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Good Craftsmanship Guide
Drainage
This Good Craftsmanship Guide
highlights key problems with drainage
and gives guidance on how to avoid
them. The problems are those most
commonly identified during NHBC’s
inspection of homes under construction.
All photographs are of defects and
were taken on real building sites.
The Guide, based on the NHBC
Standards, is intended for use by
NHBC registered builders and
inspection staff.
© NHBC 2004No reproduction without NHBC prior permission in writing.
Introduction
Drainage
Proximity to foundations 2
Gradients 4
Bedding 6
Laying/jointing pipes 7
Access 8
Backfill 13
Support and movement 15
Protection 17
Soakaways 18
Septic tanks 20
Inspection and testing 22
Contents
1
Proximity to foundations
2
Problem to avoid:Undermining foundations
What to do:where possible keep foundation bottoms lower thandrainage trenches
where possible keep drainage runs outside theinfluence of the foundation loading
3
A
more than 1m
B is within A-150mm from bottom of foundation
B
A
less than 1m
if it is necessary for drainage to be located withinthe zone of influence of the foundation loading, laythe drainage first and surround in concrete to asuitable level, before excavating foundations
Note: If A is less than 1m, concrete drainage trench up to at least theunderside of the foundation concrete. If A is more than 1m, concrete drainagetrench up to at least B.
Gradients
4
Problems to avoid:Drains prone to blocking
Drains do not self-cleanse if gradients are not adequate
What to do:install drainage in accordance with the design,normally at the following minimum gradients:
The following gradients may be used where flows exceed 1.0L/second:
Pipe diameter [mm] Minimum gradient
100 1 : 80
150 1 : 150
5
take particular care when laying drainage close tothe minimum gradients
increase gradients where ground movement isexpected (e.g. in clay soils or in made ground)
Surface water:
– 75mm and 100mm drainage should be laid at agradient not less than 1:100
– 150mm drainage should be laid at a gradient notless than 1:150
Bedding
6
Problem to avoid:Drainage settling due to inadequate bedding
What to do:bed pipes:
– on granular material, minimum 100mm deep
– directly on trench bottom but only where thebottom can be accurately hand-trimmed but is firm
remove any temporary packing from under pipes to avoid hard spots
provide adequate support for manholes andinspection chambers in accordance withmanufacturers’ instructions
soft spots to be removed and replaced with suitablematerial
pipe supported on hand-trimmed trench bottom
pipe supported on bed of granular material
Laying/jointing pipes
7
Problem to avoid:Dislodged sealing rings
What to do:ensure pipes are cut squarely
chamfer ends of cut pipe lengths
provide lubricant to ease connection and provideseal without dislodging seal
Access
8
Problem to avoid:Inadequate access for clearing blocked drains
What to do:provide access in accordance with the design
ensure all pipes can be rodded
use correct sized access manholes, inspectionchambers and access chambers
9
continued over
Notes: 1 The clear opening may be reduced by 20mm in order to provideproper support for the cover and frame. 2 Drains up to 150mm. 3 A larger clearopening cover may be used in conjunction with a restricted access. The size isrestricted for health and safety reasons to deter entry.
Min
imum
dim
ensi
ons
for
acce
ss fi
ttin
gs a
nd c
ham
bers Inte
rnal
siz
esCo
ver
size
s
Type
Dept
h to
inve
rtLe
ngth
x W
idth
Circ
ular
Leng
th x
Wid
thCi
rcul
arfr
om c
over
leve
l (m
)(m
m x
mm
)(m
m)
(mm
x m
m)
(mm
)
Rodd
ing
eye
As d
rain
but
min
100
Sam
e si
ze a
spi
pew
ork
1
Acce
ss fi
ttin
gsm
all
150
diam
0.6
or le
ss,
150
x 10
0ex
cept
whe
re15
0 x
100
150
150
x 10
01
Sam
e si
ze a
sla
rge
22
5 x
100
situ
ated
in a
225
x 10
022
522
5 x
100
1ac
cess
fitt
ing
cham
ber
Insp
ectio
n ch
ambe
rsh
allo
w0.
6 or
less
225
x 10
019
02
—19
01
1.2 o
r le
ss45
0 x
450
450
Min
430
x 4
3043
0de
epgr
eate
r th
an 1.
245
0 x
450
450
Max
300
x 3
003
Acce
ss r
estr
icte
dto
max
350
3
Access
10
Minim
um dim
ensions for manholes
TypeSize of largest
Min internal
Min clear
pipe (DN) (mm
)dim
ensions1
opening size1
RectangularCircular
Rectangular lengthCircular
length and width
diameter
and width
diameter
(mm
)(m
m)
(mm
)(m
m)
Manhole
less than 1.5mequal to or
deep to soffitless than 150
750 x 6757
10007
750 x 6752
na3
2251200 x 675
12001200 x 675
2
3001200 x 750
1200greater than 300
1800 x (DN+450)The larger of 1800
or (DN+450)
greater than 1.5mequal to or
deep to soffitless than 225
1200 x 10001200
600 x 600600
3001200 x 1075
1200375 - 450
1350 x 12251200
greater than 4501800 x (DN+775)
The larger of 1800or (DN+775)
Manhole shaft
4
greater than 3.0mSteps
51050 x 800
1050600 x 600
600
deep to soffitLadder
51200 x 800
1200
Winch
6900 x 800
900600 x 600
600
11
Notes: 1 Larger sizes may be required for manholes on bends or where thereare junctions. 2 May be reduced to 600 by 600 where required by highwayloading considerations, subject to a safe system of work being specified. 3 Notapplicable due to working space needed. 4 Minimum height of chamber inshafted manhole 2m from benching to underside of reducing slab. 5 Min clearspace between ladder or steps and the opposite face of the shaft should beapproximately 900mm. 6 Winch only - no steps or ladders, permanent orremovable. 7 The minimum size of any manhole serving a sewer (i.e any drainserving more than one property) should be 1200mm x 675 mm rectangular or1200mm diameter.
Reproduced from Tables 11 and 12 of Approved Document H to the BuildingRegulations by permission of HMSO.
ensure that manholes, inspection chambers, accesschambers and rodding eyes will be accessible, e.g.not under fences
make sure the main channel of manholes andinspection chambers is used. This will help to avoidthe build-up of solids
make connections obliquely with the flow
continued over
proprietarymanhole
Access
12
finish benching neatly with a steel trowel
use the appropriate type of cover, aligned with thesurrounding ground
slow radius bends
13
Backfill
Problem to avoid:Damage to pipes due to unsuitable backfill
What to do:place backfill as soon as the pipes have been bedded,jointed and inspected by NHBC/local authority
carry out final visual inspection before backfilling
use granular material or selected backfill from thetrench excavation free from:
– stones larger than 40mm
– clay lumps larger than 100mm
– timber
– frozen material
– vegetable matter
compact backfill in layers not deeper than 300mm
continued over
Backfill
14
provide protection for pipes under roads and drives
provide protection for shallow pipes under gardenareas
protected ifcover less
than 900mm
protected ifcover lessthan 1.2m
FLEXIBLE PIPES RIGID PIPES
less than600mmcover
75mm
100mm
2 layers handcompacted beforemechanicalcompaction
150mmabove
pipe
avoid mechanical compaction until fill is at least450mm above pipework
15
Support and movement
Problem to avoid:Damage to pipes due to differential movement
What to do:ensure pipework can move where it passes thoughsubstructure by:
– allowing at least 50mm clearance around pipe*
– providing a sleeve, or
– building pipe into wall with flexible joints within150mm of the wall
* a greater allowance may be required, for example,where settlement or clay heave may produce upto 150mm differential movement.
the opening should be protected against vermin entry
continued over
16
provide concrete under rest bend at base of SVP connection
ensure pipes under suspended floors areadequately supported along their length to allow for ground movement in clay subsoils or inmade ground
ensure an adequate void is left under pipeworkbelow suspended floors to allow for clay heave
50mm spacearound pipe
openingmaskedboth sides
PIPES PASSING THROUGH LINTELLED OPENING
flexiblejoint
flexiblejoint
600mmmax
600mmmax
150mmmax
150mmmax
PIPES BEDDED IN WALLS
Support and movement
Protection
17
Problem to avoid:Damage to access fittings and pipes
What to do:provide temporary protection to access fittings
clearly identify access points so plant movementcan avoid damaging them
Soakaways
18
Problem to avoid:Ineffective storage and disposal of surface water
What to do:lower than the building
locate soakaways at least 5m away from building.
carry out a percolation test unless the ground isfree draining and granular. If in doubt carry out a test
fill with clean hardcore 10mm - 150mm in size
provide PVC sheet or concrete blinding over the fillto prevent topsoil being washed into the soakaway
19
PVC sheet orconcrete blinding
diameter D
effective depth D
Septic tanks
20
Problems to avoid:Ineffective operation
Ineffective outfall
21
What to do:check approval from:
– Environment Agency (England and Wales)
– Scottish Environment Protection Agency (Sc)
– Environment and Heritage Service (NI)
carry out percolation test to ensure satisfactoryoutfall disposal
site at least 7m from a dwelling. In Scotland theyshould be at least 5m from a dwelling and aboundary. The tank should be within 30m of avehicle access for emptying
ensure that effluent is drained away from the building
Inspection and testing
22
Problem to avoid:Ineffective operation of drainage
What to do:check that pipe runs are straight, with adequategradients
check all debris is removed from pipe runs and chambers
check drainage system is in full working order at completion
carry out air/water testing
23
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24
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25
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Notes
NHBC, Buildmark House, Chiltern Avenue,Amersham, Bucks HP6 5APTel: 0870 241 4302 Fax: 01494 735201 www.nhbc.co.uk
HB1961 03/04
