LEAKING PITCHED ROOFS (part 1)
In past articles we have looked at the common problem of leaking flat roofs, but pitched tiled roofs can also be a source of rainwater ingress, which many owners have found out during the heavy October rains. In some cases, this may have been first time that ingress had been a problem, so how is it that an untouched roof can be OK for years then suddenly fail?
While there are many possible sources of ingress including poor flashings (which will be addressed in part 2 of this article), unlike in the UK, Spanish roofs are often only subject to relatively vertical rain, without much wind force, i.e. the combination of wind and rain is less common. As soon as the two conditions occur together (‘driving’ rain) the problem of ‘capillary’ action (water tracking up under the tiles) becomes more of an issue. In addition to this, a few hours or even a day of rain may go unnoticed (any minor water ingress to the roof slab starts to dry out before it tracks through to the living space. In contrast, prolonged rain over a few days will result in the damp affecting the interior of the house, including ceilings and upper walls as the dry underlying slab has limited capacity to soak up isolated points of water entry. In short, as some roofs are only prone to leaking when exposed to certain types of rain, failure of a roof’s weather tightness can take owners by surprise and often be very difficult to diagnose.
Why does capillary action occur?
Without getting into the technicalities of ‘cohesive intermolecular forces’ and surface adhesion, if two surfaces are loosely resting together and relatively adhesive, water will creep into the smaller space between them. Clay tiles rest in this way, but a decent pitch and good overlap will generally prevent the water from tracking up high enough so as to pass over the upper edge of the lower tile. If however, either the overlap (head lap) or the general roof pitch is insufficient (sometimes both), the roof will only cope with short periods of moderate to heavy non ‘driving’ rain (generally the norm in Spain).
Pitch:- Ideally the roof pitch for curved caly tiles will be a minimum of 25%*, and in Spain, as the tiles are generally loose laid rather than pegged, nailed or featuring nibs over battens, the pitch should be kept under 46%, so as to reduce the risk of tile slip. Most surface tiles other than those at the ridge (top), verge (gable end sides) and eaves ( bottom edge) are loose laid, while the underlying inverted tiles are bedded on a cement sand or lime mortar. Some roofs do however feature pointing to all tiles, which may help to resist slip and driving rain, though eventually the pointing normally cracks away.
Tile overlap:-The overlap of curved clay tiles should ideally be 10cm+ (min 7.5cm on a steeply pitched roof) or around 1/3 of the tile length*, unfortunately this is often not the case as a less generous roofer may have ‘stretched’ the tiles (i.e. skimped on tiles so as to save money by giving minimal overlap). Historically, when money was tight, even self builders or DIY repairers may have stretched the roof tiles, in order to save money e.g. after an earth tremor or storm related tile loss, ( a practice that will also increase the risk of wind related lift in the future).
In addition to the main issues of poor pitch and ‘stretching’, some tiles (upper or inverted
lower) may be cracked, which will allow the rain to pass through to the slab.
While screeded concrete beam and block slabs or ceramic plaque ‘bardo’ underlays will
generally resist the initial passage of water, the older reed underlays, will allow little
time, before the water drips through to the living space. The ‘quaint’ practice of storing
old paella pans, clay pots and even plastic buckets at random across the attic floor of
some older houses (often three floors away from the kitchen!) actually has nothing to do
with tradition, other than the fact that most old roofs leak.
Unlike the UK, the practice of installing a secondary underlying water barrier (e.g. bitumen felt) is still not standard practice in Spain, though various under tile (‘bajo teja’) systems are available from traditional felt to corrugated panels. A slab surface application of polyurethane foam (mainly for insulation purposes) can also help to ensure that any water that breaches the tile covering, will be tracked down to the eaves, rather than entering the living space. As a bear minimum, ideally any under tile screeds should at least include a water proof additive such as ‘Sika1’ or an upper liquid membrane treatment, but the reality is, (as roofs without a secondary underlay will be OK most of the time), such precautionary and slightly more costly details are commonly excluded. In contrast, a lack of roofing felt in the UK might mean that the roof leaks most of the time (prolonged and driving rain being far more common) and hence the underlay practice has been standard UK parcatice for many years.
It is also worth noting that clay tiles are porous, normally slowly soaking up water. In Spain, dry weather normally returns prior to full penetration of the tile thickness, but older more friable tiles may allow for faster passage. In frost prone areas tile damage can be accelerated and some higher inland areas may suffer from blown snow related ingress.
What to look out for:-
In addition to simply examining the roof surface (if safely accessed), capillary related penetration is often characterised by :-
- Non isolated water entry on only one or some sides of the pitch, namely that which faced the wind during the last downpour.
- Periodic leakage that does not occur during relatively vertical (wind free) rain.
- (On surface inspection), look for insufficiently lapped tiles or a very low roof pitch. In general, the lower the pitch, the more head lap is needed.
What can be done?
Unless a new roof structure is required anyway, increasing the pitch is often least viable. Provided that insufficient overlap can be identified as the problem, the head lap can be increased by simply buying some extra tiles and adding in a few rows. All roof tiles will have to be nudged up, and unfortunately this may involve some relaying of the lower inverted tiles if the junction line of upper tiles coincides with that of those below, however a good roofer may be able to play around with a few upper lines to ensure that water tightness is achieved.
Low pitches (under 25%) may require an underlay e.g. of corrugated ‘Onduline Bajo Teja’, the corrugation of which varies depending on the tile type, while almost flat surfaces under 20%, may require a continuous torch bonded asphalt membrane (as for flat roofs). It is worth noting that if full tile removal and an underlay is needed, corrugated insulation panels are also available, such that the headache of re-covering might be compensated for by making the temperature within the living space more comfortable, while reducing heating/cooling costs. While roof paints (normally the red type e.g. ‘Sikafill’) and screed additives (‘Sika1’) are better than no secondary barrier at all, both systems are inferior to a corrugated or torch bonded underlay as they are less capable of coping with thermal movement in the long term. The use of a continuous PU foam (the yellow stuff) over a liquid membrane or waterproofed screed will reduce thermal movement and hence extend the life of the underlay while adding some waterproofing via the resistance of the foam itself. Ideally slabs tah feature membrane shuld include some deliberate eaves area drainage to prevent the build up of water on the slab if water does pass beyond the tiles.
Any cracked or obviously decayed tiles should be replaced. To give a uniform look, it is sometimes better to lift all loose tiles and relay the entire surface while intermixing new and old tiles. (Tiles fixed to non-porous underlying membranes are best located with a low expansion PU foam before final pointing with a sand/cement mortar).
*All tile manufacturers will recommend minimum/max pitches and head laps depending on the tile type and location, so make sure your builder follows their guidelines closely.
Surprisingly many new houses suffer ingress problems due a general lack of control and the use of unskilled labour, so this is an issue that can affect both new and old buildings. Of course, capillary action is not the only reason for roof leaks and in the part 2 of this article we will take a look at other common pitched roof leak causes and how to remedy or better still avoid them.