The Hambleside Danelaw underlay lap vent has a shallow and tapered design to fit between the overlaps in the roofing underlay below the tiles or slates to introduce ventilation openings. It is retrofitted to reduce condensation in the roof space. Simple for anyone to fit from inside the loft by simply inserting between the overlaps of the underlay. The underlay lap ventilator is designed for simple professional or DIY installation from the inside of cold pitched roofs where condensation is a problem. It is designed to fit between the overlaps in the roofing underlay below the tiles or slates to introduce ventilation openings where there is no proper provision for roof space ventilation or existing ventilation air paths have been blocked, particularly when the loft insulation has been upgraded. The shallow and tapered design of the panel suits all commonly encountered rafter spacings and allows for easier fitting between the roofing felt or underlay where the overlaps are tight. Properties Airflow: 2,280mm2 airflow per vent.Material: PVC.Colour: black. Application • It is recommended that one ventilator panel is fitted between each rafter in the lowest underlay lap above the level of the insulation layer in the loft on both sides of the roof (see product brochure fig. 1).• For rafter centres of 600mm and above, the ventilation area provided can be increased by the installation of two panels between each rafter. The two panels should be trimmed along the dotted line and overlapped by interlocking the first spacing rib (see product brochure fig. 2).• In cases of severe condensation or for large roofs where the distance between eaves exceeds 10m (30ft), or the roof pitch is steeper than 35o, then it is recommended that an additional row of ventilation panels be fitted into the highest line of roofing felt or underlay lap closest to the ridge on both sides of the roof.• The panels should be fully inserted with the ribs uppermost and the two retaining tabs locating over the exposed edge of the roofing underlay. • After installation, the condensation levels should be monitored and if found to persist, then it is recommended that an additional row of ventilation panels be fitted between the next lowest roofing underlay overlap on both sides of the roof. Ventilation area per linear metre 400mm rafter centres: 5700mm2 minimum.450mm rafter centres: 5060mm2 minimum.600mm single panel: 3810mm2 minimum.600mm double panel: 6270mm2 minimum.