When we are installing a roofing underlay we are looking for suitable strength, durability and water resistance to serve for the entire life cycle of the roof. This is vital, because the failure of that membrane is the death knell for any installation. Think about it: when we install our roof, we start with the underlay placed over the rafters; we hammer in our battens on top and then fix our tiles on those battens. So it follows that if the membrane fails, it is off with the tiles, off with the battens and so off with the entire roof. An expensive repair.
It begs the question why anyone would cut corners or cost when installing the underlay but they do, and once that membrane fails it cannot be ignored.
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The cost of allowing the roof to ‘go commando’ and lose this essential undergarment is threefold. Firstly, the underlay provides an essential barrier to reduce the wind uplift load acting on the slates and tiles. This means that using a substandard, or poorly installed underlay places increased pressure on the tiles and their fixings leaving them at greater risk of being removed in high winds. In short, this could mean tiles leaving the roof and taking up residence in the garden. A brief search of Google will deliver a host of horror stories about the dangers of dislodged clay and concrete tiles from a roof (including deaths), not to mention the cost of repair afterwards.
Secondly, the underlay is also the final barrier against the ingress of wind driven rain, snow and dust into your roof space. Tiles and slates for the most part keep the weather out but, in extreme conditions, they will let small amounts of water through. This is when the underlay drape carries the excess moisture down to the eaves of the roof and into the drainage system.
Remove that last line of defense and any water getting past the roof covering is destined to destroy the record collection or that box of old wedding photos tucked away at the back of the attic. Worse, it won’t stop there because once water finds a route into a property, that ingress will continue and the first anyone will know about it is that dark damp patch of plaster on the bedroom ceiling.
Lastly, modern underlays, particularly the vapour permeable type, are also used to a lesser or greater extent to control condensation in the roof space. Most of us have cold roof spaces, which means that we insulate the ceiling of our upper floor leaving the roof tiles and membrane insulated and ‘cold.’
This means that all the warm air and moisture that we produce in our homes and we produce a great deal of it floats upwards into the loft and hits that cold barrier. What happens when steam hits a cold surface? It turns back to water! In a modern home the underlay is designed to help control this process and allow some moisture through, but if that underlay is insufficient for the task or degraded the results will be as damaging as water getting in from the outside.
When it comes to roofing underlay failure simply is not an option as the cost of repair is nothing short of a reroof. Yes: we can patch the underlay up, but if the cause of the failure is down to a substandard material then where does the patch end?
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