How to Mix 12.5% Sodium Hypochlorite (SH)
SH is the cheapest, fastest cleaner in pressure washing and the easiest one to get wrong. These are the ratios we give contractors at the counter.
Target strength at the surface
| Surface | SH strength hitting the wall |
|---|---|
| Vinyl and aluminum siding | 1% - 1.5% |
| Brick, stucco, concrete | 1.5% - 3% |
| Asphalt shingle roof | 3% - 5% |
| Heavy mold or north-facing walls | up to 2.5% siding, 6% roof |
Start at the low end. You can always hit it again; you cannot un-burn a plant bed.
Downstream (through the pressure washer)
A downstream injector dilutes roughly 10:1 [verify against your injector spec], so your bucket mix is the raw strength:
- House wash bucket: 5 gal SH + 5 gal water + 8 oz surfactant = ~6% in the bucket, ~0.6% at the wall. Bump toward straight 12.5% SH in the bucket for stubborn algae (~1.2% at the wall).
12V or roof pump (no dilution at the gun)
What is in the tank is what hits the roof.
- Roof mix: 5 gal 12.5% SH + 7.5 gal water = 5%. Add surfactant per label.
- Soft wash house mix: 1 gal SH + 9 gal water = 1.25%.
Use the SH Mix Calculator for any tank size.
Rules that save callbacks
- Pre-wet plants and rinse them after. Every job.
- Never mix SH with acids or ammonia products. Ever.
- SH loses strength in heat and sunlight. A drum left in a hot truck can drop from 12.5% to 8% in two weeks. Buy what you will use in 30 days.
- Use stainless or poly fittings on anything that touches SH. Brass corrodes.
Buying SH in Maryland
We fill fresh 12.5% SH in 5-gal buckets, 15-gal and 55-gal drums, and bulk tank fills for pickup in Gaithersburg. No hazmat shipping fees. See 12.5% Sodium Hypochlorite and House Washing Chemicals.