Leaking Pitched Roofs (part 2)

In the first part of this article we looked at capillary action and solutions for pitched roofs, (you can access part 1 via the IT website if you missed it). In addition to water driving up under tiles, the roof junction areas are a common point of water ingress on both new build and older properties.Flashing (What flashing?)

Unlike the UK where lead flashing is common, the traditional flashing methods used in Spain are far less reliable. Normally the upper course of tiles is chased into the wall slightly while the junction is ‘flashed’ with a cement sand mortar fillet. Over time the mortar often cracks allowing water to penetrate. In most cases (except where a waterproof additive has been used in the mortar mix) the junction is also dependant on a good surface paint finish over the mortar fillet. A lack of regular maintenance (common at roof level due to difficult access), will result in paint breakdown and hence failure of this critical waterproof finish. It is important to note that in Spain, the combination of UV related damage and temperature extremes play havoc with fragile building elements and a mortar flashing may crack through within days (even hours) of installation during hot weather.

So what is the solution?

As a bare minimum, the cement sand mortar mix for any roof details should include a latex additive e.g. ‘Sika Latex’. This will help adherence and help to waterproof the mortar as well as reducing the risk of cracking. The additive must be well mixed with the water first (according to manufacturer’s instructions) before making up the mortar.

Ideally junction fillets e.g. at roof to wall or roof to chimney should be painted as for main walls, such that surface water is carried to the lower edge of the fillet. The upkeep of this paint must be maintained. (use ‘Sika Fill’ blanco) or at least a 100% flexible acrylic paint e.g. ‘Jotun Tex Ultra’. Never skimp on materials at this stage, otherwise the requirement to repaint will be less than 2 years. Try and avoid the red coloured roof paint as this is far more susceptible to thermal damage. The white version will reflect the heat, reducing the risk of cracking in the mortar itself and in turn lasting much longer.

Ultimately, the junction should feature an additional flashing in the form of a torch bonded asphalt band or a zinc profiled flashing. Most tile manufacturers recommended zinc profiles or Alu/zinc banding as standard junction details, however in practice few builders use them. Some good builders will now include the underlying asphalt membrane band as standard (though this is still quite rare). The asphalt solution is cost effective and can easily be added to seal the junction between roofs and walls prior to rendering. The wall area is hidden by the render while a half tile can be laid over the tile area asphalt (using the latex additive in the mortar mix) to give a conventional finish. A traditional upper fillet of the same mix can then be made to terminate the junction (any ‘monocap’ renders or paint finishes should also run over this fillet). Following this, should any fine cracking occur, water entering the junction area will be tracked back out to the roof surface over the protective (flexible) asphalt. It is worth noting that in addition to covering up the unsightly asphalt, the upper tile will also protect it from general UV related decay.

 

It is worth noting that ‘monocap’ coloured renders are only waterproof if of sufficient thickness. Where they may taper e.g. over a mortar fillet, the render may not give adequate protection. A well maintained acrylic paint finish is actually more effective than a monocap render at such junctions by resisting water from the surface, while providing some flexibility. Ultimately if you can choose the wall surface finish, in the long term it is best to opt for a fully cured conventional cement sand mix render with a  ‘raspado’ textured surface over which a spray applied acrylic mortar such as ‘Webertene Extra’ is then applied (to include fillet areas).

Valley junctions

In many cases roof valleys comprise of an inverted tile gulley. Unfortunately as the tile overlap from the main roof is limited, this type of valley may not cope with heavy driving rain. Tiles may also crack through or suffer slip. Ideally all valleys should be made of a flexible band e.g. of zinc ‘canelon’ roll or profile, providing a continuous or well lapped valley, which runs well under the overlapping roof tiles. As a precaution, all bedding mortar mixes should include the latex additive. Again the tile manufacturers (‘Tejas Borja’ being one of the largest), provide good standard details, however, even if the Architect has included such a detail (not always the case), the builder may well ignore it.

Eaves detailing

Generally the larger the eaves overhang, the more effective the roof will be at resisting eaves area water ingress. The most common cause of eaves area ingress is a cracked or damaged inverted tile, the simple solution to which is replacement or repair with a flexible filler e.g. with ‘Sika Flex 11 FC’. In new build or re-roofing situations the latex mortar additive can also be used for the bedding and pointing of eaves tiles. If you have an option, ask the builder to extend the eaves overhang by cantilevering a number of flat brick tiles ‘ladrillos’ before finished with the latex mortar bedded curved tiles. That way a 30cm cornicing might be extended to a total eaves overhang of 50cm (much better protection for walls below and less risk of water tracking across cornicing).

Gable end details

Ideally as for eaves, gable ends should feature a substantial overhang, however it is common for eaves termination to run almost flush with the gable wall face and this technique is cheaper to execute. Where this occurs the termination ‘verge’ detail is very important. Normally a flat brick tile ‘ladrillo’ is used to cantilever out from the face slightly. Importantly this should angle downwards slightly to resist the tracking of water back to the wall and into the bedding mortar. As for all other details the latex additive should be used in the bedding mortar mix. Occasionally a curved tile is fixed over the verge, but the addition of a slight cantilever overhang is preferred.

 

As for many details a good surface paint finish is commonly required to stop water from soaking in to the upper gable area. Existing roofs that suffer verge area ingress will commonly show signs of paint key loss or render staining to the upper gable and may suffer ingress to the living space. Ideally in addition to repainting any exposed gables preferably with the ‘webertene’ product or ‘Jotun tex Ultra’, the verge ladrillos and mortar can be further protected with two coats of ‘Sika 703W’ (clear mat waterproofing agent), If the verge mortar has previously been painted, repaint it with ‘Sika Fill’ blanco or the paint product above. Be sure to brush off any loose paint, moss or algae growth first before application.

Zinc verge profiles are sometimes used but are not popular as they are not in keeping with the traditional look of many properties. An angled termination verge tile may also be used, to create a more effective drip (most common over timber rafters).

It is important to note that most manufacturers have special termination tiles for different situations and it may be worth ensuring that special tiles are purchased for critical detail areas, rather than making do with the adaptation of standard main roof tiles.


Ridge detail.

Most ridges comprise of a mortar bedded row of overlapping tiles. As for all mortar areas, cracking can occur due to thermal exposure. The use of the latex additive is again preferred for both bedding and jointing. If the ridge is angled (often the case on some older roofs) the tiles should lap over each other starting from the lowest point so as to shed water more effectively (the reverse can result in rapid ridge failure especially when no latex is used). Ideally, as for other junctions an underlay will help to ensure that water cannot penetrate to the living space, even if the ridge tiles or joints fail. As a precaution an underlay over the roof slab ridge junction can be included.

 

Summary

Many water ingress problems at roof level are due to poor initial detailing, lack of mortar additives or the failure to maintain surface protection of mortar fillets e.g. with a good paint system. Isolated defects are often easily diagnosed on inspection and may be cheap to resolve. If an issue is more extensive or a roof is being newly built or re-covered, it is well worth buying the additional materials to ensure that details are formed properly in the first place or remade in full. If you can’t see the roof surface yourself, get the roofer to take some photos of the defect area and during the works. In many cases owners may be fobbed off with a ‘solution’ that may last for less than a year, while paying for more extensive works. Most foil backed bitumen bands or other adhesive repair bands (e.g. ‘Sika Multiseal’ are only suitable for temporary repairs). (More information for general roof areas is detailed in part 1 of this article).

Finally, don’t be tempted to carry out the often dangerous activity of roof repairs yourself, instead consult an experienced roofer.

NB:- Information for advice purposes only. Proper safety precautions should be taken  and legal procedures followed when carrying out all works.

Information provided by Mark Paddon BSc Hons Building Surveying. MCIOB. Property purchase advice, Structural Surveys, building defects diagnosis and solutions, throughout the Valencia region. Free 17 page property buyer’s guide available via www.surveysspain.com  T: 962807247 M: 653733066

Free initial advice is available by e-mailing :- This email address is being protected from spambots. You need JavaScript enabled to view it.

Copyright Mark Paddon 2008

Additional information