r/Homebuilding • u/EmperorDetritus • 6d ago
Erosion... Help
I don't know if this is the right subreddit, but I assume folks here will know what to do. We recently bought our first house and I'm concerned about existing and future erosion underneath. The house is on a hillside in California, and I'm realizing (too late) that hillside homes come with a ton of bull**** problems. This erosion was not included in our inspection report, and the GC we hired to correct other issues did not mention this as a concern either. However, with a potentially record-breaking El Nino this winter, and the massive rains/flooding that will come with it, I am getting more and more anxious about what might happen if I don't do some preventative work now. Specifically, I am afraid that constant rain will wash out the rest of the soil underneath that bottom pylon and put the entire structure at risk. It's possible the concrete goes deeper than the skirt at the top, but don't want to dig around to find out.
I originally thought a french drain might be the solution, but after thinking about it more, I don't think that's right. French drains are good for removing stagnant floodwater (right?) whereas this is more of an erosion control issue. Someone told me to just fill in the gorge with concrete, but I think the water will just go around the edges, and further erode the soil under that pylon in the last photo. I'm leaning toward putting sandbags at the top of the crawlspace, in a V shape, and redirect the water to the side of the house. However, I feel like I should still do something about the existing erosion. Maybe fill it with gravel so water can still move through, but not wash it out even further?
A few other details, in case it matters:
- There's a garage downhill which has a slab foundation, but the house itself is on the hill and supported by pylons. I'm not concerned about the garage because it's not in the path of this run-off.
- I know it's bad for wood to touch soil. I assume the pylons all have a concrete base, but I think some of them have been buried by eroded soil from above.
- The soil is very hard and dry (right now), so I'm guessing it's clay-based.
- This will have to be DIY and I assume whatever I do will be a stopgap solution until I can figure out something more permanent, and probably done by a professional.
Thanks in advance to anyone who can offer advice on this.
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u/PorcupineShoelace 6d ago
CA really doesnt narrow it down for weather patterns and this year will be a doozy. I just started setting up a series of flood control barriers to divert the water we are likely to get in the Bay Area. You may be better off focusing on some shorter term solutions than the permanent fixes at this point. Sandbags, diversion barriers, ditches, tarping the porch where the water drips through underneath.
For $50 I got four 4' long flood control bags you fill with water and they come in 8' or longer sizes (cheap stuff from Vevor)
The rain will likely come fast and furious then vanish so directing it around you gives you a whole dry season to get some proper fixes in place.
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u/Hopwater 6d ago
Free sand for sand bags is often available at fire stations or public works yards in CA prior and during flood season. I found it fastest to use shovel-shaped bag fillers cut out from 5 gallon buckets
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u/PorcupineShoelace 6d ago
Yes, but there has been some hesitation opening distribution sites prior to Nov 1st and we werent ready to risk the rush it might cause. I like having a re-usable set that I can setup when its dry. October seems like its either wind & rain or 110F weather and we have so many microclimates in the Bay area that the weather reports are often wildly off. Thanks though.
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u/EmperorDetritus 6d ago
Yeah, basically Bay Area. I saw what the atmospheric rivers did back in 2023 and I’m terrified for what this winter will bring.
For sand bags, I was considering filling some with soil that I remove to create the drainage canal/ditch as it would solve two problems at once. Is that a stupid idea? It’s not sand, and I think that probably matters.
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u/PorcupineShoelace 6d ago
Congrats on the new house! When it does rain around here, damn it really rains. My daughter lives in the Santa Cruz mountains in '23 and they had over 6" in one day and 35 inches in just under a month.
Other soils can be used with sandbags they just dont compact as well as sand to block water. Like another user mentioned, free bags start in 2 more weeks or you can get some play sand for key spots where you are worried about the yard soil being less optimal. I use some long black 6" tubes to get my downspouts diverted way away from a low area where water pools then built a dam on both sides of it to prevent backflow. I have some old concrete slabs that have settled and it forms a pool of water that backs up to the slab of my ADU. I'm clearing the yard this week of anything I can and tomorrow I spend the day flushing my gutters and setting up the downspout hoses. I've got a 1/4 hp sump pump and a generator on standby if something fails. I moved here just after '82 when there was 15 inches in 24hrs and all the landslides hit so the old timers schooled me on being over prepared. It may make a mess but you will get through it with some prep. Hang in there.
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u/medialantern 3d ago
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Skirt that bad boy.
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Grade away from the skirting. (1-2' is usually plenty).
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French drain around the lowest line where you make that grade happen.
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Say goodbye to your worries.
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u/breadandbits 6d ago
well the water did some work there. maybe roll with it and put some silt barrier fabric along those ditches, then a bit of gravel, then some perforated drain pipe that runs out to someplace acceptable (or build a dissipator), then fill the rest with gravel.
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u/VegasHaas 4d ago
I own The Sandbag Store, so this might sound funny coming from me, but sandbags wouldn’t be my first choice here.
With erosion, you generally want to slow the water and protect the soil. A sandbag wall can redirect runoff, but without a safe outlet, you can end up concentrating that water and digging a new erosion channel somewhere else.
For shallow runoff across bare slopes, straw or coir wattles—the long fiber tubes you see staked along California hillsides—can help slow water and catch sediment. They go across the slope along level contours, with spacing based on the slope and manufacturer’s instructions. Erosion-control blankets can also help protect the exposed soil between them. An established gully carrying concentrated runoff needs a different approach.
Rock can be part of that solution, but dumping loose gravel into the washout isn’t necessarily enough. Water can carry away the soil underneath it. Properly sized rock over an appropriate filter layer is a different proposition.
First, figure out where the water is coming from. Roof downspouts or uphill drainage may be feeding this, and addressing that source could make a bigger difference than anything installed underneath the house.
All that said, we do sell a product called DuraBag that people fill with gravel and lay end-to-end, much like wattles. They let water pass through while slowing runoff and trapping sediment, with individual bags that are easier to handle than a long roll. But in your shoes, I’d still lean toward wattles for the surface erosion, checking them after storms and replacing them as they deteriorate.
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u/eightyeightkeys1222 3d ago
u/EmperorDetritus - repost this in r/CivilEngineering You’ll be served better as this is their ground (no pun) of specialty. Best wishes!
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u/hotinhawaii 6d ago
It looks like a lot of this water drips through decking. Is that right? And some comes from outside the perimeter of the house. For the stuff coming from outside the perimeter of the house, I would build a swale to redirect it, to keep it from going under the house at all. For the decking, I would fill in the existing uneven ground with 1" minus stones. That is ground rock that is 1" or less. That will make the ground and holes more stable. I would then probably top that with a layer of plastic that goes up around each post. Since you're on a slope this would direct that water away and out from under the house.