101 Level 1 Principles of building construction, information and communication

© 2013 City and Guilds of London Institute. All rights reserved. 1 of 15

City & Guilds Construction

PowerPoint presentation

Presentation 4: Flat roofs

Unit 101: Principles of building construction, information and communication



AimIntroduce learners to flat roofs.

Objectives

• Identify at least two components of a flat roof.

• List the two methods of obtaining fall on a flat roof.

• State the level of degree that determines a flat roof.

• State two of the materials used in covering on a flat roof.



What is a flat roof?A flat roof is one of the three types of roof structure. It is generally used on garages or small extensions, rather than a main house roof.

A flat roof has on average a life span of approximately 15 years.

The definition of a flat roof is any roof which has a slope of less than 10 degrees or a fall ratio of 1:6.



Components of a flat roof



TaskSketch and label a flat roof.



Fall on a flat roofThe direction of the slope of the roof is called the fall.

The fall of the roof will depend upon the:

• type of roof construction and its covering

• location of the building

• position of any surface water drain.



Fall on a flat roof continued

The amount of fall should be enough to clear away any water and direct it to the guttering.

If the fall is not enough, the water may form pools on top of the roof.

Water pools on the roof will increase the weight of the roof and can lead to water penetration.

There are three basic methods of obtaining the fall of a flat roof. The chosen method will depend upon the type and use of the building and the internal ceiling finish required.



Sloping roof joistsIn this type of construction, the slope or fall is obtained by sloping the roof joists. This type of construction is used where there is no level ceiling required.



Tapered firring piecesIn this type of construction, the slope or fall is obtained by attaching tapered pieces of timber to the roof joists. This type of construction will produce a level ceiling.



Deepened joists (parallel fittings)This type of construction involves parallel firring pieces. The firrings are diminished section and are nailed at right angles to the fall of the roof.



Roof joistsAll the roof joists used in the construction of flat roofs must comply with the requirements of current building regulations.

These regulations govern:

• the sectional sizes and grades of timber

• dead loads imposed upon sectional areas of roof

• maximum clear span for imposed loadings

• spacing of joists in relation to covering and imposed loads.



Flat roof coveringWhen covering a flat roof, a sequence of events must take place:

• When the joists and firrings are installed, the chipboard sheets can be cut and secured to the joists using annular ring nails.

• Once the boards are laid, the roof must be inspected for any sharp edges/parts and swept.

• Hot bitumen is then poured onto the roof and the underfelt is laid in the same direction as the fall of the roof.

Image courtesy of www.shutterstock.com. Reproduced with permission.



Using underfelt Once the fist layer of underfelt has cooled, the next layer of underfelt can be laid using the same process as the first layer, but in the opposite direction to the fall of the roof.

This ensures crossed joints and more strength.

Image courtesy of www.constructionphotography.com. Reproduced with permission.

Flashing providing waterproofing



Using underfelt continued

When two layers of underfelt are laid, the top layer, which is green mineral felt can be laid, using the same processes again but in the same direction to the fall on the roof.

The eaves of the felt are tucked and doubled to create a drip which directs water into the gutter. The other edges are raised preventing water from leaving the roof.

Flashing around a chimney

Image courtesy of www.shutterstock.com. Reproduced with permission.



Any questions?

Education

101 Level 1 Principles of building construction, information and communication