Designing Slabs for Privies - The Water, Sanitation and Hygiene

Water for the WorldTechnicalNote No. SAN. 1.D.lThe slab is the floor of the privy. Materials NeededIt covers the pit and has a holethrough which to defecate. Designing Measuring tape - To check dimensionsa slab involves selecting the type ofof previously constructed Itemsslab (squatting or sitting), deciding(pit, base around pit, or pourwhichimprovements the privy will have,flush bowl, for Instance)calculating the dimensions of the slab,and determining the materials, labor,and tools needed to build it. Theproducts of this process are designdrawings of the slab and improvements,Selecting Slab TypeIf any, and a detailed materials list.These items should be given to theperson in charge of construction.This technical note describes howto design a slab and arrive at theseend-products. Read the entire technicalnote before beginning the designprocess.The type of slab selected dependson whether the users prefer to squator sit when defecating.Squatting Slab. The main featuresof a'squatting slab are a hole, a pairof footrests, and a lid to cover thehole. See Figures 1 and 2.TOP View

The hole is generally key-holeshaped, about 400mm long, and 125mmwide at the narrow end. The wide endis a circle 175mm in diameter. Theback edge of the hole usually shouldbe about 150mm from the back wall ofthe privy which, depending on thedesign, may be at the edge of theslab. If the distance between thewall and the hole is less than 150mm,there may not be enough space tosquat. If the distance is more, thereis a greater risk of soiling thefloor. The distance between the edgeof the hole and the edge of the slabmay be greater than 150mm if the privyhas a vent pipe.--Metal fastenersTop ViewIt I Hand’eut out to fit footrestsSince the footrests ensure that theprivy user is positioned correctlyover the hole, their placement is important.They are oval-shaped, about300mm long, 125mm wide, and 25mm high.The lid should cover the hole butnot fit inside it. It should have ahandle. See Figure 2.Sitting Slab. The main features ofa sitting slab are a hole, a pedestalor riser, a seat, and a lid to coverthe seat.The hole is 250-300mm in diameterand should be about 150mm from the backwall of the privy which may be at theback edge of the slab, depending on thedesign. See Figure 3.The pedestal is 275-350mm high andhas the same inside diameter as thehole. The thickness of the pedestalwalls depends on the materials used.The seat is attached to the top ofthe pedestal. Its outside measurementsare equal to or greater than the outsidemeasurements of the pedestal.The seat has a hole in the center 200mmin diameter. A second seat with asmaller hole (150mm) in diameter canbe included for children.The lid covers the seat and is oftenattached to the back of the seat witha hinge. See Figure 3.Figure 2. Lid for Squatting SlabDeterminingImprovementsThe main improvements to a privyare a vent pipe, a pour-flush bowl,an off-set pit, or a combination ofthe three. Any privy improvementwill modify the slab design.Vent Pipe. If the privy is tohave a vent pipe, the pit must be about300mm longer than a pit for an unimprovedprivy (see "Designing Pits forPrivies," SAN.l.D.2). The slab mustalso be longer by about 300mm. Thismeans that the distance from the backedge of the squatting hole to the edgeof the slab is 450mm--150mm for thebasic design plus 300mm for the ventpipe. See Figure 4. The slab has ahole lOO-150mm in diameter, dependingon the size of the vent pipe, and ispositioned as in Figure 5. The ventpipe can be made from a sheet of tinor galvanized metal and should betopped with a fly-proof screen.Pour-Flush Bowl. If the privy isto have a pour-flush bowl, the squattinghole may not be key-hole shaped.The shape of the hole must conform tothat of the bowl, and often the bowlsare prefabricated as shown in Figure6. The bowl should be positioned toflush forward, to prevent erosion ofthe pit wall.

350~5OOmmSide ViewTop VlewTop Vlew-PEDESTAL350~5OOmmTop ViewTop View29-74I-. Minimum: Im. .,., . . -, . ...: . . .; ,., .;,.. . . -. (., . . . . . , . . ‘.‘,End VlewA‘5mmT-End ViewSEATLID$3;iat;limensionsFigure 3. Sitting Slab with Pedestal, Seat and Lidhiaher tha Protective conePipe extendsthroughMinimum:75mmT.___,______,-_r __-__ .--I. 150mm ”,. ‘.’WITHOUT VENT WITH VENTFigure 4. Comparison of Privies with and without VentSQUATTING SLAB SllTlNG SLABFigure 5. Top View of Slabs Showing Vent Hole Placement ,3

Off-Set Pit. The slab for an offsetpit rests on a platform made ofwood, bricks, or concrete and has ametal chute attached to the hole.The chute can be made from a sheetof tin or galvanized metal. It entersthe pit below ground level at a downwardangle of 50° to 600. The upperend is mortared to the bottom of theslab and encircles the squatting hole.The lower end narrows to about 200mmin diameter and extends about 1OOmmbeyond the pit wall. See Figure 7.The pit must have a cover which can bemade in one piece or in sections. Ifthe cover is made of concrete, itshould be made in sections for easierhandling as shown in Figure 8. Thecover must be strong enough to preventpersons from falling into the pit.Combination. If there is a combinationof improvements, each improvementwill modify the design of theslab as described above. For example,a privy with a vent pipe and a pourflushbowl must have a longer slab toaccommodate the vent and a speciallyshaped hole for the bowl. There Isone exception. The slab design foran off-set pit is the same whether ornot the pit has a vent pipe, becausea vent pipe used with an off-set pitextends through the pit cover, notthrough the slab. See Figures 7 and 8.CalculatingDimensionsUnimproved, ventilated, or pourflushprivies must have slabs thatcover the pit and overlap each edge-,),..: *‘, ,limm‘C.;.‘.’ ,::., .’ ,e : .: : ,..’‘:.!. ‘. .. :?~ ;,. * : ,: .’ , I,.: .:, ‘.‘\ ’ : L.’ ::_,.,,.’ ‘.’L , .. ‘.. ;, ,,,:. :;. , _.,‘,’ ‘:. ,:.*30mm.I%----340mm----~-‘/j30mm,.:. ,., : 30inm 30hm.‘.:.30TmJ--t.;. .’I_ . . .,‘s. :’ ‘.I I1------440mmg0mm9, LYm--30/mmI..,‘I, : ‘,f. [4 L---rT5PRECASTCONCRETEGALVANIZEDMETALFigure 6. Pour-flush Bowls for Squatting Slabs4

y at least 75mm. The length of theslab for these kinds of privies equalsthe length of the pit plus 150mm andthe width of the slab equals the widthof the pit plus 150mm. Worksheet Ashows the steps in calculating slabdimensions. For example, suppose thepit is 1.5m long and 1.2m wide. Thenthe length of the slab is 1.5m + 0.15m= 1.65m. The width of the slab is 1.2m+ 0.15m = 1.35m. See Worksheet A, step#l. For an off-set pit, the slabshould be about lm square. Thethickness of a slab depends on thematerial used to make it. A reinforcedconcrete slab is 50-75mm thick.The cover for an off-set pit mustbe large enough to cover the pit andoverlap each edge by at least 75mm.The length of the cover equals thelength of the pit plus 150mm, and thewidth of the cover equals the widthof the pit plus 150mm. For example,suppose the pit is 1.7m long and 1.2mwide. Then the length of the coveris 1.7m + 0.15m = 1.85m. The widthof the cover is 1.2m t 0.15m = 1.35m.See Worksheet A, step #3.race for vent pipeSide View&Platform\ Figure 7. Off-Set Pit PrivyI--PitEnd View (Rear)1 150mm :I-+ Slab IiiChute length 1 m minimumDetail of ChuteIrm/Minimum:75mmbBase Cover sections Hole for vent.Top ViewSide ViewFigure 6. Cover for Off-Set Pit5

Worksheet A. CalculatingDimensions1. Slab (sitting or squatting) for unimproved, ventilated, or pourflushprivy:Length of slab = length of pit /.g m + 0.15m = /.&rnWidth of slab = width of pit + 0.15m = /.gsrnThickness of slab (if2. Slab (sitting or squatting) for off-set pit privy:Length of slab = l.OmWidth of slab = l.OmThickness of slab (if concrete) = 50-i'5mm3. Cover for off-set pit:Length of cover = length of pit A7 m + 0.15m = /.pf mWidth of cover = width of pit /.A m t 0.15m = /.j!mThickness of cover (if concretF75mm4. If cover is in sections:Length of each section = width of cover = /.srnWidth of each section except one = 300mmWidth of one section = 300mm plus necessary width to totalentire length of the cover = 300mm t 50- mm = 350 mmCombined widths of sections = total length of cover =300 + 300 + 300 + 300 + 300 t 35Omm = 185Omm(NOTE: To calculate quantities of concrete, see "Designing SepticTanks," SAN.2.D.3.)If the cover is reinforced concrete,it should be made in sections. Thelength of each section equals the widthof the pit plus 150mm and the width ofeach section, except for one end section,is 300mm. The width of one endsection must be 300mm plus whatevermeasurement is necessary to add up tothe total length of the cover. Forexample, suppose the total length ofthe cover must be 185Omm. Then thecover would be made in six sectionswith widths of 300 t 300 t 300 t 300+ 300 t 350mm = 185Omm. See WorksheetA, step #4.The thickness of a cover for anoff-set pit depends on the materialit is made from. A reinforced concretecover should be about 75mmthick.A vent pipe is 2-2.5m long andlOO-150mm in diameter. A chute foran off-set pit is at least lm long,with an average width of 200mm.Worksheet B shows how to calculatethe dimensions of the materials neededto make vent pipes and chutes for offsetpits.6

Worksheet B. Calculating Quantities of Material for Vent Pipe and ChuteVentPipeGenerally made from a sheet of tin or galvanized metal. The sizeof the sheet:Length = height of privy shelter (from "Designing Privy Shelters,"SAN.l.D.3) plus O.6mWidth = diameter of vent pipe times 3.3Example: Suppose that the height of the privy shelter is 2m andthe diameter of the vent pipe is 150mm. Then the sheet of tin neededto make the pipe will have these dimensions:Length = 2m + 0.6m = 2.6mWidth = 150mm x 3.3 = 500mm(NOTE: The method used to calculate the width allows the edges of thesheet to overlap about 25mm when the pipe is made.)-------------------------------------------------------------------------Chute (for off-set pit)Generally made from a sheet of heavy tin or galvanized metal. Thesize of the sheet:Length = 1.5 times the distance from the front edge of the pitto the farthest edge of the hole in the slab.Width = distance around the hole plus 25mm(NOTE: The distance around the hole equals 2 times the length plusthe width.)Example: Suppose the hole in the slab is 150mm wide and 400mm long,and that the distance from the pit to the edge of the hole farthestfrom the pit is 700mm. Then the sheet of tin needed to make the chutewill have these dimensions:Length = 1.5 x 700mm = 1050mmWidth = distance around hole + 25mm= 2 x (150mm + 400mm) + 25mm= 1125mm(NOTE: The "width" of the sheet may be longer than the "length.")

When the type of improvements anddimensions have been determined, preparedesign drawings similar toFigures l-9, showing correct dimensionsand top and side views of, theslab and improvements. Give thesedrawings to the person in charge ofconstruction.MaterialsListSlabs can be made from a varietyof materials, including reinforcedconcrete, wood or bamboo. Generally,they are made from concrete, becauseconcrete is strong, long-lasting, andeasy to clean.A concrete slab must have reinforcingmaterial, such as steel bars1Omm in diameter, wire mesh, or splitbamboo. To calculate the quantity ofsteel bars needed, draw a sketch similarto Figure 9, showing bars in place,and count the number and lengths of thebars. If wire mesh is us.ed, the quantityis approximately equal to the areaof the slab (length times width).The reinforcing material must notblock the hole in the slab. No partof it should stick out through theconcrete.A common mix by volume for concrete The tools and labor required tois one part cement, two parts sand, build a slab depend on the materialsthree parts gravel, and about 2/3 used. If it is made of reinforcedpart water or enough water to make concrete, at least one worker shoulda fairly stiff mix. The cement should have some knowledge of or experiencebe Portland cement. The sand should with concrete (mixing, pouring, andbe clean and sized fine to 6mm. The building forms). Common tools forgravel should be clean and sizedworking with concrete include hammer,6-25mm. The water should be clear. saw, and nails for building forms;For details on calculating quantities container, shovel, tamping rod, andfor concrete mix, see "Designing Sep- trowel for mixing, pouring, andtic Tanks," SAN.2.D.3. smoothing concrete.25mm 150mm ISOmm150mm 150mmVent pipe hole/pedestalTop ViewEnd View1 ::... T ._. ."-Top View With Vent HoleTa -.. .#

If the slab has a seat and pedestal.the Dedestal can be made frombrick; concrete blocks, or wood, andthe seat can be made from wood. Onepiece,ceramic seat-and-pedestalunits may be available.A cover made from wood should beprovided for both sitting-type andsquatting-type slabs. The cover forthe seat and pedestal may be attachedto the back of the seat with hinges.A pour-flush bowl may be madefrom galvanized metal, concrete,molded rubber, or ceramic material.These units may be prefabricated andready to install. A skilled craftsmancould produce a galvanized metal orconcrete pour-flush bowl using thedesign information in Figure 6. Ametal bowl must have smooth, roundededges dulled by a file. A concretebowl must be cured in water for a week.A pour-flush bowl can be secured tothe slab with concrete mortar.A vent pipe can be made of galvanizedmetal by a semi-skilled workmanusing tinsnips, pliers, metal screwsand a screw driver (or other means ofsecuring the metal), and black paintand a brush to paint the pipe black.Or, a section of bamboo with the nodesknocked out could be used as a ventpipelA chute for an off-set pit can bemade of galvanized metal by a semiskilledworkman using tinsnips, pliers,metal screws and a screwdriver, orother means of securing the metal.A cover for an off-set pit can bemade from wood, metal, or reinforcedconcrete. If concrete is used, thetools and skills of the workmen arethe same as for a concrete slab.When the materials needed have beendetermined, prepare a detailed materialslist, similar to the sample inTable 1, showing types and quantitiesof all materials, tools, and laborneeded to construct the slab and improvements,and the estimated costsbased on local prices. Give the materialslist and design drawings,similar to Figures l-9, to the personin charge of construction.Table 1. Sample Materials ListITEM DESCRIPTION QUANTITY ESTIMATED COSLabor Foreman 1Laborer (some experience with concrete)Laborers (to move constructed slab)Supplies Portland cementSand: Clean, size fine to 6mmm3m3Gravel: clean, SIX 6-38"m m3Water: Clear, drinking water. preferred ZlitersWood (for concrete forms)Nails (for concrete forms)Reinforcing bars -mm long-mm long(or wire mesh) m2Wood (for lid)_-___________-------------------------If seat and pedestal:Bricks (for pedestal)Mortar (cement, sand, water)Wood'(for seat and lid)-----________-------------------------Pour-flush bowl (prefabricated)Galvanized metal (for vent pipe)Galvanized metal (for chute)Metal screws 01' bandsScreen (for vent)Tools Measuring tapeShovelContainer for mixing concreteTotal Estimated Cost =