Digging a circular pit. 05m/h. The average lifespan of a septic system is estimated at 15 to 40 years, but it may last longer if properly maintained. More complex soakaway construction with additional depth, pipework, size or reinforcement. The drainage should be installed at a low point in the affected area to allow it to drain surface water away to storm drain, or a Soakaway. The water samples for the relationship between the bacteriological quality of obtained from the five locations, were taken to the Cross River borehole water and its relative distance to soakaway point. This is nearly always level with the ground. #4. 1,000-gallon septic tanks are 8 feet 2 inches long, 5 feet 8 inches wide, and 5 feet 1 inch deep. In general the following rules can be used as a preliminary assessment; Slopes steeper than 25% can often experience negative effects and should be. I du. . Soakaway Drainfields. So if I have a soakaway 1m x 1m as close to the corner of a plot as possible that means that I need a minimum of (5m + 1m + 2. The magnitude of any negative effects is determined by the size and infiltration volume of the soakaway , the distance to the slope, the slope gradient and the local geology. When rainwater exits your downpipe, it should enter a gully. (When calculating the size of a soak pit, the volume of water storage available for a rock-filled hole is 0. . A soakaway must be located far from the house. Re: Biodigester Soakaway In Waterlogged Area by phabulousangel ( m ): 8:38am On Sep 28, 2022. Must be 15 metres away from any house or building. . 50 metres of a chalk river or aquatic local wildlife site. Flush your toilets, do your laundry, all with rainwater which will never present you with limescale problems and you use far less detergent as it is so soft. The two septic tank Got Filled and the Soakaway. ) from a sub-surface weeping tile effluent disposal field or evaporation mound or an outdoor pit privy, Canada, Alberta. ukA simple, trapped gully will be installed on the edge of the patio nearest the soakaway, and a pipe will link the gully (which could be a linear channel , if preferred) to the cells that are about to be installed. Water will always find its way to the lowest point in the landscape, so it is common sense that the area most affected is the place you should build your French drain. 1 Ground Conditions Since the soakaway relies for its operation on outflows infiltrating into the surrounding ground, it is vital that the ground. 100x1. Buying a house in the country with a well and septic system on an acre of land. #4. 2. Blocked Soakaway ·A quick one. We always start our patios 3 or 4 inches (100mm) away from the house. near. There doesn’t seem to be a definitive guide online for minimum distances for soakaways, but what I’ve gleaned is: - Minimum 5m from the property / structure - Minimum 5m from the road (which I assume. It must be at least 50 metres away from any water. Regulation, i. 2. 4 Design and Construction. Table 2 Soakaway Area Requirements at. Water goes into thePipeline connections to the chamber shall be in accordance with Clause 507 (3 and 15) of the Department of Transport's "Specification for Highways Works". The designer for my extension put a soakaway in the middle of my front garden but the neighbours garage was only a few metres away, and there's probably only about 6 m between the edge of my proposed extension and the. . Hi all, I was recently looking into purchasing a small building plot , problem is it has a neighbours soakaway right on the rear boundary and is virtually impossible to get the 7m distance and still build anything worthwhile, it' s a real shame as is a very nice plot. 5 years old. So access. filled the hole at 1820, and by 2217 it had possibly (possibly) moved 2cm, but I suspect thats more hope than anything. Hogarth House 1994 - one of the best septic system books around, suffering a bit from small fonts and. Design Considerations. Discharges in a flood risk area should also be avoided. If the pipe starts in a trench 300mm deep the bottom of the pipe will be 600mm below the ground when it gets to the soakaway. Before I had this work done the guttering on my side always overflowed every time it rained and poured al over my. Example: 3. Re: How Do I Build A Septic Tank / Soakaway In This Situation? (pic Attached) by Nobody: 10:20pm On Jan 05, 2017. 3. Same distances apply to the soakaway. This publication is available in html and also in. 10-meter-distance (34-foot) from a stream or ditch It should be 50 meters (164 feet) away. existing ground / formation level. If you have replaced your treatment system but all these points. The soak pit should be kept away from high-traffic areas so that the soil above and around it is not compacted. 1 For disposal of non-domestic wastewater, the setback distance shall be determined by a hydrogeological study in accordance with Section 17(b) of Chapter 3, but shall not be less than the distances shown in Table 4. Within 6 months. For example, 50m2 roof area requires a rubble filled soakaway of (50m 2 x(50/3000))/0. In many cities it is possible to maintain the sewer system, which routes the sewage from the septic tank of each home to a treatment plant. 26 (£30. Is there some formula to work out whether a small soakaway would be more than adequate? My only concern about the soakaway (after readng a few sites, ie . 2. They are: Soakaway should not be within 2 metres of a neighbouring boundary. A septic tank soakaway is a series of relatively shallow (a minimum of 200mm. Soakaway Pit Design. MCQ Menu Toggle. For more complex designs, soakaways can take up to two days to install and therefore are likely to cost around £2,500. A soakaway must be 2. You’ll need a new bottle gulley under the new down pipe. Yes, they have cut our shared surface water drain, and propose to divert the shared down pipe across to their side to flow onto their new extension roof, then through the guttering and down pipe on the end of extension to their new soak away. V soak = 15 × 300/1000. Answer: As far as building regulations are concerned here is what they say: "Septic tanks should be sited at least 7m from any habitable parts of buildings, and preferably downslope. F – Distance From House Wall. Hello House, We digged septic tank and Soakaway for 6 flats. Unfortunately, in my case that's quite far from the back of the house so it would involve extensive pipework, including under my driveway, and be very expensive. Log In. This is so you can pour a 150mm slab of concrete over the top as a base for the new paving. No thanks for the thread hi-jack BaS. it is more often than not related to the soak-away system rather than the septic tank itself. 10 metres away from the open drain. 2m diameter ring. For deeper excavations please seek specialist advice. Additionally, each downspout should drain a maximum of 35 feet of gutter. It has oil central heating, can anyone give me an idea of running costs? It's a good size Victorian farmhouse. If you have to install a new ST, it must be 5 metres from a building, 5 metres from a boundary or road, and 10 metres from a watercourse. Let us know if you still have further questions. To calculate the size/volume of a rubble filled soakaway, the same calculation can be used but the result divided by 0. A percolation test can be conducted by taking the following steps: Dig a 300mm square hole to a depth of 300mm below the proposed invert level of the drainage field. Mary Long-Dhonau explains: “ [The size of. On 2022-01-31 by Tyler . Soak pit construction procedure. Can I put a soakaway next to my house?. Hi all, I was recently looking into purchasing a small building plot , problem is it has a neighbours soakaway right on the rear boundary and is virtually impossible to get the 7m distance and still build anything worthwhile, it' s a real shame as is a very nice plot. Menu. The cost to empty a septic tank is usually priced between N45,000 to N90,000. I was wondering if the distance from a house to the soakaway should be at least five meters. If you are dealing with a boundary, it must be at least two and a half metres far from that. They are typically formed from square, or circular, pits filled with rubble or lined with dry jointed masonry or perforated concrete ring units. septic tank distance from house - septic tank distance from house How far should a septic tank be from a house? How far should a soakaway be from a building? How far should the soakaway be from a well, spring or borehole? This video will answer those questions for you. The pipe end entering the soakaway must be at a depth of 25mm minimum and the end of the pipe cut at 45° Septic Tank – distance from house in England & Wales. Which body is responsible for enforcing the location of the soakaway being a sufficient distance from the boundary, and also the new building, in accordance with Council Policy? Is. IIRC, the standard mentions "about 5m" from a building. Hey all, My driveway project is about to start. The soakaway provides a slow method of diffusion which stops pooling and helps to prevent flooding. The soakaway should then be filled with 10mm limestone chippings; Soakaway Installation. ACO Water Management 's Oliver Collins. Septic tanks should be at least 7 metres away from any habitable parts of the building. Soakaway Crates: Polystorm 61 tonne Polypipe Soakaway Crate - 1m x 0. Pit should be at least 2mts deep by at least 1mt square. These septic component clearance distance tables below give distance requirements between septic components and wells, streams, trees, property boundaries, lakes, etc. Sizing of a soakaway The length of the soakaway drains is determined by the number of bedrooms in the house and the porosity of the soil. This is to avoid any issue or possible interaction with either your property or. We are happy to help, call us on 0800 3101092 or via our contact form. Common guidelines require at least 50' clearance distance between a well and a septic system tank or 150' between a well and a septic drainfield or leaching bed but you will. The condensate pipework may terminate above or below ground level. 3m. Tank passed County Council inspection around 2016. 50m from water abstraction points, boreholes, springs. Lift the cover and pour water down the the downspout hole. 0 m = 15 m 2. But there is a third system of drainage of the. 0 3. See more of Soakaway crates on Facebook. Minimum soakaway drainage field distances. The design of a typical soak pit generally consists of a pit of approximately 1 cubic meter with a depth of 1. Beyond that and the minimum clearance distance is that we have already given above on this page, distance is going to be determined by the site and the available space. The Nicholls team can also work. 6m deep. septic tank distance from house How far should a septic tank be from a house? How far should a soakaway be from a building? How far should the soakaway be from a well,. You may require a larger soakaway for a house. 5 3. To build a soakaway, you will need drainage pipes and gravel. 68mm downpipes are simply connected to the top inlet. Septic Tank Regulations. It’s also recommended that your septic tank be located within 30 metres of an access point to allow for your tank to be easily emptied when required. Country. 00 7. The excavation can proceed in a similar manner to any deep drainage excavation. These are used to hook up the crates to prevent movement while the side-filling materials are added. hi guys!!!!! ive noticed water in my cellar it seems level with area where the soakaway leaves the house. In order to carry out a BRE 365 Soakaway Test you will need to excavate some trial holes, and fill then with water and watch how quickly in soaks away. Similarly if the soakaway drains a driveway, the catchment area is the area of the hardstanding. 5 meters from any building and any boundary. I note that your conservatory entrance is built approx. #3. These are used to hook up the crates to prevent movement while the side-filling materials are added. French Drains – How They Work. My property was constructed in around 1969 and the roof guttering runs along the fascias of the property and then flows into a vertical downpipe. 00 ISBN 978-1-4064-2338-9Many houses have the roof downpipes connected to soakaways. We should not construct a soak pit nearer or in a high traffic area; failing to do so may lead to soil compaction. No thanks for the thread hi-jack BaS. The Building Standards technical handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004 and are available in two volumes, domestic buildings and non-domestic buildings. Log In. 7. It does not have to be beautiful, to scale, nor expensive. The distance between a soakaway and a house is very important when planning the. roughly 2m x 1m, x 1. 4. Our FSA Registration number (FRN) is 750007. This is measured below the pipe invert so you' generally dig 1. com Limited are authorised and regulated by the Financial Conduct Authority (FCA). 5m to 4m. Answered 8th Oct 2013. Soakaways collect all surface water run-off. Five metres is the accepted distance, but this also depends on the calculated size of the soakaway. Location. Like 0. Soakaways should be built a distance of at least 5. if its not under building regs you can do it any distance, min 5m is the ideal I suppose. for land drain & trench soak away, pipe bedding and surround to be 20mm. Soakaways should be constructed in a circuit to make a continuous loop. This closes the pores and fissures at the excavation interface leading to. For example:-. We do this for two reasons. A t = Area of the drainage field in m 2. 2 metres deep. To avoid water collecting in the soak away leaking into and damaging the foundations, The soakaway should be situated at least 5m away from any building. Regulation, i. After this, add the remaining rubble so that it surrounds the drainage pipe entering the pit. Published in Soakaway Design – Groundwater Level. According to the new building regulations any soakaway should be 5m from any foundations and at least 2. Soak pit must be located away from the drinking source, usually 30m distance away. A t = p X V p X 0. . A final option for driveway drainage is building a trench – or channel – drain. A soakaway is a hole dug into the ground that is filled with coarse stone and rubble or plastic crates. Most gullies used in the UK are manufactured with a bend. Alternatively, if you plan to sell your property before that date, you must upgrade the tank before. And tumble your outlet into it. The pipes are laid no deeper than 700mm – 800mm in the ground. Fill the hole with approximately 150mm of water and allow it to seep away completely; no need to measure the time. The two septic tank Got Filled and the Soakaway. The owner also said they had a soakaway in the garden rather than a septic tank. However, in many areas, building as close as four feet away from your property line is allowed without any issues. If you have a combined system, the dyed test water show up in the chamber from both foul and surface sources. below the ground) underground perforated pipes laid in gravel trenches that allow septic tank effluent to soak away over a large area. ie Web: . Wonder if I can get a discount. I did wonder about the soakaway issue which is why I want to trace the drain run. Log In. 3m deep and 1m x 1m. Must lay a layer of sand and gravel at the bottom to disperse the effluent flow. Hole 300mm square to depth of 300mm below where pipe will go into the soakaway. Step 5 – Add Gravel and Complete the Soakaway. After about two metres distance from the bungalow the pipe descended a lot more rapidly, and, so I now had to dig a constant path to follow it. South. This ensures aerobic. 978-1-78544-328-2. Once the hole is dug to the correct depth, add a layer of gravel at the base, approximately 1000mm. We've just pruned back a large rosebush to find that the guttering downpipe has rusted away and separated from the section that goes underground. Reaction score. 25 = 22. Should be fairly obvious if you get to a soakaway dead end. In the following information we will be focusing on the city of Auckland which is the most populated and quickest growing region in New Zealand. For existing developments, house connections from Manhole number 1 to the main sewerage system should be made at the earliest opportunity, and usage of the septic and sewage holding tank stopped. 2m3 per crate) £36. Soakaways are used to. Please contact BlockBusters Drainage and Plumbing Services on. The Building Standards technical handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004 and are available in two volumes, domestic buildings and non-domestic buildings. Soak Pit Design. So Is the site totally undev. septic tank distance from house How far should a septic tank be from a house? How far should a soakaway be from a building? How far should the soakaway be from a well,. BRE Digest 365 is the most commonly used document. septic tank distance from house How far should a septic tank be from a house? How far should a soakaway be from a building? How far should the soakaway be from a well,. This discussion moved to SEPTIC CLEARANCE DISTANCES - TEXAS 18 Aug 2015. If you cannot slope the slabs away you may wish to consider acho type drainage channels against the property which may be connected. Permeable pavement with infiltration close to house foundations The key elements of the Part H rules are that the soakaway must be 5 meters from a building or road. This publication is available in html and also in. In the range of septic tanks we sell, there's a 2,700L 4-person capacity, a 2,800L 4-person capacity, or a 2,800L 5-person capacity tank. You can buy these off-the-shelf to sit between your drainage pipe and soakaway crates. There is always a work-around to it. Created Date: 1/5/2005 7:46:36 AMInstalling a channel drain starts with a narrow trench cut into the concrete or asphalt. The key elements of the Part H rules are. Sizing of the soakaway will depend on how much rainwater it will have to take - however many councils will have a standard size that they are happy with. Soakaway Crates: Aquacrate 20 tonne soakaway crate - 1m x 1m x 0. 15 m (50 ft. . 8m from ground level. It drew on extensive research by Trinity College Dublin and the National University of Ireland Galway under the national environmental research programmes administered by the EPA: Environmental Monitoring, R&D sub-programme. Always cover the soakaway crates with a geotextile membrane to maintain the performance of the system. 1 March 2017. 5 1. It needs to be a significant distance away, normally at least 5m. a Single Dwelling House and the 2000 EPA Wastewater Treatment Manuals: Treatment Systems for Single Houses. A soakaway has to let the water soak away if it is to be successful. 3. If above ground the condensate pipe must be protected with a waterproof lagging. When the soil starts to build up in the soakaway, you can cover it with heavy duty fabric. The gravel should be sufficiently large to allow water to seep through to the bottom of the structure. When water hits the channel drain, it's sent off in another direction, well away from the house or garage. . a - is the internal surface area of the soakaway to 50% effective depth, excluding the base in m 2. As assessment of the property will also need to take place to calculate the distance of the drainage field from obstructions, ditches, surrounding buildings and water abstraction points such as such as surface water and groundwater. Refill the hole and time how long the water takes to seep away from the 75% point to the 25% point (this is 150mm). . Full drainage design suite (50% Discount) including 7 spreadsheets; Trench Soakaways consist of long narrow trenches filled with gravel or single size stone. Just like changing the oil in your car, maintaining your septic system extends its life for a small cost compared to the expense of installing a new system, which typically runs $15,000 or more. Percolation test procedure for surface water soakaways. Connect the effluent carrying pipe to the soak. You can either infill with clean hardcore or crates (see what BC will accept: some insist on crates) but either way line the hole with Terram. generally on a house extension, the gullies are joined by 110mm pipe below the ground and then 1 pipe goes to a soakaway. Ask at the builders merchants, and explain what you want it for. The remaining liquid drains from the tank by an outlet pipe and soakaway. For soakaways serving areas greater than 25m² reference should be made to BS EN 752-4, or BRE Digest 365 Soakaway design. 5m-1. The trenches for the soakaway should be between 300mm and 900mm wide with a distance of 2m between trenches. if its not under building regs you can do it any distance, min 5m is the ideal I suppose. 2. 2m MUST be supported to eliminate risk of bank collapse. 5 meters down, as a general rule of thumb for a driveway with a width of around 100 meters. Common guidelines require at least 50' clearance distance between a well and a septic system tank or 150' between a well and a septic drainfield or leaching bed but you will see. 00 3. First published in 1991 as a replacement for the old BRE 151, BRE 365 has become the standard methodology for soakaway design in the UK. SUN ALLIANCE HOUSE, MOLESWORTH STREET, DUBLIN 2, or by mail order from GOVERNMENT PUBLICATIONS, POSTAL TRADE SECTION, UNIT 20 LAKESIDE RETAIL PARK, CLAREMORRIS, CO. The Soakaway Design Guide (July 2000) D4 f18. shorter distance is safe then it can be used. The distance from the road path to my house front is 6. into the main drainage. On 2022-01-31 by Tyler . It was invented back in the 1800s by Henry Flagg French The president of Massachusetts (agricultural college). This means, in total, your soakaway is 1600mm or 1. Step 2: If you do, apply online (you can do this if you have started or even finished the work on your property) Step 3: Book an inspection. 2 Small wastewater systems that discharge to the same aquifer that supplies a public water supply well He has built an external chimney attached to a wall directly opposite our gable end wall with two rainwater down pipes each into their own soakaways. . g. It serves the function of letting the wastewater coming from the septic tank to slowly soak into the underlying ground. Ground percolation tests may help to prove that siting the soakaway nearer to the building would not be an issue. 6m trench: 22. in you case you could run a pipe across the front of the building and join at 1 corner. The pre-settled sewage from the septic tank is released to the underground chamber, where it infiltrates into the neighboring soils. Part of. See more of Soakaway crates on Facebook. If you have opened a foul only drain, the. 19 July 2009 at 4:39PM. If the discharge is to a soak away a sampling point must be provided before the soak away. Septic tank and soakaway: An in-house full flush-toilet connected via pipe and plumbing fixtures to an underground watertight settling chamber (the ‘digester’) with a liquids outlet to a subsoil drainage/soakaway system. Don’t delay we can help you today! Call 0800 181 684 for all your drainage needs. Hi, we had a new combi fitted 10 months ago. If you’ll be using a private well for drinking water, however, note that many state departments of health require a minimum of 50 feet between a new septic tank and a well, according to APEC Water. The soakaway should be sized to contain the 1 in 10 year rainfall event. Permission from the EA will only be needed if this discharge exceeds 5m squared. Here’s a summary of the regulations for the installation of soakaway crates: Should be located at least 5 metres from the wall of any building. Using my garden hose he filled the hole with water, he then came into the house to inspect the building work. The 2015 edition takes effect on 1 October 2015 for use in England. Does this mean I can site my soakaway much closer to the boundary (2-3 m) than Building Regs require. 2 metres from a boundary. And the last thing you want is for that to happen in the depths of winter! So, if your condensate pipe is going to be external, ensure it’s larger than 22mm. How far away should a Soak away be from the house when laying imprinted concrete,and where should drains be fitted when drive. Same distances apply to the soakaway. You can buy USB borescope camera’s dead cheap off Amazon, in the region of £15, just plug into your phone or PC with USB. you have not changed the volume of the discharge or, if you have, it’s not more than 2 cubic metres (2,000 litres) to ground. The fitter assured us that it was all done correctly and that it has been fitted into a soakaway. So Is the site totally undev. INLET AND OUTLET TEES. ie Web: . Other binding regulations that have to be adhered to include being environmentally. Ironsides New Member. You should recieve some clips or ties with the crates. The main services for the house will be approx 20m away and I will be running mains water and power over that distance to the annexe. If you find the pipe, go out another 3 metres. So for a three bedroom house with a maximum occupancy of five people, using the above example of VP 18: 18 x 5 x 0. 6m 8. They should also be sufficiently far away enough from any other drainage fields, mounds or soakaways so that the soakage capacity of the ground is not exceeded. For deeper excavations please seek specialist advice. Steps: To build a basic stand-alone soakaway, dig a pit in your soft turf no less than 1m2 and 1. flameport said: You cannot build a soakaway next to your house. 5 3. 2019/11/10 Kristina Daddio . Failure of soak-aways can. 100 depth below invert = 1. Protecting property foundations. 5 metres (that is halfway between the house and the 'lack of pipe'). Mary Long-Dhonau explains: “ [The size of. 6. Ideally you prefer a soakaway rate of 1 x 10-5m/s in order to achieve the half emptying time. the distance between the pan outlet and the bottom of the Rest Bend must be less than 1. Design Considerations. Soakaway capacity to hold maximum daily rainfall (taken as 40mm) Storage depth minimum 1. Skip to content TastingBritain. Then. Most septic tanks will either drain in to a local watercourse or ditch, a soakaway system or a drainage field. Where an existing soakaway shows signs of instability it is recommended that after the remedial measures have been completed the bottom two metres of the chamber be sealed with a bituthene coating protected by a 150mm thick dwarf wall formed from Class ST4 concrete. ) So: V stor = 0. Septic Tanks and Soakaway Page 11 ANNEXURE 1 – SABS 0400-1990 Tables 1, 2 and 3 ANNEXURE 2 – Diagram of a Concrete Septic Tank ANNEXURE 3 – Diagram of Various Storage Tanks ANNEXURE 4 – Diagram of a Trench Soakaway ANNEXURE 5 – Diagram of a Circular SoakawaySeverntrent Screwfix Select. 500-gallon septic tanks can support a 2-bedroom home and are 79 inches long, 45 inches wide, and 56 inches deep. My septic tank is 15' feet from house following leach is 280' feet away from Septic tank and farthest end of leach field is almost 7' foot drop in elevation. Our HQ is based in Guildford, but we cover the whole of Surrey, Parts of Berkshire, Hampshire, West Sussex and South West London. Step 2. A inspection chamber must be installed between the septic tank and. The regulations state that a rainwater soakaway must be located at least five metres from the wall of a building and at least two and a half metres from a boundary. Building, planning and design. Fill the test hole with water to a depth of at least 300mm (12in). Common soakaway designs include trenches, pits, and modular systems made from various materials like concrete, plastic, or stone. How Far Should a Soakaway be From the House? You need to ensure its far enough from the structure to prevent damage — 5 metres is the. Hi everyone, Thanks for the info ref the soak-away test, suitability in clay and the common 70's practice. The distance you need to be away depends partly on the soil so if you are wanting to get close to the house (say, closer than 2 or 3 times the wall height) you would definitely want to talk to an engineer. Mary Long-Dhonau explains: “ [The size of your soakaway] will vary depending on a number of factors, including the size of your driveway area, number of. This can be caused by natural movements in the soil or say by driving a heavy car etc over the drainage field. FAQs about clearances for septic tank, drainfield, soakaway bed, & wells:. Standard regulations states that for any house up to and including 3 bedrooms, the minimum size sewage system that can be installed is for 5 persons, with one extra person added for each extra bedroom. In order store, the Wastage or sewage for 10-30 Days Septic tank is constructed and the same is designed. As luck would have it, the site of the soakaway is. Try to ensure that the sides and bottom of the excavation are straight and square. Continue to 5 of 12 below. Whats the minimum distance you would want to place your drain field from your septic tank A Soak Pit, also known as a soakaway or leach pit, is a covered, porous-walled chamber set in the ground that allows water to slowly percolate. This is normally an infiltration field through which. 5m = 19. Devon. This is to prevent any negative effects from the infiltrating water on the house foundations. I had actually noticed a bit of rainwater getting in to the crawlspace under the house in. Small bore solid-free sewer: An in-house toilet discharging to a septic tank (or on-site digester) with liquids disposal via Excavate a hole 300mm square to the proposed depth of the pit and trench. 0m; Soakaways to be honeycombed brickwork, hollow concrete rings, or storage cells; Volume required = design area x rainfall (0. If you need to do a percolation test then: 1. 42, the M5-15 min can be found as follows: M5-15 min rainfall = 20 mm x Z1 (for 15 min duration) Read Z1 from Table 1 in the column for the required rainfall duration, D, (eg 15 minutes), and.