Home Contractor's CornerAn Inspector Calls The lay of the land: a focus on the crucial role of underlays

The lay of the land: a focus on the crucial role of underlays

by Matt Downs

This month the Inspector gets on top of the subject of underlays and why they are so important.

The last thing you want to see when climbing into a loft is the underside of the roof tiles. That thin sheet of material that sits between you and the roof covering, be it an old 1F bituminous felt or a newer vapour permeable polymer membrane, is there for a reason.

When we are installing a roofing underlay we are looking for suitable strength, durability, tear- and water-resistance to serve for the entire service life 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 would anyone cut corners or cost when installing the underlay? But they do, and once that membrane fails it cannot be ignored as invariably the risk of roof leakage escalates significantly.

The important role of underlays

Let’s just remind ourselves about the important role that underlay plays in the overall scheme of things. Firstly, it 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 increases the wind load 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 or next-door’s garden or even worse the neighbour’s conservatory or parked car. 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 the roof space. Tiles and slates for the most part keep the weather out but, under certain combinations of wind and rain, they will let small amounts of water through. This is when the underlay when draped sufficiently between rafters allows these small amounts of moisture to drain down to the eaves of the roof and into the drainage system.

Remove that last line of defence and any water getting past the roof covering is destined to destroy the customer’s 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 first to dampen the insulation in the roof reducing its thermal resistance and then often the first anyone will know about it is that dark damp patch of plaster on the bedroom ceiling.

Lastly, modern polymer-based underlays, particularly the vapour permeable type, are also used to a lesser or greater extent to reduce the risk of condensation occurring in the roof space. Most of us have cold roof spaces or lofts, which means that we insulate the ceiling of our upper floor leaving the roof tiles and roofing membrane uninsulated and close to the external air temperature.

This means that all the warm air and moisture that we produce in our homes – and we produce a great deal of it – rises upwards into the loft and hits the membrane below the roof tiles. During the cold winter months the external air temperature is such that the membrane temperature can fall below the dew point. 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 in the form of water vapour 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 is simply not an option as the cost of repair is nothing short of a reroof. Yes: we can patch tears in the underlay up with suitable tapes, but if the cause of the failure is down to a substandard choice of membrane then the underlying problems cannot be solved by tape.

The British Standard Code of Practice for pitched roofs, BS 5534, is clear in its recommendations:

  • Choose an appropriate underlay for your roof taking into account the wind exposure of the building and site location (all reputable underlays are provided with a zonal chart which will tell you if they are suitable, if in doubt ask your merchant to point it out).
  • Follow the manufacturer’s installation instructions to the letter. Underlay might seem like a simple product to install but the cost of getting it wrong can be catastrophic, so don’t. Read the instructions and if in any doubt at all call the underlay manufacturer for guidance.
  • Restrain your horizontal underlay laps properly. Where the underlay laps horizontally you must either restrain it with a batten fixed over the lap or by using a proprietary adhesive or tapes guaranteed for your particular roof installation. Any old glue or tape will not do since its unlikely to be fit for purpose in terms of its durability and again listening to your manufacturer here is paramount.

If there is any part of the roof where no expense should be spared then the underlay is it. It is your last line of defence against the weather, your safeguard against condensation damage and an essential wind load barrier for tiles.


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