power point flat roofs l1

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City & Guilds Construction

PowerPoint presentationPresentation 4: Flat roofs

Unit 101: Principles of building construction,

information and communication



Aim

Introduce 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



Task

Sketch and label a flat roof.



Fall on a flat roof

The 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 joists

In 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 pieces

In 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 joists

All 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 covering

When 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?

power point flat roofs l1

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