Extension stage 9: Roofing
Roofing is the most complicated part of a build, it must join the existing building perfectly, be structurally sound, in-keeping with the rest of the property and be very durable.
It should also be remembered that it is too dangerous to construct a roof in bad weather so to minimize the risk of delays, most people have this done over the summer months.
Roofing can be split into two categories, flat roofs and pitched roofs, planning permission and purpose will dictate which one is most suited to each build.
These are built by laying a deck of wooden joists which are supported by the load bearing walls of the new extension and to hangers on a wall plate attached to the existing property.
Very rarely are roofs totally flat as they have to allow water to run off into guttering systems, the term applies to any roof with an incline of less than 10 degrees. They are normally the cheapest option and if lucky at the planning permission stage, it may even be possible to turn the roof into a terrace with
access from the upstairs rooms.
On the downside is their reputation for having a very short lifespan with some lasting less than 10 years before repairs are needed, flat roofs are also only suitable for single story extensions. See below for the various options:
- The most common and cheapest roofing is roofing felt layers bonded with bitumen, this material has a short life span as the constant expanding in hot weather and contracting in cold weather will eventually cause cracks. To counter this it is possible to line the roof with a reflective paint to decrease temperature change, but the effectiveness of this varies.
- Another option is glass re-enforced polyester which is more resistant to warping or changing size depending on heat.
- The least popular soft roof option is Metal sheet coverings such as lead, zinc and copper mainly due their higher cost. However it is known for a lead roof to last over 100 years, so it may be a long term saving despite the initial outset. To prevent expansion causing splits or buckling, expansion joints are left at regular intervals.
- If it is to be walked on regularly like a roof terrace, a hard covering like asphalt or specialist slabs will be required.
To ensure flat roofs comply with Building regulations they must be insulated. The easiest and most common method is to lay vapor barrier sheets on top of the deck and then thick foam insulation boarding is laid on top of that, this is then finished off with layers of weatherproof felt.
These are more expensive and complicated than flat roofs but last longer, look better and are more popular with planning permission. Read below to see the different options:
- Since the 1970s pitched roofs have usually been built using pre-manufactured trussed rafters which are simply lifted into position, attached to the load bearing walls and kept straight using a central beam running at the top of the triangular trussed rafters. The downside of these pre-manufactured rafters is that the high cost is only viable because of the decreased labor cost of installing them compared to other techniques. However, they are only easy and quick to install if the extension requires a standard size roof. If not a carpenter will have to spend a lot of time modifying them in a way to make them fit without damaging their structural integrity, therefore making them uneconomical. Other drawbacks include that a crane is required to lift them in place and that the loft space created cannot easily be converted to living space.
- Extensions usually require a relatively small roof compared to the home builds which pre-manufactured trussed rafters were designed for, therefore it may be cheaper to have a carpenter custom build the roof from scratch by cutting the timber to size on site. This type of roof construction is called a cut roof.
- Lean-to roofs are especially popular for small ground floor extensions because of their simplicity. Lean-to rafters are secured to the existing property, and the beams slope downwards onto the load bearing wall parallel to the house. This is sometimes called a mono pitch roof or a half Gable roof. for the walls that meet the existing property at right angles, brick walls are built upwards to meet up with the rafters.
- Pyramid shaped roofs were very popular in the 1930s They are also known as hipped roofs and are very complicated and expensive to construct compared to a lean-to or pitched roof. They may be useful for gaining a south facing Velux window where the garden is South facing.
There are two ways to ensure a pitched roof meets Building regulations on insulation. The first is called a cold roof because it leaves the loft space cold by placing the insulation on top and around the ceiling rafters, this makes the space unsuitable for living because it will be very cold. The second option is called a warm roof because the loft space is warm due to the insulation being achieved through placing foam insulation boards between the rafters, this is more expensive but allows the space to be converted for living space.
There are a lot of different options for Pitched roof coverings, for a full listing of the types, see below
- Plain tiles – these are clay or concrete rectangular and curved slightly to assist water run off. They are attached using nails ever 4 or 5 rows.
- Pantiles – these large S shaped tiles are popular in the East of England and are hung by their nibs and overlap at the sides.
- Interlocking tiles – these cost effective and easy to lay concrete tiles as the are single lap (they lock in place with eachother).
- Natural slate – this light and durable covering is nailed twice per slate and can last over a hundred years as reflected by the higher cost compared to other tiles.
- Artificial slate – made from composite fibre, cement, slate dust and glass fibre resin, they are cheap and can look authentic.
- Stone slates – this is the most expensive roof covering as it is heavy slabs of irregular sized stone and is commonly used in the Pennines.
Whatever type of roof is chosen, it is important to consider drainage and guttering as gutter pipes may be obstructed by windows, doors, vents and flues.