Pitched Roof Construction and Types of Roof Tile

Roofs are categorized as being either pitched or flat. A pitched roof can be constructed in two 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 without 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, 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.

Roof Trusses

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.

Insulation

A roof space is not designed to be hot (unless of course it has been converted). The heat is meant to stay in the building and now, with the latest amendments to Part L of the Building Regulations (The conservation of fuel and power) it is expected that (when all condensation and boarding out problems are considered) your existing loft insulation is topped up to at least 200mm.

This is to be placed between, and over the ceiling joists. As mentioned in our sequence of events section, it is important not to cover cables and light fittings with this insulation. The insulation will stop heat from the building rising through it.

Insulating a roof in this way means that the loft space itself is always quite cold. It is therefore necessary to insulate water tanks and pipes as in this “cold roof” design it can often be as cold in the roof space as it is outside. This is why many burst pipe situations (See repairing a burst pipe project P13) originate in the loft.

When a roof is designed, as most roofs are, as a cold roof, it is important that the roof is adequately ventilated. If air remains still for any period of time it warms up and in that warming it collects water vapour. The warmer the roof space the more humid (containing water vapour) the air.

When that warm air hits any colder surface such as the underside of the tiles or even the water tanks in the loft, It condenses. This means the vapour turns to water. The water soaks into timbers in the roof and can cause much damage.

The insulation in the loft should therefore be stopped short of the edge, or eaves, of the loft floor as can be seen in the diagram. Roof construction, in cold roof scenarios, allows cold air to pass through the eaves into the loft.

This should keep the loft at a constant temperature thus avoiding condensation. The air is admitted through air vents known as soffit vents which are placed in the soffit board between the facia board and the external wall of the house.

Pitched Roof Covering

As already stated, pitched roofs are usually covered with tiles or slates which fix, or clip over, battens. These battens sit on a roofing membrane and are fixed to the rafters below. The battens are fixed at regular intervals according to the gauge (distance between battens) specified by the tile manufacturer. This in turn will vary according to the angle, or pitch, of the roof.

Each tile must overlap the tile below it and this is the critical factor in working out how to tile even the porch roof we mentioned above. The table below shows the lap and spacing for a variety of common tiles.

If you are unsure which tiles you have, simply zoom in on them with a digital camera and the local Builders Merchants will be able to identify them for you. Before reading the table below there are things you need to know.

The first column in the table, Tile Name, may sound strange but every tile has a name to distinguish it from the others. The two main players in the roof tile market are Marley and Redland.

Marley started making roof tiles in 1924 and Redland in 1919, both are still going strong with a huge range of tiles and slates. Different types of tile vary hugely in size with the small clay, or concrete “Plain tiles” at only 265 x 165mm compared to the largest of roofing slates at a giant 600 x 300mm.

Both Marley and Redland manufacture similar tiles but they do not quite interlock with each other. It becomes very important then to identify your tiles correctly.

The next column is the size of the particular tile. Tiles are always longer than they are wide.

Next is the minimum pitch. The angle a roof sits at is called the pitch and this pitch angle is measured from the horizontal. A flat roof therefore would be 0 degrees. As you can see from the diagrams, one tile overlaps the one below it and if the pitch is too shallow for a given the, the wind and rain can drive up under the tile. Each make of tile has a minimum pitch onto which it can be used safely. Sometimes by increasing the overlap of the tiles (Headlap) the pitch can be reduced.

The maximum pitch speaks for itself but is included because roof tiles are not meant to “hang” on their battens. The volume of weight must press down onto the roof surface so ordinary roof tiles should not be put on a roof that is too steep.

The minimum headlap is the smallest amount one tile can overlap the one below it. Sometimes this figure has a tolerance and, together with the gauge (distance between the tops of the battens) the roof tiles can be adjusted so the top course of tiles finishes right up at the peak, or Ridge, of the roof.

Redland

Interlocking Slate Tiles

Concrete Plain Tiles

Clay Plain Tiles

Sandtoft

Concrete Tiles

Clay Tiles

Slate Tiles

Marley

Plain Tiles

Interlocking Slate Tiles

Interlocking Tiles

Forticrete (Anchor) Tiles