North Poway’s granite country teaches a simple lesson the hard way. Rock that looks eternal can weather into grit. That grit—decomposed granite, or DG—drains well and digs easy. It also washes out from under flatwork and footing edges when water runs in the wrong place.
This post is for hillside and estate owners on DG pads and cut slopes. It pairs with our note on Friars clay and fill. Different terrain, same city, same need to keep water from becoming a structural tool.
How granite becomes DG
Poway’s Public Safety Element describes Woodson Mountain Granodiorite as very resistant, forming high areas with large rounded boulders and exfoliation domes. It describes Green Valley Tonalite as highly susceptible to weathering, forming low gentle areas. Same broad “granitic” family, different behavior.
A Forest Service paper by Durgin on landslides and weathering of granitic rocks lays out a useful sequence: fresh rock, corestones in weathered material, decomposed granitoid, then saprolite. At the decomposed stage, the rock has largely become soil-like. On a residential lot that often means sandy DG that ravelson cut faces and erodes under concentrated runoff.
Our earlier Mt. Woodson boulder post focused on corestones and rockfall. Here the subject is the soft stuff that used to be rock—and still sits under many pads, driveways, and retaining-wall backfills.
Why water wins against DG
DG has little cohesion once it is fully weathered. A roof leader that dumps onto a cut face will cut a gully. A broken drip line against a stem wall will underground a void. A driveway that sheds stormwater onto a fill edge will nibble support from the downhill side.
The Safety Element lists saturation among the factors that induce ground movement, including prolonged rainfall, over-irrigation, leaky pools or pipes, and leach line discharge. It also flags oversteepening and undercutting of slopes—exactly what unmanaged runoff does when it carves DG.
The County’s Poway hazard annex keeps geologic hazards in the local planning conversation even when citywide rankings look modest. On a single hillside lot, rankings matter less than whether your downspout has been mining the same spot for ten winters.
Do / don’t list for DG pads
Do
- Extend downspouts to discharge well away from footings and slope faces
- Maintain positive grade away from the house on all sides you can control
- Inspect cut slopes after big rains for new rills and undermining
- Keep irrigation off steep DG faces
- Clear hillside brow ditches and wall drains so they actually run
- Photograph voids under stairs, patio edges, and deck posts when you first see them
Don’t
- Let a single scupper carve a slot beside a garage
- Build planters that hold water against the slab on a DG pad
- Cut a new path or bench below a rocky or DG slope without design help
- Assume rock outcrops nearby mean the pad itself is hard rock
- Ignore a leaning timber wall that retains DG—drainage inside the wall is part of the structure
Reading a DG cut face
Walk the cut above or beside the house after a storm. Fresh rills that were not there last year mean concentrated flow. Undermining at the base of a boulder or corestone means the soft matrix is leaving first—the same pattern discussed in rockfall education, now viewed as a drainage defect. Wet seams that drip for days after rain can signal perched water on less-weathered rock beneath DG.
Mark those spots on a simple sketch. Your future self, or an evaluator, will thank you.
Driveways, brow ditches, and wall drains
Hillside driveways are accidental aqueducts. If the driveway drains toward a garage slab or a top-of-wall, every storm tests the DG. Brow ditches at the top of cuts only work if they are continuous and outlet somewhere safe. Wall drains and weepholes only work if they are not clogged with fines washed from DG backfill.
A useful winter habit: after the first big rain, walk every outlet with a camera. If an outlet is dry while the ground above is wet, the drain is not doing its job.
When flatwork voids appear
A hollow under a stair tread or patio corner is easy to dismiss. On DG, it is often the visible end of a water path. Probe gently, photograph, and keep people and loads off unsupported edges until someone assesses bearing. Filling a void with random soil without fixing the water path usually recreates the void.
Coordination beats DIY walls
Timber and block walls on DG fail in familiar ways when drains are missing or grade behind the wall loads the stem. Redesign belongs with licensed professionals. We coordinate with licensed engineers as needed; we do not treat a blog checklist as an engineered wall design.
Footings, walls, and the edge of the pad
Hillside houses often combine spread footings, stepped foundations, and retaining walls. DG loss at a footing edge shows up as settlement cracks upstairs. DG loss behind a wall shows up as tilt, bulging, or washouts at weep holes. Treat drainage as part of hillside foundation care and retaining wall care, not as landscaping alone. Our foundation drainage page covers the broader checklist.
If a wall needs repair, cost factors include access, height, surcharge from slopes or driveways, whether drains must be rebuilt, and what a soils or structural review requires. No honest flat price covers every Old Coach or Garden Road condition.
After fire or long drought, then rain
Burned slopes lose plant cover that once slowed runoff. Drought-dried DG can crust and then erode when the first heavy rains arrive. Walk the property after those first storms. Look for new debris fans, exposed footing edges, and outlets that used to be buried.
Stay calm. Most of what you find is maintenance information, not a midnight emergency. Sudden voids under a primary bearing wall, or a wall that moved overnight, are the moments to call for help the same day.
Where we fit
We work hillside residential foundations and related structures across Poway. We coordinate with licensed engineers as needed when slope stability, wall design, or engineered drainage is part of the solution. Serving the whole city means DG pads and Friars fills both get honest scopes.
Ready for a look? Start with a Free Quote. Bring photos of the downspouts, the cut faces, and any voids under flatwork. On DG, the water story is usually visible if you walk the lot on purpose.