FPASA BULLETIN FD 12
FIRE RESISTANCE RATINGS OF LIGHTWEIGHT
WALL CONSTRUCTIONS
Increased use is being made of lightweight or dry-wall methods of construction for the internal
walls in multi-storey buildings.
There are both advantages and disadvantages inherent in the use of lightweight wall
constructions when compared to the use of conventional plastered brick walls. The greater
mass and nature of brickwork and most concrete blockwork ensures that, for internal walls, a
far greater resistance to fire, impact loads and noise transmission is achieved.
The quality of site workmanship seldom affects these properties in the case of plastered
brickwork or blockwork. However, workmanship is of the utmost importance insofar as the fire
resistance of lightweight walls is concerned and the fire-resistance time given in the table will
be achieved only if the workmanship is of a high standard. The details at cross-wall junctions
need to be designed and then executed on site in a manner that will ensure that the fire
resistance at these positions is the same as that of the wall.
The values in the table are intended merely to indicate to designers what fire resistance ratings
can be expected from different types of lightweight wall constructions. General terms have
been used to describe the types of wall constructions that utilise patented components such as,
for instance, studs, board fixing devices, etc, which have achieved similar fire resistance ratings
in wall furnace tests.
Notes to table
1.
2.
3.
4.
5.
6.
7.
8.
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GP board means gypsum plasterboard.
UA-C sheet means unpressed asbestos-cement sheeting, wet-cured or autoclaved. The
same fire behaviour will not be achieved if pressed sheets are used.
X-rated GP board means a special type of gypsum plasterboard which has a small
percentage of glass fibre incorporated in the gypsum core of the board.
When two layers of cladding material are used on one side of a wall, a joint between the
cladding sheets of the second layer must not be on the same stud as a joint in the first
layer.
When fixing cladding materials to the studs or nogging pieces, it is important that the
distances between fixing screws or nails should not be more than 300 mm both on
gypsum plasterboard and asbestos-cement sheet materials. Also, when two layers of
cladding are used on a side, the first layer must be fixed in the same manner as if only a
single layer were to be used. The screw fixing of the second layer should be staggered
vertically relative to the fixing of the first layer.
By “steel studs” is meant cold-rolled galvanised steel.
Class I impact is achieved by swinging a 30 kg sandbag twice against the face of the wall
from a height of 0,45 m above the point of impact.
When timber studs are used in walls, the walls are considered to be semi-combustible.
Type and thickness of cladding
as well as studs used in the
construction of walls
Stud
spacing
(mm)
1 x 12,7 mm thick GP board on
either side of 65 x 35 mm steel
studs
1 x 12,7 mm thick GP board on
either side of 75 x 38 mm timber
studs. Boards fixed vertically or
horizontally
2 x 9,5 mm thick GP boards on
either side of 65 x 35 mm steel
studs or of 75 x 38 mm timber
studs.
2 x 12,7 mm thick GP boards on
either side of 65 x 35 mm steel
studs or of 75 x 38 mm timber
studs.
1 x 12,7 mm thick GP board on
one side and 2 x 9,5 mm thick GP
boards on other side of 65 x 35
mm steel studs or of 75 x 38 mm
timber studs.
1 x 12,7 mm thick X-rated GP
board on either side of 65 x 35
mm steel studs. Boards fixed
vertically or horizontally.
Composite cladding on walls
1 x 9,5 mm thick GP board and 1 x
9,5 mm UA-C sheet on either side
of 65 x 35 mm steel studs.
Unpressed asbestos-cement
board cladding to walls
1 x 9,5 mm thick U-AC sheet on
either side of 65 x 35 mm steel
studs.
As above but with mineral wool in
the cavity.
1 x 9,5 mm thick UA-C sheet on
either side of 75 x 38 mm timber
studs with cavities filled with
mineral wool.
2 x 9,5 mm thick U-AC sheets on
either side of 65 x 35 mm steel
studs.
1 x 12,7 mm thick U-AC sheet on
either side of 65 x 35 mm steel
studs or 75 x 38 mm timber studs.
As done above but with a nominal
50 mm thick mineral wool
insulation blanket in the cavity.
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Impact
resistance
Class
Remarks on construction
450
Fire
resistance
rating
(min.)
30
<1
600
450
20
30
<1
<1
Board fixed vertically or horizontally
with or without glass wool or mineral
wool in the cavity
This type of construction is considered
to be semi-combustible
600
20
<1
405
60
1
600
30
<1
450
60
1
600
30
1
450
30
1
(or < 1)
450
60
1
450
60
1
450
20
<1
600
20
Cladding sheets fixed to the steel
studs with screws, so that the joints
between the boards on each side of
the wall are staggered.
450
30
<1
As above
600
450
30
30
<1
As above
405
60
1
450
30
<1
Boards can be fixed either vertically or
horizontally or vertically and
horizontally. The joints between the
boards must be staggered.
Cladding sheets fixed to studs with
steel screws at 300 mm centres
600
450
20
30
>1
600
30
First layer of cladding can be vertical
with second layer horizontal or viceversa.
As above. If both layers are fixed
vertically, steel studs placed back-toback and fixed to each other at 300
mm centres must be used.
The double layer of boards must be
used on the face most likely to be
impacted during normal use eg the
side bordering on a passage. The
other side will have an impact of <1.
If boards are fixed horizontally, then
horizontal joints between boards must
be backed by and fixed to nogging
joints.
Cladding materials can be either
horizontal or vertical. Joints between
boards must be staggered.
As above.
References:
Anon, Fire Protection of buildings, FPASA Journal “Fire Protection” Vol. 4, No. 2 June 1977
E G Williams, Fire resistance ratings of some common lightweight wall constructions, FPASA journal
“Fire Protection” Vol. 7 No. 1 March 1980.
Published by
Fire Protection Association of Southern Africa
(Incorporated Association not for Gain)
(Reg. No. 73/00022/08)
P O Box 15467
Impala Park
1472
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