flatroofdrainagedesign.pdf

1 DESIGN FACTORS

1.1 FALLS AND DRAINAGE

1.2 THERMAL DESIGN

1.3 VAPOUR CONTROL DESIGN GUIDE

1.4 TRAPPED MOISTURE

1.5 MOVEMENT AND MEMBRANES

1.6 WIND

1.7 WIND ATTACHMENT DESIGN GUIDE

1.8 FIRE

1.9 ROOF RENEWAL

THE RUBEROID BLUE BOOK 1

INTRODUCTION

It is generally accepted as good practice for flat roofsto be designed to clear surface water as rapidly aspossible and it would be exceptional nowadays for a

roof to be designed without falls.The ponding of rainwater is often observed on old

flat roofs. As well as being unsightly and increasingthe dead load on the roof, the consequences ofwaterproofing failure are obviously more serious if thearea involved is not properly drained and allows areservoir of water to collect, ready to feed into thebuilding.

Falls may be formed in the structure or can becreated within the specification above the deck. Falls inthe structure can be achieved by adjusting the height ofsupporting beams or purlins, by using taperedsupports, or by the addition of firring pieces before thedeck is laid. The latter method is normally used withdecks such as woodwool, timber, precast concrete andmetal decking. In the case of an in-situ cast concreteslab, falls are normally provided by the use of ascreed.

Preformed tapered insulation boards also provide auseful method of forming falls on a level roof deck,though they may not be suitable if a complex pattern offalls and cross falls is required.

DESIGN OF FALLS

Flat roofs should be constructed to a minimum fall of 1in 80. To achieve this the designer needs to adopt adesign fall which will allow for deflections andinaccuracies in construction.

Some designers arbitrarily double the finished fall andadopt 1 in 40 as the design fall, assuming that this willalways produce a finished fall of at least 1 in 80. Analternative approach is to choose an intermediatefigure of 1 in 60.

On many occasions it is both practical and economicto design falls to 1 in 40, but on some buildings it willprove an unnecessarily severe design criterion. Withscreeded roofs in particular, doubling the screed depthat the highest points merely to allow for inaccuracies inthe construction could cause an unnecessary increasein the thickness and cost of the roof system.

As an alternative, the designer should consider theaccuracy and deflection of the roof in question andmay find a reasonable compromise would be to take 1in 80 as the finished fall, and add an arbitraryadjustment for construction inaccuracies, such as 25mmfor concrete roofs or 15mm for metal decks.

Having chosen a minimum finished fall and anallowance for inaccuracies, consideration should thenbe given to the effects of deck deflection which mayhave a favourable or adverse effect on drainage flow.

Outlets in the central area of the roof may bepositioned at or near the point of maximum deflectionof the deck, and any deflection would therefore assistthe drainage flow. In practice, however, there isusually a need to position internal downpipes againstcolumns or walls for support and protection, and thiswill mean that the outlets will be positioned away fromthe natural low point of roof deflection. Under thesecircumstances, the effect of mid-span deflection will beto reduce the fall to the outlet, and this should be takeninto account when calculating the design fall.

When allowing for these deflections it should notnormally prove necessary to allow for deflections fromimposed loads on the roof. The falls will ensure thereis no significant load from standing water, and it isonly necessary to take account of the dead loaddeflection.


2 THE RUBEROID BLUE BOOK

Design fall 1 in 40

Deflection leaves sufficient fall

25mm tolerance for construction inaccuracies

1 in 80 fall

Mid-span deflection of deck aids drainage

Mid-span deflection of deck restricts drainageDesign fall 1 in 80

Deflection can produce ponding

Assuming that the deck takes a circular shape whendeflecting, a reverse fall will be avoided entirely byraising one end of the deck by four times thedeflection. For example a typical deflection for metaldeck under dead load is span/650 in which case anadditional fall of 4/650 or approximately 1 in 160will compensate for deflection adverse to drainage.Some decks however are so stiff that their deflectiondue to dead load can be ignored.

When the falls are provided by screeding, the deckdeflection will be taken out by the application of thescreed and no allowance need be made for deflection.

Where deflection is favourable to drainage, itshould only be necessary to include an allowance forconstruction inaccuracies. The design fall could bereduced in line with the anticipated deflection but thiswould not be wise unless the designer is confident thatthe dead load deflection can be accurately predictedand the construction can be completed within designtolerances.

CROSS FALLSAt the junction of two roof surfaces with differentdirections of fall, a valley will be formed, known as across fall, and the effective slope of this will be lessthan that for the main falls.

Many designers favour the adoption of 1 in 80 for thecross fall, which on a square roof produces a main fallof 1 in 56. Similarly, if 1 in 40 is adopted for thecross fall, the main fall will be 1 in 28. Theimplications of this approach are a substantiallyincreased volume and cost of screed and an increasedparapet height to accommodate the extra depth ofscreed.

The alternative approach is to accept that a smallreduction in the cross fall will not impair the efficiencyof drainage. Assuming a finished fall of 1 in 80 to themain area, the cross fall will be 1 in 113 which isunlikely to cause any great volume of residual waterafter rainfall.

CONVERSION TABLE FOR FALLSThe fall is most commonly expressed as a ratio, such as1 in 80, or as an angle, although it is sometimesconvenient to describe it in terms of a percentage slopewhere by definition 1 in 100 is 1%. This is convenientfor calculation as it expresses the fall in centimetres permetre run.

The relationship between falls, angles andpercentage slope is indicated in table 1.1.

TABLE 1.1

Fall ratio Slope angle % Slope

1 : 120 0.5° 0.81 : 100 0.6° 1.01 : 80 0.7° 1.31 : 60 1.0° 1.71 : 40 1.4° 2.51 : 38.2 1.5° 2.61 : 28.6 2.0° 3.51 : 19.1 3.0° 5.21 : 14.3 4.0° 7.01 : 11.4 5.0° 8.71 : 9.5 6.0° 10.51 : 8.1 7.0° 12.31 : 7.1 8.0° 14.11 : 6.3 9.0° 15.81 : 5.7 10.0° 17.6



Adjustment of fall for adverse deflection

Adjustment of fall for favourable deflection

Span

D

D

4D

Allowance for adverse deflection eg 1 in 160



1 in 80 fall

1 in 80 fall

Cross fall

Main fall

ROOF DRAINAGE

The design of falls and drainage patterns will have aconsiderable influence on the depth of the total roofconstruction or roof zone, which should be afundamental consideration at the very earliest stages ofconception of a building. It is only after assessing thedepth of roof zone that the designer can decide thelevels of all other aspects of construction above thelevel of the flat roof.

It is a common mistake to underestimate the depth ofthe roof zone, and only too often it is found on site thatskirtings under windows and thresholds are too low andfalls are inadequate. Unfortunately, designers tend tocompromise on these aspects rather than increase theheight of the higher level construction or decrease thesize of windows or doors to ensure that good designprinciples can be adopted for an adjoining flat roof.

Flat roofs may be drained by two basic methods:towards the outer edges and into external gutters, ortowards internal gutters or outlets within the main roofarea. Straight falls to external gutters are simple toform by sloping the roof deck, by screeding or byusing tapered insulation boards. Internal drainage willbe achieved by straight falls to gutters or a pattern offalls and cross falls to outlets.

When the falls are created by a screed, it shouldalways be possible to drain the whole roof efficiently,with falls and cross falls to outlets and without the useof gutters. If the falls are formed in the structure, apattern of falls and cross falls will be difficult to achieveand straight falls to a gutter or to outlets will normallybe incorporated. Falls between outlets can beprovided by the addition of tapered firrings to thepurlins between outlets or by introducing a fall in thepurlins themselves.

Where internal gutters are to be used, they should alsobe laid to falls and this may lead to a considerabledepth of gutter at the low point. Dead level gutters arenot normally recommended as they can hold aconsiderable quantity of standing water. It is better toomit the gutter and accept a construction which has flatsections of roof between outlets. Indeed, one of theadvantages of flat roofs is the opportunity to avoidgutters and include a continuous wall-to-wall waterproofcovering. As a generalisation, a well designed flatroof will contain a good number of outlets and nointernal gutters.

TAPERED INSULATIONMost major insulation suppliers are able to design andsupply suitable tapered insulation systems. Theseprovide both insulation and falls and are of particularimportance for re-roofing existing roofs, many of whichdo not have sufficient falls and probably do not havesufficient insulation.

Tapered insulation can provide falls in one directionto a gutter or level valley. Also falls in two directions toform falls and cross falls, but the intersection should beat 45° to avoid complex geometry.

Skirting heights are often a limiting factor for addedinsulation as it is necessary to leave a skirting height of150mm whilst being sure that the top of the skirtingdoes not rise above damp course level. If it is notpossible to keep below damp course level the schemeshould be abandoned or the entire wall face above theskirting should be fully protected from rainfall. Theprotection can be metal cladding or in the case of lowparapets, the waterproofing can be taken to the top ofthe parapet and tucked in underneath the damp coursebeneath the coping. If this leads to a doubtful detailthe waterproofing can be taken up and over the top ofthe wall.

Another possible design solution is to introduce agutter along the foot of the skirting with little or noadditional insulation in the sole of the gutter. Thisgutter may not be well drained and may suffer pondingbut at least the rest of the roof can be to satisfactoryfalls and cut to falls insulation can displace pondingwater on the majority of the roof area.


Lowest level for dpc, cills and thresholds

Tapered firring along purlins

Outlet position

150mm allowance for skirtings

Screed to falls

Structural deck

Roof

zon

e

Surface finish, eg slabsWaterproofingInsulation

Tapered insulation can be used to produce falls and crossfalls

45°

DRAINAGE CRICKETSDrainage saddles or crickets may be used to improvedrainage between outlets where a roof is installed tostraight falls to an otherwise level valley. Crickets willdisplace standing water and provide a modest fallbetween outlets. They do not form fully efficient fallsand cross falls and cannot be expected to completelyeliminate ponding. In effect, crickets introduce a newvalley with improved falls.

Pre-cut crickets are usually made available as partof a cut to falls service by insulation suppliers. Themain falls are formed with tapered insulation, to falls of1:60 or other suitable design fall. The crickets arewedge shaped to a fairly steep slope, probably 1:40.These are overlaid on the main insulation to a diamondshape on plan.

In practical terms there will be a limit to themaximum width of cricket between outlets, and this willlimit the effective fall which can be achieved.

Table 1.2 gives the maximum width of cricketrequired in a valley, for a range of distances betweenoutlets, the main fall of the roof and the crossfallrequired. In all cases the design fall will only beachieved if the original substrate is level, and theoutlets are not positioned at high spots in theconstruction.

Crickets can also be used to improve local areas ofponding on existing roofs, particularly when re-roofing.In this case it will be necessary to carry out a survey ofthe levels involved, and to design special cricketsaccordingly.

DRAINAGE LAYOUTSInternal rainwater pipes are usually positioned againstthe main columns and the options for positioning outletswill be limited. The outlets should be positioned todivide the roof into convenient drainage areas so faras this is possible.

If the level at the outlets is taken as zero, then thepattern of drainage can be drawn and the levels at thehigh points of the roof calculated.

There are many different approaches to the designof drainage patterns. The four typical drainage layoutsoverleaf show solutions for the design of drainage for arectangular roof with two outlets.

For illustration purposes dimensions are based on afinished fall of 1 in 60 and any allowance forconstruction tolerances and deflections will depend onthe type of specification used.

It can be seen that the effect of the gutter is toincrease the height of the roof zone.



Main fall

Main fall

Distance between outlets

Width

Maximum width of cricket required in valley (m)

TABLE 1.2

Main fall1: 40 60 80

Cross fall1: 100 110 120 130 100 110 120 130 100 110 120 130

Distance 5 2.2 2.0 1.8 1.6 3.8 3.3 2.9 2.6 6.7 5.3 4.5 3.9between 10 4.4 3.9 3.5 3.2 7.5 6.5 5.8 5.2 13.3 10.6 8.9 7.8outlets (m) 15 6.5 5.9 5.3 4.9 11.3 9.8 8.7 7.8 20.0 15.9 13.4 11.7

20 8.7 7.8 7.1 6.5 15.0 13.0 11.5 10.4 26.7 21.2 17.9 15.625 10.9 9.8 8.8 8.1 18.8 16.3 14.4 13.0 33.3 26.5 22.4 19.530 13.1 11.7 10.6 9.7 22.5 19.5 17.3 15.6 40.0 31.8 26.8 23.4

Cross fall


4. Falls formed by tapered insulation, with crickets between outlets3. Straight falls to valley gutter

2. Straight fall to internal gutter. Gutter should also be to falls1. Screeded roof with main falls and cross falls

Typical drainage layouts

5m

6m

12m

6m

12m

+100

0

0

Outlet

Outlet

+100

+100

+100 +100

+100 +100

20m

5m

+83

+83

0Outlet

0Outlet

+83

+183 +183

+183 +183

+183 +183

20m

6m

12m

5m

0

0

0Outlet

0Outlet

0

+100 +100

+100 +100

+100 +100

20m

6m 2.9m

12m

5m

+78

0Outlet

0Outlet

+100 +100+48 +48

+78+48 +48+100

1:12

0

1:12

01:

120

1:12

0

1:1201:120

1:1201:120

+100

+78+48 +48+100 +100

20m

RAINWATER OUTLETS

BS 6367:1983 Code of practice for drainage of roofsand paved areas, and the Plumbing EngineeringServices Design Guide issued by the Institute ofPlumbing give advice on calculation methods employedto design the roof drainage.

It is normal to adopt a rainfall rate of 75mm perhour as the basis of design for flat roofs in the UK,provided that any overflow will not cause damage tothe building or its contents. The likelihood of this rateof rainfall occurring for two minutes is shown in rainfallmap 1. It can be seen that there is less likelihood ofthis rate of rainfall being exceeded in Northern Ireland,Wales, Scotland and the north of England than in therest of England. From the rainfall maps, it can be seenthat, surprisingly, it is in the drier areas of the UK thatthe intensity of short bursts of rainfall is greatest.

The industry intends to move towards standardrainfall rates expressed in litres per second and relatedto specific return periods. Work is proceeding on thepreparation of a suitable presentation of data and islikely to be available with the appropriate Europeanstandard in due course.

The rainwater will flow over the roof area as arelatively thin surface film, perhaps only a fewmillimetres deep, depending on the length of run to theoutlet, the texture of the roof surface and the fall. Therecommended fall of 1 in 80 will ensure that the waterremains a thin layer on the roof if suitable outlets areprovided.

Rainwater discharges into the outlets at a ratedepending on the head of water at the outlet. It willcollect in the gutter or on the roof until the head ofwater at the outlet has built up sufficiently to dischargethe rainwater as fast as it falls on the roof. A smallincrease in the head of water will produce a substantialincrease in the rate of flow and it does not matterwhether the head is produced on a dead level roof, aroof to falls or in gutters or sumps.

The flow of the water into the outlet can be of twotypes; weir flow or orifice flow. Weir flow is the freeflow of water over an edge with an unrestricted drop.The flow of water into outlets will be by weir flow whenthe water is relatively shallow, and can be assumed toact when the depth of water does not exceed half thetop diameter of the outlet. For greater depths of water,weir flow is prevented and orifice flow takes over.Orifice flow causes a vortex or swirl to form and theefficiency of the outlet is reduced.

Tapered outlets are more efficient than those with auniform diameter. If the vertical dimension of the taperis at least equal to the top opening and if the diameterof the top opening is not more than one and a halftimes the downpipe size, the calculation of flow can bebased on the top diameter. This is called the effectivediameter.

If the taper of the outlet is greater than that givenabove, calculations for flow should be based on amaximum design outlet diameter of one and a halftimes the diameter of the downpipe. The alternative isto subject the outlet to hydraulic tests to establish therelationship between the rate of flow of the water andthe depth of water above the outlet. Mostmanufacturers of proprietary systems will supplyinformation on flow rates, quoted in litres per second.Table 1.3 shows the relationship between area drainedand flow rate for a rainfall rate of 75mm and 150mmper hour.

Roof area drained for different flow rates (m2)

TABLE 1.3

Flow rate L/s Design rainfall mm/hr

75 150

1 48 242 96 483 144 724 192 965 240 1206 288 1447 336 1688 384 1929 432 216

10 480 24011 528 26412 576 288



Roof to falls

Gutter or sump


Redrawn from British Standard BS 6367:1983 Code of practice for drainageof roofs and paved areas.

Based upon the Ordnance Survey map with the permission of the Controller ofHer Majesty’s Stationery Office, Crown copyright reserved.

Rainfall map 1: Period in years between events of 75mm per hour for 2 minutes

Key Years

100

50

10

5

1

0.5

5050

50

10

10

1

1

5

5



Redrawn from British Standard BS 6367:1983 Code of practice for drainageof roofs and paved areas.

Based upon the Ordnance Survey map with the permission of the Controller ofHer Majesty’s Stationery Office, Crown copyright reserved.

Rainfall map 2: Period in years between events of 150mm per hour for 2 minutes

Key Years

500

100

50

10

5500

500 100

100

100

100

50

50

GRAVITY SYSTEM

The traditional rainwater disposal system allowsrainwater to fall freely through the downpipes underthe influence of gravity. The size and position ofdownpipes is chosen so that water drains fast enoughto prevent an excessive head of water at the outlet. Therainwater pipes do not fill up with water and the flowis by “trickle down” the sides of the pipes. Theproportion of water in the downpipes at maximumdesign flow is in the order of one third full, theremaining two thirds is air.

Bends in the downpipes and horizontal runs arekept to a minimum. Horizontal runs which cannot beavoided should be installed to falls.

All internal downpipes should have rodding eyes atfloor level, positioned so that a blockage between thedownpipe and the surface water drainage system canbe cleared by rodding.

The junction between the outlet and the internaldownpipe should be sealed or caulked as a precautionagainst backing up, but experience suggests that theseseals are not always effective in the long term. Thesurface water drains must be of sufficient size to carryaway all the water from the roofs immediately or therewill be a danger of water backing up the downpipes.

The traditional gravity system usually entails a largenumber of downpipes from roof to ground. Acomprehensive system of ground drains is also requiredto connect up with all the downpipes.

GRAVEL GUARDSGravel guards are normally necessary on all outletswhere the downpipe size is less than 150mm diameter.If the downpipe is 150mm diameter or more and itdischarges as a straight drop from the outlet with asingle bend to the main surface water drainage system,it may well be regarded as a self-cleansing system.Although gravel guards will normally be used they arenot necessarily required and may be omitted if thedownpipe can be regarded as self-cleansing.

The provision of gravel guards introduces the needfor routine inspection and cleaning.

AREA DRAINED BY OUTLETSThe majority of flat roofs are drained to rooftop outletsonly, and the crucial aspect of design is the depth ofwater at the outlet. On a roof to falls during a storm,the water will collect over the roof area local to theoutlet to form a natural sump, and a head of water willbe formed.

If the fall is 1 in 80, a head of 50mm will beprovided by a natural sump which extends 4 metresfrom the outlet. It should be appreciated that this willonly occur for a few minutes during the part of thestorm which gives rain at 75mm per hour.

The roof area drained by a single outlet can becalculated in accordance with BS 6367:1983.Alternatively as an aid to design, table 1.4 gives theroof areas which will be drained by straight dropoutlets or tapered outlets. The table is based on arainfall rate of 75mm per hour taking into account weirand orifice flow as appropriate, and the capacity ofthe downpipe. For a rainfall rate of 150mm per hour,the values in the tables should be halved. A similarpro-rata adjustment can be made for other rainfallrates.

Where there is a substantial area of wall projectingabove the level of the roof and draining onto the roof,this must be allowed for when calculating the total areato be drained. In the case of only one wall, theeffective additional area can be taken as half theexposed vertical area of the wall. Further guidance forother configurations of walls is given in BS6367:1983.

Flow through gravel guards can only be accuratelypredicted by hydraulic testing. In the absence of testinformation, it is generally accepted that a flowreduction of 50% can be taken as a reasonable safeassumption. Table 1.4 is worked on this basis to showthe nominal effect of gravel guards.

The flow of water through outlets will be reduced ifwater cannot approach them from all directions. Table1.4 can be taken to represent the case of an outletwhich is placed sufficiently far from a wall or sides of agutter to allow a flow of water between the wall andthe outlet.

It will be seen from the table that the roof area to bedrained is influenced more by the head of water at theoutlet than by other factors. BS 6367:1983recommends a design head of up to 30mm but anincreased head will provide increased flow and maywell be appropriate with large outlet sizes. It issuggested that 30mm be used for the design head ofwater at outlets with downpipes up to 100mm, and50mm design head for downpipes of 125mm to150mm.

These design heads will be associated with asignificant load from the depth of water on the roof.30mm of water produces a load of 0.3kN/m2, and50mm produces 0.5kN/m2.


50mm head of water

4m

Areas of roof drained by outlets

TABLE 1.4



Pipe diameter

Pipe diameter

Pipe diameter

Pipe diameter

Area of roof drained (m2) by one straight drop outlet without gravel guard

Pipe Head of water mmdia. mm 15 20 25 30 35 40 45 50

50 19 29 40 44 47 51 54 5765 24 37 52 68 80 86 91 9675 28 43 60 79 99 114 121 127

100 37 57 80 105 133 162 193 226150 56 86 120 158 199 243 290 339

Area of roof drained (m2) by one straight drop outlet with gravel guard


50 9 14 20 22 24 25 27 2865 12 19 26 34 40 43 45 4875 14 21 30 39 50 57 60 64

100 19 29 40 53 66 81 97 113150 28 43 60 79 99 121 145 170

Area of roof drained (m2) by one tapered outlet without gravel guard


50 28 43 60 79 85 85 85 8565 36 56 78 103 129 158 170 17075 42 64 90 118 149 182 217 250

100 56 86 120 158 199 243 290 339150 84 129 180 237 298 364 435 509

Area of roof drained (m2) by one tapered outlet with gravel guard


50 14 21 30 39 50 57 60 6465 18 28 39 51 65 79 94 10875 21 32 45 59 75 91 109 127

100 28 43 60 79 99 121 145 170150 42 64 90 118 149 182 217 255

Length of taperto equal orexceed topdiameter

Top diameterto be not lessthan 150% ofpipe diameter

Height of slotsto exceedhead of water

Height of slotsto exceedhead of water

SYPHONIC SYSTEMS

The drainage layouts of large buildings can oftenbenefit from the use of a syphonic disposal system.The syphonic principle normally uses short drops fromoutlets to exposed horizontal pipes under the roof deck.These discharge rainwater into a downpipe to ground.The pipes run at full bore at the design flow rate.

Rainwater is collected through specially designedoutlets which are open only at the periphery, andinclude a central baffle. This arrangement allowswater to enter by weir flow and prevents air enteringthe outlet through the centre. Additional vertical bafflescan be incorporated to prevent the formation of avortex or swirl.

In practice under fast flowing conditions the outlet iscovered over with turbulent water at a quite substantialhead. The head to drive fully efficient syphonic flowneeds to be in the order of 50mm to 80mm dependingon the design of the outlet. In order to produce thishead, it is common practice to place the syphonicoutlet in a gutter or sump.

When rain is not falling heavily, the outlet will notbe covered over with water, and a sufficient head forsyphonic flow may not develop. Air will enter thedownpipes through the outlets and the system willremain a traditional gravity system with “trickle down”drainage. In heavy rain the amount of air contained inthe pipes will reduce and syphonic flow will developwhen the proportion of air is down to about 40%. Theair will then move along with the water, and whenmaximum storm conditions occur the pipes will bealmost full of water with very little air content.

The entire syphonic system will usually feed intoonly one or two downpipes. The individual outlets feedinto horizontal pipes located immediately under theroof. A large number of outlets can be connected bythe horizontal run into a single downpipe. Thesyphonic system is designed with relatively smalldiameter outlets feeding into carefully designedhorizontal and vertical runs which are sufficientlyrestricting to ensure the system runs with pipes full ofwater, and with a minimum of included air. The speedof flow is restrained only by the friction of the wateragainst the pipe walls, and the restriction formed bybends and junctions. It is the careful design of thefriction and resistance which controls the flow rate andmaintains the syphonic action. The essence of thedesign is to make sure the pipework is small enoughfor the system to run at full bore.

The design procedure is complex and will normallybe carried out using technology and computerprogrammes developed by suppliers of proprietarysystems.

The increased efficiency of syphonic systemsgenerally allows a reduction in the number or size ofoutlets on a roof, but the location of the outlets needscareful design in order to maintain a balanced system.It will be wise to consult the suppliers at an early stage.

The pipework of a syphonic system is undersignificant positive or negative pressures and thesystem must be balanced to avoid excessive pressureswhich could cause severe damage. All joints aresealed and may be welded, usually in the factory, withcompleted sections delivered to site with specialconnectors for site connection and welding. The extra

cost of the relatively sophisticated units is offset by thereduced numbers of outlets and downpipes and thegreatly simplified ground drainage.

Syphonic outlets are designed to give specific flowrates when the system is at full bore. Standard outletswith flow rates of 6 litre/s and 12 litre/s are common.At a rainfall rate of 75mm per hour these can drain aflat roof area of 288m2 and 576m2 respectivelyproviding the system has been designed accordingly.

The depth of coverage of a syphonic roof outlet willdetermine its capacity, at 35mm its capacity is typicallyhalf its normal rating, at 50mm it is equal to its nominalrating, and at 80 to 100mm its capacity may insuitable circumstances be increased to twice its nominalrating.

With a syphonic system it is particularly importantto choose an appropriate design rainfall rate, becausethe system will usually have only a small margin ofexcess capacity above its designed capacity.

Any enclosed roof area should be drained by atleast two outlets to allow for the risk of blockage.Regular inspection is necessary to ensure gratings arekept clean and clear.

SUMPS AND GUTTERS

Sumps and gutters allow a shallow flow of water overa long periphery into a deep collection space. Thedepth of water can then build up to form a substantialhead at the outlet to drive the maximum amount ofwater into the rainwater drainage system. In order toachieve weir flow, the depth of the sump or gutter mustbe equal to the design head of water at the outlet plusat least 25mm.

During a 75mm per hour storm lasting for a fewminutes, sumps and gutters will fill extremely fast untilthe depth of water is equal to the design head. At thisstage an equilibrium will form and the water willdischarge down the outlets as quickly as it arrives.

AREA DRAINED BY SUMPSThe area drained by an outlet/sump assembly needs tobe considered in two stages. The area drained by theoutlet itself must first be calculated or taken from table1.4. Secondly, the sump size must be chosen so thatthe rate of drainage into the sump is matched to therate of drainage through the outlet.

Table 1.5 gives the area drained by a given headand periphery of sump. From this it is possible tojudge the periphery which is necessary to dischargewater into the sump at the same rate as the outletdischarges water into the downpipe. There is noreason why sumps should be made larger than this andno increased flow will result from the outlet/sumpassembly, unless the size of the outlet is also increased.


25mm

Design head of water

As with rooftop outlets, the designer must decide whatdesign head he will allow for flow of water into sumpsand again this is a rather arbitrary decision. BS 6367:1983 recommends 30mm or less.

The area of roof which will drain to sumps of thesize given in table 1.5, assumes that the flow of wateris from all directions.

When the sump is positioned in such a way that theflow of water to one or more sides is obstructed, theeffective perimeter of the sump will be reduced pro-rataand reference should be made to the effectiveperiphery column.

Area of roof (m2) drained into sumps

TABLE 1.5

Sump Effective Head of water mmsize peripherymm mm 5 10 15 20 25 30

500x500 2000 45 126 232 358 500 657600x600 2400 54 152 279 429 600 789700x700 2800 63 177 325 501 700 920800x800 3200 72 202 372 572 800 1052900x900 3600 80 228 418 644 900 1183

1000x1000 4000 89 253 465 716 1000 1315

GUTTERSGutters have such a long edge that shallow weir flowover the side will always take place regardless of theroof area to be drained and there is no need to checkthis by calculation. It is only necessary to calculate thesize of outlet to discharge water at the required rate.

Lined gutters are no more than a waterway and thesize of the gutter is immaterial other than the provisionof a suitable depth to provide the design head of waterin the gutter plus 25mm to ensure free weir flow intothe gutter. If lined gutters are thought to be necessary,it is important to make sure that they are suitablyshaped for the installation of a satisfactorywaterproofing. It is recommended that the sole of thegutter is at least twice the maximum depth of the gutterand not less than 300mm wide after insulation andwaterproofing.

Parapet gutters should have the skirting heightagainst the parapet at least 75mm higher than themain roof area to accept overfilling of the gutter.

It must be remembered that if designing for 75mm perhour, the design depth is likely to be exceededoccasionally in the southern part of England where150mm per hour will occur from time-to-time. In theseareas a reserve of head will occasionally prove usefuland a flat roof will always provide this by forming areservoir if the gutter overflows. The level of waterbuilding up on the roof is only likely to rise a fewmillimetres, not enough to cause any concern and notenough to call the height of upstands into question.

If the roof membrane is not taken continuouslythrough an internal gutter, but is merely turned into itwith a drip edge, there must be a risk of rainwaterentering the building in the event of overflow of thegutter. Under these circumstances a rainfall rate of75mm per hour will not be sufficient, and the rate to beused for design will depend on probability based onthe design life of the building and safety factorrequired as given in BS 6367:1983. An emergencyoverflow should be included in the design to ensurethat in the event of overfilling, the overflow of water isto the outside of the building in a position which willcause no harm. The overflow should be arranged tooperate at 5mm above the water level associated withthe design head.

Sumps and gutters may tend to block from silting upor from blown or washed leaves, twigs or industrialresidues and regular maintenance inspection becomeseven more important.



75mm

flatroofdrainagedesign.pdf

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