour and air permeable membranes, there is the likelihood of condensation forming within the batten cavity. Therefore, it is important that the underlay be laid to ensure that any water in the batten cavity can be drained safely into the gutters.  

Manufacturer’s details always show the underlay fully supported at eaves, whether it be by a plywood board or proprietary plastic underlay support tray, though it is surprising how often these are omitted.  

It is not good practice to continue the general underlay into the gutter as it will degrade through exposure to sunlight, therefore the common solution is to install proprietary plastic eaves support trays which not only support the underlay behind the fascia at the correct fall, but they also extend into the gutter and are UV-stable. The alternative solution is to fit a timber support and lay a minimum 300mm wide strip of 5U bituminous underlay at eaves, which being resistant to UV degradation can extend into the gutter.

Bellcast eaves detail
The rafter pitch on the roof surveyed is 45 degrees, therefore it should be quite simple to provide a support for the underlay at the eaves. Unfortunately, the fascias are set too high, resulting in a ‘bellcast’ eaves detail. This does not contravene BS 5534 recommendations for double lapped plain tiles, provided that the tiles in the bellcast section of roof are still set above their minimum recommended roof pitch, which in this case, they are. But it does make correctly detailing the eaves support and underlay more difficult. A fall in the underlay must be maintained behind the fascia; if this is not possible, it would be necessary to lower the fascias. The eaves detail was further complicated because there are over fascia ventilators installed, therefore an air path from the ventilators to the roof space must be maintained.

The distance between the top of the fascia and fascia ventilator to the first course under-eaves tiling batten was too short to fit standard plastic underlay support trays without having to cut each tray.  Therefore, the solution found was to install specially cut softwood timbers to support the underlay to the correct fall towards the gutter, shaped to maintain the air path from the eaves. The diagram below shows the detail.

Diagram shows air path through eaves ventilator.

When constructing a roof, the fascias or tilt fillets should ideally be set to support the tiles at the same relative pitch as the tiles above. This becomes even more important where the roof pitch is at, or close to, the minimum recommended roof pitch of the tiles. If the tiles in the eaves courses are set below their minimum recommended pitch, there is a risk of water ingress through the tiling at the point in the roof where there is most water as it runs down from the roof slope above. For double lapped plain tiles, a change in pitch – which is what a bellcast eaves is – affects tiles in several courses up from the eaves, not just at the point of the change in pitch. For single lapped tiles, a bellcast eaves is not recommended because it directly affects the way the tiles overlap and can interfere with the headlaps’ normal water-shedding function.

In summary:
• Set the fascias or tilt fillets to support the eaves course tiles at the same relative pitch as the tiles above.

• Ensure there is full support behind the fascia for the underlay to lay at an adequate fall towards the gutter.

• Do not extend the roof underlay into the gutter. Install proprietary underlay support trays or use strips of 5U bituminous underlay which is more resistant to UV attack.

• Ensure a clear air path is maintained for ventilation at eaves into the roof space.

www.johnmercerconsultant.co.uk

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