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Home  Home Maintenance House and Home Flat Roof Defects
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Written by MARK PADDON
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Flat roofs are common in Spain and often form useful terraces above the living space. In higher density developments nearly all homes come with a ‘solarium’. There are many types of build method and covering, but this article will focus on the most common type and it’s associated problems.
Build method:-
The typical traditional flat roof in Spain is comprised of steel reinforced concrete (‘SRC’) beams with hollow concrete or ceramic block infill. The upper surface is normally levelled with a thin concrete or sand/cement screed over which an asphalt waterproof membrane is laid. This is then normally protected with another layer of screed over which, brick shaped ‘baldoso’ tiles are laid.
Common problems:
Initially this type of roof is normally effective at keeping the rain out for many years, but after 10 years (sometimes earlier) a number of problems can begin to arise. While thermal movement is one of the most common causes, the earlier use of less advanced materials helps to accelerate the failure process. The sequence of failure is normally as follows:-
1. Thermal movement. Extremes between winter and summer, day and night, create expansion and contraction in the concrete beams and slab, (steel beams will be even more prone to movement). This causes regular stresses on asphalt membrane and tile joints above.
2. Surface cracking. The tile joints and even the tiles themselves will quickly give in to the movement (sometimes within the first season, or following an exceptionally hot summer or very cold winter) and hairline cracks will appear.
3. Membrane cracking. Initially the asphalt membrane will be more flexible and hence, more tolerant to some minor movement, but over the years it becomes brittle as compounds evaporate out from the material. Regular movement will cause fatigue along the movement lines and eventually break though the asphalt.
4. Surface decay. While the membrane tires over the years, foot traffic and weathering will also break down the water resistant surfacing of the tiles and the cracked joints will allow for passage of water directly to the membrane. Point loads such as water tanks can also cause problems.
5. Water ingress. Once water gets to the failing membrane, it will soon track through to the living space. Ceilings will get stained or later risk collapse, the SRC beams will start to decay and rainy days may call for a few strategically placed buckets. Poor laying or bowing of the roof can also create pooling areas. Which will normally cause more extensive ingress.
Movement and membrane failure will also commonly occur at parapet wall junctions, chimney stack junctions and level changes. The roof is also prone to heave above load bearing walls, which can also result in membrane failure. Despite the strong nature of SRC beam roofs, they will actually flex slightly under loading, and hence years of BBQ’s and sunbathing on the terrace will also aggravate the surface and membrane. While leak areas are commonly flashed with bitumen backed foil, this too will fail over time and often only serves as a temporary and often unsightly solution. Sika do however now make a tile red coloured version ‘Sika Sealcolor, rojo teja’.
Solutions
In the short term, a secondary membrane can be added over the existing tiles in the form of a roof paint, which is best reinforced with a polyester mesh. Commonly ‘weber.tec hydrostop’ or ‘Sikafill’ paints are used for recovering, however there is a better material available ‘Sikafloor 400N Elastic’ which is designed to seal the roof terrace, while being more resilient to foot traffic. Commonly people choose darker red colours, however lighter colours will serve to reflect heat from the roof slab and reduce future movement. The 400N product is specially designs to resist and reflect UV rays and only comes in lighter colours. It is important to realise that painting over the existing surface will only last for a few years (or less) as thermal movement will still occur in the slab. Clear resurfacing treatments area available for the tiles, however they are generally not thick enough to tolerate movement and hence will crack through (normally at the tile joints, within the first season). If your roof surface is not leaking yet, you can add life and waterproof properties back into the tiles by painting with a product such as ‘Monestir Mitrol’.
A long term solution would require re-asphalting e.g. with –‘Sika Dan Elastomero’ preferably with insulation and screed over and finished with relatively light coloured ceramic tiles, set on flexible waterproof grout – ‘Sika Bond T-8’. To save the removal of tiles, the new membrane can be laid over the existing roof surface, over which an insulative layer can be laid (e.g. polyurethane foam or ‘Arlita’ aggregate). Arlita can be loose laid and protected with a polythene layer then finished with a reinforced screed, or wet laid (mixed with sand/cement and water), to form a lightweight slab (e.g. of 5cm depth). Both surfaces can be finished with ceramic tiles (preferably porcelain to resist frost damage) which should essentially be set on a flexible grout (‘Sikabond T8’, ‘Weber.col Superflex’ or ‘Weber.col fluid’), and jointed with ‘Weber.color flex’.
Failure to add upper insulation to the roof structure or failure to use flexible grouts and jointing compounds will result in seasonal thermal movement in the roof slab and hence, new leaks. Despite this fact, many new roofs and terraces in Spain are still being installed without insulation and the use of cheaper inferior grouts and jointing compounds is common. It is worth noting that insulation will also keep the living space warmer in the winter and cooler in the summer as well as preventing ceilings from cracking along beam lines.
It is worth noting that some roofs that appear to be set on SRC or steel beams may be supported with timber beams. Though these will suffer minimal thermal movement, they will be affected by changes in moisture content and will general flex to greater degree under loading. Hence failure of the upper membrane is normally still an issue.
As a rule, flat roofs either do, did, or will one day leak and the addition of a pitched roof can be preferable in the long term. Protection via insulation and the use of specially designed modern materials can however result in a much longer roof life than traditional build methods.
If you have a building structure related question you can e-mail Mark for free initial advice :–
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NB:- Information for advice purposes only. Proper legal and safety procedures should be followed for all property purchases and building works.
Information provided by Mark Paddon BSc Hons Building Surveying. ICIOB. Structural surveys and purchase advice in the Valencia community. www.surveysspain.com T: 962807247 M: 653733066
ã Mark Paddon 2007
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