Roofs are categorized as being either pitched or flat. A pitched roof can be constructed in three ways. Trussed roof: Pre-made sections called trusses are placed on top of the load bearing walls or supports. Traditional Roof: Sections of the chosen material, usually timber in domestic construction, are built together in-situ. Or by combining both methods.
The word truss means tied together and roof trusses are sections (again, usually of timber) fixed solidly together to form the angled shape required for the pitch of the roof. Most pitched roofs have an equal pitch (symmetrical pitch) on both sides of the ridge but there are many variations on this theme. A mono-pitch may just have one sloping side coming down from a wall, an inverted pitch or Butterfly roof has two sides sloping inwards to a valley at the bottom of the pitches, an asymmetrical pitch with one side of the roof slope at a different angle to the other. An asymmetrical butterfly, or a lean to roof. A lean-to roof is the most commonly constructed by our many thousands of customers on the web site and even though it is a much smaller project than a huge, main roof, the principles are exactly the same. The timbers must be the right size to support the structure. The tiles must be put on in the correct way and the top of the roof must be sealed against water penetration.
Whatever the roof, it is generally designed to give you, and the inside of the property, the best protection possible from the weather. Roof design is quite a complex field and involves many calculations regarding the strength of the materials used. A roof has to withstand very high wind speeds and snow loading and each roof is designed to carry the covering, eg tiles, that is put on it. A conservatory roof designed for clear plastic (Poly carbonate) roofing sheets would not be able to carry the weight of concrete or clay roof tiles. It is important for the DIY'er to realise that a roof is constructed the way it is for many reasons and it is not safe in any way to alter that composition with consulting an architect.
We have had so many instances of people wanting to convert their loft into a bedroom or living room, or even just put in a loft window. Timbers have just been removed, the roof weakened and the roof has started to sag. Even experienced roofing carpenters work on the principle that for every roof timber that is removed, at least two have to be put back. The skill of the tradesman is knowing where to put them.
We will go through each roof part on a pitched roof in later paragraphs but as an overview it is useful to know that roofing in the UK is generally covered with small sections like tiles and slates because they are easier to get up onto the roof, safer to handle when you are up there and finally, are small enough to allow for contraction and expansion in the dramatic temperature changes we get in this Country. These parts are laid, much the same as bricks, in an overlapping way but not for strength as with the bricks, but so two joints do not fall on top of each other to allow water penetration. The covering is usually fixed onto battens which are spaced out up the roof . Each batten is nailed to every rafter it passes over.
Underneath the battens is a roofing felt. There are various makes of felt but each one serves as both a vapour and a dust barrier. That is, it stops warm air from inside the roof space hitting the cold underside of the tiles where it may have condensed. Water, condensing on tiles, is the single most reason for rot in roof timbers. It also stops dust and road fumes etc entering the roof space.
Many people (unfortunately some Cowboy Builders included) think (and tell customers) that roofing felt is a secondary waterproof layer for the roof. It is not and in fact ventilation holes are deliberately left in the felt in some roof constructions.
Bearing in mind the felt under the tiles, and the pitch of most roofs, it is almost impossible to tell where, when a roof is leaking, where it is leaking from.
The water can get through a broken tile or slate and run down the felt until it collects in a sagging bit of felt, or just drips through an unnoticed puncture in the membrane. Water can be getting in because of a broken ridge tile but not be evident until it is seen running down the far wall in the bedroom. This makes leak diagnosis on a roof an expensive pastime and results in many people trying to find the leak themselves.
Never attempt to work on a roof without a scaffold. Tiles are constantly under the hammer from our weather and as such can be very slippery even on the driest of days. We have witnessed many falls from height in 35 years of building and we have not found a single human being yet that bounces. If a professional wants to wander about on your roof without a scaffold, and he is insured against the damage he can cause to your roof on his way down, its his problem, but do not try it yourself.
Most ordinary house roofs in this country are formed by roof trusses. These trusses are designed for each particular type of dwelling and as many of our houses are built to the same style, so there is one very popular truss type. This is the Fink Truss. The fink truss is a duo pitch truss, that is it has two sloping sides meeting in the middle. Roof trusses are placed on top of the load-bearing external walls of a building. They are placed at regular, equal intervals to suit the type of load they are to carry. The heavier the load, the narrower the spacing or the larger the timbers used to make the truss. A normal spacing for a roof truss in a domestic situation is 600mm.
Roof trusses remain upright because they are tied together by binding timbers which are fixed to the underside of each truss. The end truss or couple of trusses is fixed to the inside skin of the gable end (see roof diagram above) wall to make sure that the trusses do not achieve the "domino" effect. When a roof is battened for tiling this also helps the tying together.
The bottom, horizontal timber of a roof truss is also a ceiling joist. As far as its load bearing capacity is concerned it is only designed to hold up the ceiling of the room below and perhaps a few empty suitcases in the attic. It is not designed to be walked, slept or danced on and neither is it designed to carry the entire contents of the last five offices you worked at. Our later section on loft conversions covers what you may and may not do in the loft.
Water tanks placed in the loft are placed on strengthened platforms which spread the weight over a number of trusses.