Roof Flashcards
Roof purpose
The most important function of a roof is to protect the house
from RAIN, WIND, DUST, HEAT & SNOW.
The more extreme the external conditions are, the steeper the
pitch of the roof becomes
WEATHER EXCLUSION:
a) Rain
Where there is a high rain intensity level, a pitch of a roof
may have to be increased and a thicker waterproof
membrane added for additional resistance to water
penetration. (Some overlapping tiles are more effective that
others such as butt
-joined tiles).
b) Wind and Dust
The effect of wind
-pressure on a pitched roof depends on the
angle of the pitch (see examples). For a ridge type roof of
between 20 and 30 degrees the suction will be more or less
even on both slopes.
On steeper pitches: the suction occurs on the side away from
the wind.
On shallow pitches: the suction occurs on the windward side.
THERMAL INSULATION:
A roof with no insulation will readily allow the
transfer of HEAT. In winter, heat will be lost
through the roof and in summer, heat will be
transmitted through the roof into the room.
Thermal insulation can be provided in three
ways:
- A layer of insulation material external to the
pitched roof structure, but beneath the tiles.
Sisalation (very thin!) or Alucushion on top of
rafters. - An insulation material beneath the main structure
(above ceiling): fibre
-glass slabs. - Flat roof: built up roofing would include insulation
material
CONDENSATION
Condensation causes the timber to get mould
and fungi: this can be prevented through
insulation and through ventilation. Airbricks
are built into the wall inside the roof structure
to ensure airflow.
COST
The costs for the roofing products will always
play an important part in the roof design, not
only for the construction but also for the
maintenance.
The Roofing Industry offers a wide range of
“roof packages” which are designed to work
together, e.g. which decking goes with which
pitch
-degree.
The cost of timber is very high and to choose
the right size of timber for the construction is
best left to the Roof Specialists as well as to
choose the best way of construction.
IMPLICATIONS ON SUPERSTRUCTURE
The National Building Regulations give “deemed
to satisfy rules” in Part L, which cover
:
1. Fire resistance
2. Construction and fixing of structural
components
3. Maximum spans for trusses and rafters
4. Connecting devices
5. Waterproofing
6. Minimum roof slopes and sheet and laps
7. Flashing
8. Flat roofs
APPEARANCE
The factor affecting the appearance is mainly the Architectural style of house, e.g. Spanish or Cape Dutch. The style of the roof is normally chosen by \: a) The pitch b) The roofing materials: Colouring and texture, e.g. tiles, thatch, sheeting c) The use of the roof area (roof garden, traffic, fire escape route for flat roofs)
Ceilings SANS
The minimum height for a ceiling by SABS
standards is 2400mm AFFL, but 2600mm to
2700mm is the more commonly used height. The
height of the room increases with the size of the
room.
Design Criteria:
Aesthetics Access Weight Acoustic properties Fire properties Cost
Trimming
This is a term to describe the framing of joists
around an opening or projection. Various joists
can be used to connect the members together.
(stair openings, chimney, flues, etc.)
Common joist:
A joist spanning from support to
support
Trimming joist:
Span as for common joist but is
usually 25mm thicker and supports trimmer
joists at openings
Trimmer joist:
a joist at a right angle (90
degrees) to the main span supporting the
trimmed joists and is usually 25mm thicker than
a common joist.
Trimmed joist:
a joist cut short from an opening
and it is supported by a trimmer joist. It spans in
the same direction as common joists and is of
the same section size.
Trimmed joist:
a joist cut short from an opening
and it is supported by a trimmer joist. It spans in
the same direction as common joists and is of
the same section size.
