How to read where water goes on your own lot

Almost every expensive landscape mistake we are called to fix started as a drainage question nobody asked. Plants that drowned. A patio that sank. A bed that turned into a trench. Soil against a slab that stayed wet for eleven months of the year. In each case the water was doing the same thing it had always done, and the landscape was built as though it were not.

The good news is that a homeowner can learn most of what a designer would want to know, without instruments, in a single storm.

Go outside during the rain

Not after. During. Put boots on in the middle of the next real downpour and walk the perimeter of the house. You are looking for four things, and they are all obvious once you are out there and invisible from a window.

• Sheet flow — the wide, shallow film of water crossing open lawn. Note which way it runs and where it stops.
• Concentration — the places where sheet flow narrows into a stream. That is where a swale or a drain wants to be.
• Standing — the low spots that fill. Mark each one with a stick; you will not find them again when it is dry.
• Discharge — where the water finally leaves the property. If you cannot find this, that is the finding.

Then go out again four hours later

This is the measurement that matters most and the one almost nobody takes. A landscape that drains is dry underfoot a few hours after the rain stops. Water still sitting the following morning is not a weather event, it is a grading problem. Turf will thin there. Roots will suffocate there. Mosquitoes will breed there. And every plant you put in it will be a plant you buy twice.

Check the first ten feet

The strip of ground immediately around the house does more work than the rest of the yard combined. It should fall away from the slab — roughly six inches of drop across the first ten feet is the figure most builders are held to, and a great many properties no longer have it, because years of mulch top-ups have quietly raised the beds.

A simple test: lay a straight board out from the wall, put a level on it, and shim the far end until it reads level. The size of the shim is your fall. If it is nothing, or if it is negative, you have found the reason for most of what else is going wrong.

Look at the mulch line

Mulch should sit two to three inches deep and it should sit below the top of the bed edge, not above it. Beds that have been topped up every spring for a decade sit high, hold water against the foundation and shed rain outward rather than absorbing it. Resetting a bed to the correct height is unglamorous, cheap, and frequently the single most useful thing that can be done to a property.

What to do with what you find

Most of what a walk in the rain turns up is fixable with grading rather than engineering. A shallow swale — a broad, grassed dip you would mow straight over without noticing — moves an enormous amount of water and costs a fraction of a piped system. Piped drains earn their place where there is genuinely nowhere for the surface to carry it, or where the fall runs out before the property line does.