Drainage Against a House Wall: Gravel, Channel and French Drains

The Principle: Get Water Away, Fast

Before comparing the three tools, it helps to be clear what any of them is trying to achieve. Brick and stone are porous, and if the ground at the base of a wall stays wet, that damp works its way up and in. Older houses had a step down from the damp proof course to the ground, so rain ran off; modern paving and raised beds often bury that step, and the damp proof course with it.

So the job is the same: keep the ground at the wall lower than the damp proof course, let water move away, and give it somewhere useful to go. The method changes with the site. The principle doesn't.

Three Ways to Keep the Wall Base Dry

Here's how a designer chooses between them: none is simply better than the others.

Gravel Margin

The simplest option is a gravel margin: a band of clean, free-draining stone between paving and wall, keeping the level down and the wall clear of soil and mulch. It's cheap, unobtrusive, and lets the wall breathe.

The honest weakness: a gravel margin isn't a drain. It doesn't take water anywhere; it just stops the ground piling up wet against the brickwork and lets rain find its own way down. On heavy clay, water can sit under that gravel for days with nowhere to go.

Channel or Slot Drain

Where paving needs to come close to the house, often because a threshold demands a level, flush finish, a gravel margin isn't an option. A channel or slot drain does the job instead: a linear channel with a grating at the paving edge, collecting surface water and carrying it away.

The honest weakness: it only works if it has somewhere proper to discharge to, and it needs clearing. Grit and leaves collect in the channel, and a blocked drain does nothing at all, or worse, holds water exactly where you were trying to protect.

French Drain

Where the problem isn't surface water off paving but the ground itself staying saturated, a French drain does a different job: a trench with a perforated pipe bedded in free-draining stone, wrapped in geotextile fabric, collecting water in the ground and carrying it away.

The weakness is twofold: it's only as good as where it discharges to, and if the geotextile is skipped, or degrades unseen, the pipe silts up underground, where you won't know until the ground stays wet again.

The Question Everyone Forgets: Where Does the Water Go?

All three of these are collection methods, not destinations. A gravel margin lets water down. A channel drain and a French drain carry it somewhere else, which is the part most often missed.

Water needs to go to a soakaway sited properly away from the house, to a surface water drain, or to a designed feature such as a pond or rain garden. What it must never go to is the foul drain; connecting surface water into foul drainage isn't acceptable, and in most cases it isn't legal either. What you're permitted to connect to needs confirming for that property. I won't guess at it in a design, and nobody working on your garden should either.

Falls: Everything Moves Away From the House

Whichever you use, the ground and paving around it must fall away from the wall, never towards it. That's non-negotiable in any drainage design near a house. I won't give a gradient here; the right fall depends on the material and the site, and needs setting out properly. The direction, though, is never in question: away from the house, always.

Kent Clay Makes This Harder

I'd be doing readers a disservice if I didn't say plainly that a lot of Kent, and parts of East Sussex, sit on heavy clay. Free-draining stone and pipework work by letting water move through the ground; clay barely lets it move at all.

A soakaway dug into clay can perform poorly, filling in wet weather and draining away slowly, because clay walls won't let water disperse the way sand or chalk would. That doesn't make drainage pointless on clay; it means designing for that limitation from the start, sometimes with a larger soakaway, a different discharge point, or a proper assessment by a drainage contractor. Soakaway design and garden drainage as a whole are their own subject, one I'll come back to.

The Maintenance Nobody Writes About

This is the section that decides whether any of it still works in five years, the one most articles skip.

A gravel margin left alone slowly fills with soil and leaf litter until, eventually, it's just soil against the wall again, the exact problem it was built to solve. A channel drain packs with grit and leaves until the grating does nothing. A silted French drain is an expensive trench doing nothing, and being underground, you won't know until the damp reappears.

None of these fail because the design was wrong; they fail because nobody kept them clear. Design with access in mind, so a margin can be raked out and a channel cleaned, treated as routine as clearing gutters.

Designer, Builder and Drainage Contractor

As a garden designer, my job is choosing which of these tools suits a site, specifying it properly, and getting the levels and falls right on the drawing. What I don't do is dig the trench, lay the pipe, or make the physical connection into an existing drain.

That work belongs to a builder or a genuine drainage contractor, and wherever there's a connection into existing drainage, involving Building Control or the council is the right call, not an optional extra.

Conclusion

Drainage against a house wall isn't complicated in principle: keep the ground low and dry, pick the right tool for the site, and send the water somewhere proper that stays clear. It's the follow-through that gets missed.

If you're looking at a patio, a raised bed or a new level near your house and you're not sure which of these applies, it's worth talking it through before anything gets built.