lapVent - a revolution in roof space ventilation systems

Tel: +44 (0) 1202 979912

lapVent - a revolution in roof space ventilation systems lapVent - a revolution in roof space ventilation systems

Product Overview

Ambient air passes freely through the natural surface fissures in most normal, unsealed slate or tile roofs - the surface of a conventional free-laid pitched roof is far from airtight. In addition, the whole of any one section of roof is also interconnected by numerous batten cavities and gaps running between draped underlay and tile/slate battens.  Air permeates largely unhindered and quite naturally at normal atmospheric pressures, as BRE wind-loading trials and research confirms.  This remarkable system was born out of that knowledge and through long and thorough experience.



Product Overview

1. Step flap valve
The adjustbale self righting flap control extreme negative and positive wind loads.

2. Mesh
An expanded aliminium mesh acts as a filter, fly screen and difuser.

3. Lightweight body
None-load bearing body forms a controlled airway into the rof space wthout breaching or weakening the roof defenses.

4. Retaining wall
Two retaining walls deflect cross flow and maintain a clear resevoir area ijn the batten cavities.

5. Storm baffles
Six troops  prevent the entry of driven rain and snow and self draining ramps stop pooling dirt and water (thwarting plant growth).

6. Diffuser blades
Three blades dampen airflow  and deflect moisture droplets.

7. Underfelt

8. Batten cavity

9. Cladding


Safety Flap Valve

safety flap valveThe balanced, adjustable self righting flap acts as a valve that controls both negative and positive wind loads. There is a cut out section at the lower end of the hung flap for adding weights (i.e., lead) for making fine or extra adjustments where required, for example where roofs have a pitch steeper than 500. The inside surface area facing the meshed opening, serves as a splash-board for any incoming airborne pollutants, as well as acting as an effective disperser of airflow. A raised rim running around the inner face of the flap is there to direct any run off caused by humidity and guide moisture down into the body and then to atmosphere. Two holes, one on the body flap bracket and one on the flap itself, have been put there so that screws can be inserted to shut the vent down if ever necessary.



meshThe main inlet/outlet aperture is filtered by a fine mesh measuring some 37,150 mm2, this provides a comparatively large 'free flow area' of 20,043 mm2. The expanded aluminium mesh acts as a fly and airborne detritus barrier, as well as a baffle to calm airflow and to further extract more condensation from the movement of warm, damp air as it passes through the vent. Because of the configuration of this type of mesh, air goes out faster than it enters (see BRE technical info), thus extracting moist air faster than it allows dryer fresh air in. This is yet another unrivalled feature, which means that even more moisture can be extracted from incoming air, a particularly good thing, especially on hot sunny days after a rain storm when roofs are actually seen to steam. Other parts of the vent are also just as unique and purposely designed to regulate the in/out airflow specifically for energy conservation purposes.


Lightweight Body

lightweight bodyThe 'none load bearing' lightweight body of lapVent is shaped to form a weatherproof, unexposed, controlled airway which safely interconnects inside roof voids with atmosphere. By using laid down methods of installation, there is no interference with essential structural detail or weakening of insulation products and materials. As with the rest of the components which make up the vent, the body is manufactured out of durable, top quality, high impact, recycled ABS plastics and being entirely unexposed to the ravages of the weather, it should have an extremely long life expectancy. Therefore, this ventilation unit is given a lifetime guarantee to match the overall duration of the roof, its fittings and materials.



Central Walls

central wallsTwo retaining walls deflect any cross-flowing air as it permeates below the outer cladding of a roof and form a clear reservoir area within the batten cavities, between slate/tile batten and underfelt. The walls also provide further surface zones for the collection of airborne moisture and they stand clear of the ramps to form a gulley. The front end is sloped to avoid the vent snagging on the underside of any battens when being inserted into the underfelt.


Storm Baffles

central wallsSix ramped traps prevent any entry of storm driven rain and snow and by design allow the vent to be self-draining so that no pooling muck and water collects to encourage plant growth (the 'flower-pot' effect!). The shape of these storm baffles, one end flat, the other sloped, also acts as a 'damper' and 'venturi'. The upright feature slows down the inflowing air, deterring moisture from entering the roof so reducing energy/heat loss, and the sloped parts speed up the outflow thus extracting internal moisture relatively swiftly - it sucks in slowly and blows out quicker!



Diffuser Blades

diffuser bladesThree blades dampen down the airflow and deflect moisture droplets away harmlessly towards the gutter or to atmosphere. They are connected to the upper part which creates a cover to the ventilator chamber; each element in turn forms a 'ceiling' for moisture to condensate on. Although the underside of roof slates and tiles are naturally and constantly wet throughout the different seasons and times of the day, this ventilation system will not add to it, mainly because of the many barriers that have been put into place to avoid it. What moisture there is in the air will be turned to condensation long before it can land on tiles or their battens. At this side, on the exterior of the roofing underlay, even if anything reached other parts of the cladding or its substructure, it would naturally and rapidly be evaporated to atmosphere in due course.



underfeltThis type of ventilation system sits unobtrusively between two overlapping runs of any sort or make of underfelt, without impairing its performance or effectiveness. A standard minimum lap of 100mm must be maintained and the re-covering should be done with either equal or superior material. It would of course be advantageous if an extra run of underfelt is laid below the unit to eaves, but not entirely necessary as the run off from condensation would be minimal. Unless it is a very short run to the eaves, any trickle of water will quickly evaporate naturally to atmosphere before getting very far down the roof - if proper underfelt of average quality is used, a small amount of moisture coursing down it occasionally will cause no more harm than if the vent was not there.



Batten Cavity

batten cavityThe batten cavity is a continuous gap that is ever present beneath the under surface of a roof's slates, or tiles and above the outer surface of any insulating underlay. Air flows around and up and down this empty void quite freely and without much hindrance. Even heavy growths of moss and lichen and hundreds of year's worth of dust and dirt cannot stop free laid, small element, unsealed roofing materials from breathing quite freely and naturally! Building Research Establishment wind-loading test figures show that on an average, two thirds of the force of any wind and the prevailing atmospheric pressure hits the external surfaces of a roof's underlay. This ability to 'breathe' is a characteristic that has long preserved many an aged old pile and it is one that the lapVent system now reproduces.


External Cladding

The many different shapes and varieties of roofing material these days mostly adhere to one major principle; and that is that they are generally freely laid in an overlapping manner, individually, one upon the other in a 'weatherproof', not 'airproof' manner.

natural welsh slatebelgium clay fully interlocking tileUnless nailed directly to sarking boards, as in the Scottish manner, roofing slates and tiles of all sorts lay loosely against each other, if they did not, then the constant day to day expansion and contraction of a roof would eventually do damage. An unyielding surface would either disrupt the weatherproof set of the cladding or even possibly destroy the material itself. If not blanket-bedded on, or painted over with a ruinous mastic type sealant, slate, stone, clay and concrete roofs will breathe quite freely - as they have done for hundreds of years. The original roof of a very sound 17th century building is seen to pass air quite liberally through its surface just as well as it always has done, even under centuries of grime! A few modern roofing materials may be less irregularly fitted than their illustrious counterparts, but they still breathe by design.

interlocking concretepurbeck roofing grade stoneThe acid test is to view an un-insulated roof from below and 'look downwards'. Position the eye correctly and very carefully and daylight will be seen through every perpendicular abutment and bottom edge of just about every individual slate or tile - even extremely close fitting man-made artificial slates have at least 2mm gaps at 'each' perpend. Hold the two sides of any one type of popular roof cladding closely together, hold it up to the light and it will be seen just how much of a gap there really is. Even the overlapping sides of interlocking tiles 'leak' air copiously if not sealed.

hand-made clay plain tileman-made plain concreteAs for dirt and moss and any other plant growth altering the general permeability of a roof, that is somewhat misleading as it has little to no effect. Because of an overpowering 'musty' smell that could not be cured, a Dorset bungalow was about to be actually demolished and rebuilt a few years back until 'lapVent' was installed (see pictures below). The problem was a lack of roof-space ventilation, although it was a very high specification building and this 30 year old, modern concrete interlocking tiled roof had more muck and plant life on top of it than is usually normal!

dorset bungalow with internal image of lapVent installation