What is a Drainage Plan?

There is one thing that all home builders in the Pacific Northwest are equally familiar with, RAIN! When it rains, water runs off the roof and into the gutters and downspouts and to the ground. But then where does it go?

This is where a drainage plan comes in.  A drainage plan is a proposed method for containing rainwater on your property, so that it doesn’t pool up at your foundation, or run off onto your neighbors property, perhaps even causing flooding. Many counties require drainage plans to control any increase in the amount of rainwater that runs off each piece of property as a result of the development of that property.

Why Drainage Requirements?

Almost any of the land development involved in building a stick-built home increases the amount of rainfall that may run off after a heavy rainstorm. For example, when an excavator grades the site, in the process of grading he will eliminate natural pockets and depressions in the soil which hold water until it is absorbed into the soil, and so instead of absorbing into the soil, the water will naturally runoff. Removing trees and brush reduces the amount of water absorbed by roots and the subsequent evaporation through the plants leaves. Covering ground with the house, driveways, and other impervious covers (definition: “not able to be penetrated, as by water”) prevents the underlying soil from accepting surface water. While bare land may readily absorb soil, a lawn of grass may make it less absorbent. Trucks driving over the ground during and after construction compacts the surface. These and other factors may combine to decrease the ability of native soil to absorb rainfall and cause the water to ‘stand’ or ‘pool’. If the pooling goes on long enough, it can eventually spill over into the crawl space, or into your neighbor’s property. To avoid this, a drainage plan is required to show that this has been addressed.

This is in your best interests. One of the most expensive homeowner insurance claims is the ‘water damage’ claim. Some insurers will actually cancel or raise your insurance premiums with only one water damage claim while they might tolerate two or three claims of another type. The reason is that repairs from water damage are expensive and often the damage goes into areas that may come back to haunt you and the insurance company later on. Additionally, if runoff from your site were to damage your neighbors’ home (which can happen) then this might leave you subject to liability claims. So having drainage management plan prepared by a licensed specialist is a protection for you.

Most of the time this sort of plan will be required before the building department will issue you a building permit. True Built Home has a list of subcontractors who can assist you with preparing a drainage plan and site plan.

The standard method of controlling stormwater runoff from the roof is to run the water into downspouts and then into a drywell. The criteria for this is that an area on your property must be available for the drywell which does not conflict with any septic drainfield, structure foundations, or public wells.

The soil must be sufficiently permeable (“Capable of being passed through or permeated, especially by liquids”). If your soil is such that you can use a conventional septic system or a pressurized sewage disposal system (due to excessively porous soils) the soil is usually considered to be sufficiently permeable. If you meet the criteria for this method your site plan should clearly show the direction of the water flow. This will usually suffice as a drainage plan. The standard method can be used if the following criteria are met:

Alternative Methods

If your proposed development does not meet the above criteria (e.g., the soils are not sufficiently permeable) or if you would prefer not to use drywells, you may propose an alternative drainage plan. Alternatives may include (but are not limited to) the following:

  • Detention Ponds
  • Underground Storage Tanks
  • Infiltration Trenches Retention Berms

A letter of certification from a licensed civil engineer which states the soil permeability and topography of the site will prevent any increase in stormwater runoff. To use an alternate method, the proposed development must be evaluated for its effect on stormwater runoff. The drainage system must be inspected by the Building Division prior to certifying the structure for occupancy.


This article represents general information about site drainage and in no way reflects instructions or an actual drainage plan. You are responsible for damage caused by stormwater runoff due to your development.


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