Top Retaining Walls Christchurch builds H5 treated pine retaining walls in Christchurch, and we are fussy about the treatment level because it is the single easiest thing for a builder to get wrong. H5 is the grade for timber buried in wet ground. H4 is not, no matter how similar the two look stacked on a rack.
New Zealand grades treated timber by where it is allowed to sit. H4 covers timber in contact with the ground in normal conditions, like a fence post. H5 covers timber in ground contact where it stays wet, which is exactly what a retaining wall post does, because there is saturated soil pressed against it for its whole life.
Poles for retaining walls come 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 on delivery and we will 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 |
The rails between the posts sit above ground level, so H4 is the right call there and it costs less. The posts go in the ground and must be H5. Builders sometimes run H4 posts because a supplier had them in stock, and the wall looks perfect for eight years before the posts start to go at ground line.
Every time we cut a pole we brush the fresh end with a timber preservative, because the cut face is untreated heartwood and it is the first place rot starts.
On the flat western and northern suburbs, where the ground is deep Canterbury gravel, an H5 pole wall is a straightforward job. Good bearing, easy digging, and the free draining gravel behind helps the drainage system do its work.
Out east through New Brighton, Bexley and Aranui the water table often sits within a metre or so of the surface. H5 still works there, but the discharge point for the drain coil matters more, and sometimes we have to run the coil further than you would expect to find a fall.
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. Use H5 for anything below ground. H4 is fine for the rails above ground.
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 hole. Once they are concreted in you cannot check.
Not for durability. The treatment does that job. Some owners stain the visible face for looks, and if you do, leave it a few months so the timber dries out enough to take the coating.
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.
