Top Retaining Walls Lower Hutt builds H5 treated pine retaining walls across Lower Hutt, and we are fussy about treatment level because it is the easiest thing on a job to get wrong and the hardest to fix later. H5 is the grade for timber buried in wet ground. H4 is not, however similar the two look stacked on a rack at the yard.
New Zealand grades treated timber by where it is allowed to sit. H4 covers ground contact in normal conditions, which is a fence post. H5 covers ground contact where the timber stays wet, which is exactly what a retaining post does, because saturated soil is pressed against it for its entire life.
Retaining poles fall under NZS 3605 and get treated to the H5 specification in NZS 3602. Every pole carries a brand or tag showing the grade. We check it when it comes off the truck and we are happy to show you.
| Post treatment | H5 to NZS 3602, poles to NZS 3605 |
|---|---|
| Rail treatment | H4, above ground only |
| Post diameter | 150mm to 200mm small end diameter depending on height |
| Cut ends | Brushed with timber preservative on site |
| Fixings | Hot dipped galvanised |
| Expected life | 25 to 40 years with drainage working |
| Without drainage | Commonly 10 to 15 years even at H5 |
Rails sit above ground, so H4 is right there and it costs less. Posts go in the ground and have to be H5. Builders occasionally run H4 posts because that is what the yard had in stock, and the wall looks perfect for eight years before the posts start going at ground line.
Every time we cut a pole we brush the fresh end with preservative. The cut face is untreated heartwood and it is the first place rot takes hold.
The Hutt Valley catches serious rain coming off the Tararua Range, and the southerlies drive water sideways into anything standing up. Weathered greywacke clay on the hill suburbs holds that water rather than letting it through, so a post up in Normandale or Kelson spends more of its life saturated than the same post would on free draining valley gravel.
H5 handles it. What it cannot do on its own is survive with no drainage behind the wall, which is why we quote the two together and will not separate them.
Every wall we build gets drainage. At least 300mm of clean free draining metal, a perforated coil at the base running to a real outlet, and geotextile over the top so silt cannot block it. It is not an optional line on our quotes.
No. H4 is rated for ground contact but not for the constant wet ground contact a retaining post lives in. H5 below ground, H4 for the rails above, every time.
Treated poles carry a brand, tag or stamp with the hazard class on them. Ask any builder to show you before the poles go in the ground, because once they are concreted in you cannot check.
Not for durability, the treatment covers that. Some owners stain the visible face for looks. If you do, give the timber a few months to dry out first or the coating will not hold.
Our old wall was rotten right through and leaning into the neighbour's place. They pulled the lot out, put proper drainage behind the new one and had it finished in three days. Swept the site clean every afternoon before they left too. Tidiest crew we have ever had here by a long way.
They sorted the consent, which was the bit I was dreading. Dealt with the council so I never had to, and kept me in the loop the whole way through. The driveway sits right above it and has not shifted a millimetre.
Steep section, shocking access, and they still made it look easy. They suggested two smaller walls instead of one big one, which saved us thousands in engineering fees. The terrace in between is now my favourite part of the whole garden.
Call [PHONE] or answer three quick questions. The site visit and the written quote cost you nothing.
