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A Roof over Your Head

Your roof is the crowning glory over your house, yet an area of building few people know much about. Over the centuries they have adapted to cater for changes in the climate and the addition of numerous high tech appliances in the home as well as modern advances in construction materials. George McGarvie a Civil Engineer from the UK gives us some inside information into roofing solutions in Bulgaria.

Types of Roof

Roofs in general are categorised as being either pitched or flat and a pitched roof can be constructed in one of three ways: as a trussed roof, which is constructed from pre-made sections called trusses, which are placed on top of the load bearing walls or pre made supports; as a traditional roof made of timber sections, which are built together in-situ; by combining both methods. Whichever method of construction you decide upon, it should be designed to give the interior of the property, the best protection possible against the extreme changes that Bulgarian weather has to offer.

Roof Design

Roof design is quite complex and involves many calculations regarding the strength of the materials used to create a sound, durable roof. It must withstand very high wind and drifts of snow loading and is designed to carry the covering used in its construction e.g. roof tiles. Consequently, it is not safe to alter the composition of any roof without consulting relevant specialists.

Rebuilding or Refurbishing your Roof

Trussed roofs are rare in most areas in Bulgaria with traditional roofing being the preferred method of construction in older properties. The reason for this choice comes down to the age of most houses here and the materials that were available to the builders at the time. The differences in construction in Western European countries and Bulgaria are small but there is more technical input in Western European roof construction, whereas Bulgarian builders tend to over-engineer and verge on the side of caution. Both are equally acceptable although don't expect to see a regime of safe working practices with Bulgarian builders, I can't remember the last time I saw anyone wearing a hard hat let alone using scaffolding!

The best way to ensure you get a good construction is to use a builder that has been recommended to you and inspect some of his previous work. Old advice I know but it works. I've found that local 'one man band' builders are generally better than companies are, and cheaper, but pay for materials only and around a 5 to 10% retainer up front with the final balance to be paid upon completion to your satisfaction.

Checking the Pitch

In Bulgaria the pitch or slope of your roof is very important because of the snow fall during the winter months. If the pitch is too shallow you will find that during the thaw, water will back up and enter the roof space through the tiles. Therefore it is important to check your pitch is steep enough to cope. There are no hard and fast rules serving pitching angles as they vary with climate and design but I would never accept anything less than a 33 degree or 1:3 pitch. A Bulgarian architect told me that the minimum in Bulgaria is 22 degrees; a 45 degree or 1:2 would be my choice if possible. Additionally, make sure that the roof tiles are designed for the pitch you choose. The shallower the pitch, the more overlap needed in the tiles.

Using Under Felt

Under felt or membrane is an important part of your pitched roof. Yet many people think that roofing felt is merely a secondary waterproof layer – this is a myth and in fact ventilation holes are deliberately left in the felt in some roof constructions. Under felt is placed underneath the battens (the wooden straps across the joists for tile placing. There are various makes of felt but each one serves as both a vapour and a dust barrier stopping warm air inside the roof space from hitting the cold underside of the tiles where it may condense. Water, condensing on tiles, is the single biggest cause of rot in roof timbers. Most under felts will channel water that leaks through the tiles away from the loft area to the eves and help keep the area dry but this is not their primary function and they should not be relied upon as a method of waterproofing. They can make it very difficult to detect the position of a leak as the water can be channeled some distance away from the source.

Ensuring Adequate Ventilation

The biggest single difference I have encountered between roof construction in the East and West is that they don't ventilate the roof space. My guess is that the simpler way of living and the lack of many modern appliances that create vapour like power showers, air con, baths, washing machines, gas heaters and of course washing hung over radiators, has meant that generally, ventilation was not required in the way it is today. Ventilation with today’s wealth of modern appliances is paramount - if air remains still for any period of time it warms up and collects water vapour. The warmer the roof space the more humid the air. When that warm air hits any colder surface such as the underside of the tiles or any metal pipes in the loft, it condenses and the vapour turns to water, which soaks into the roof timbers causing much damage.

Today, there are far higher volumes of water vapour is generated thanks to modern appliances, which were and still are absent from many Bulgarian lives so much so that ventilation is essential now and something you will need to rectify if you are moving into an old property. It is better to use smaller vents spread around the eves and ridge than having one big ugly vent at the top and bottom. It is also more efficient if they are evenly spread throughout; more ventilation is better so to achieve a balanced look put more in if you want to. Bear in mind that if you use a mesh covering over your vents, this can decrease their performance by about 40%, so increase the size of your vent to correct this depending on the vent covers you intend to use. To ensure good ventilation you should also limit the insulation in the loft area to stop short of the eaves to allow for good air flow. Roof construction, in cold roof scenarios, allows cold air to pass through the eaves into the loft and out through the ridge. This should keep the loft at a constant temperature thus avoiding condensation.

A Case Study

Vic is thinking of constructing a new roof 7.5 m x 7.5 m and was looking for advice before committing to the project. The ratio I would recommend for working out how much ventilation you will need is 1:300. Some standards are more and some are less but I strongly urge you to use 1:300 or more. On a roof Vic’s size the flat area is 56.25 sq m. divide this number by 300 to get 0.1875 sq m. To convert this to sq cm, multiply it by 100 x 100 giving 1875 sq cm of venting required. This should be split between the eves (soffits or where the roof sits on the wall) and the ridge (rounded tile at the top of the roof) 50:50 so 937.5 sq cm ridge vent and 937.5 sq cm eves vent. 40:60 is also fine with the 40% at the ridge.

Remember, virtually all wood infesting insects only thrive in warm, damp conditions so keep your loft vented and dry!
Photographs courtesy Craig Thomas