A deck can look spot-on from above while the timber underneath starts failing years too early. The deciding factor is often not the decking board, but whether you selected H3 vs H4 treated timber correctly for the subframe, bearers and posts.
For Sydney outdoor projects, both treatments have a place. H3 is the usual choice for above-ground external framing. H4 is made for timber that contacts the ground or sits in conditions where moisture exposure is far higher. Choosing more treatment than the job needs can add unnecessary cost and weight. Choosing less can leave structural timber exposed to decay and termite attack.
What H3 and H4 timber treatments mean
The H rating is a hazard class. It describes the conditions the timber treatment is designed to handle, rather than the timber's strength or visual grade. A treated-pine label may also show structural grades, dimensions, treatment type and retention information. These are separate details, and all need to suit the build.
H3 treated timber is intended for exterior, above-ground use. It is suitable where timber is exposed to weather, condensation and termites but can drain and dry between wet periods. It is commonly used for deck joists, bearers, pergola members, garden edging above soil level, cladding battens and other external framing.
H4 treated timber is intended for in-ground contact. The treatment penetrates further or is retained at a higher level to deal with constant moisture, soil organisms and termite risk. It is the standard choice for posts set into the ground, retaining-wall structures where appropriate, and other timber that will be in contact with soil or freshwater.
Neither hazard class replaces good construction practice. Water must be able to escape, fixings must suit treated timber, and timber connections need to be detailed so they do not trap moisture.
H3 vs H4 treated timber for decks
For a conventional raised deck, H3 structural timber is generally the right material for the joists, bearers and perimeter frame. It is designed for exterior exposure and is more economical than using H4 throughout the structure. Provided the frame is above ground, well ventilated and not sitting in wet soil, H3 will suit the job.
H4 comes into the picture where the deck is supported by posts installed into the ground. Those posts need to tolerate soil contact for the life of the structure, so H4 is the sensible minimum treatment class. A deck may therefore use H4 posts below and at ground level, with H3 bearers and joists above.
Low-level decks need a closer look. If the underside has poor airflow, stays damp after rain, or is very close to soil, an H3 subframe can be exposed to much harsher conditions than a typical raised deck. Raising the frame, improving drainage, installing a gravel base, or using a pedestal system can often be a better long-term answer than simply changing every component to H4.
For decks over concrete, H3 framing is normally suitable where it is kept off the slab with packers, pedestals or other supports that allow drainage. Do not place standard H3 timber directly onto a slab that ponds water. The timber may be technically above ground, but prolonged wet contact creates the same practical problem: it cannot dry.
A simple selection guide
Use the exposure condition, not just the project name, to choose the treatment:
- Choose H3 for above-ground deck framing, pergolas, external wall battens and bearers that are clear of the soil.
- Choose H4 for posts set into the ground, timber embedded in soil and members likely to remain in ground-contact conditions.
- Review the whole support detail for decks close to earth, over poorly drained concrete or in permanently shaded locations.
- Check engineering requirements before substituting sizes, spans or post locations on structural work.
H4 is not automatically stronger than H3
A common mistake is to treat H4 as an upgrade in every respect. H4 treatment improves resistance to the hazards associated with ground contact. It does not automatically mean the piece of timber has a higher structural grade, a larger span capacity or a straighter finish.
Structural performance still depends on species, stress grade, size, span, support spacing and load. A 90 x 45 H3 MGP10 joist and an H4 post are designed for different roles. Select the treatment class for exposure, then select the timber grade and section size for the load it must carry.
There can also be practical trade-offs. H4 timber is often heavier and can cost more than H3. It may be less necessary for above-ground framing that is properly built and protected. For visible work, remember that treated pine is primarily a structural product. If appearance is central to the project, use a suitable decking board, cladding product or finish over the structural frame.
Details that help treated timber last
The right hazard class gives the project a sound starting point. The installation details determine whether it performs as expected. This matters particularly beneath composite decking, where the surface is low-maintenance but the timber subframe still needs attention.
Keep framing clear of standing water and build in fall where required so water moves away from the structure. Do not fill the space beneath a low deck with organic material that holds moisture against the frame. Leave sufficient airflow around joists and bearers, especially along enclosed sides and near walls.
Joist protection tape is a worthwhile addition over horizontal framing members. Deck boards and fasteners create small areas where water can sit on top of joists. Tape helps protect that upper face, which is one of the most exposed parts of a subframe. It is an added layer of protection, not a substitute for selecting H3 or H4 correctly.
Any site-cut ends, drilled holes or notches should be treated with an approved brush-on preservative compatible with the timber treatment. Factory treatment protects the original timber, but cutting exposes untreated end grain. This is especially relevant for H4 posts trimmed on site and H3 deck joists cut to length.
Use compatible galvanised, stainless steel or suitably coated fixings and connectors. The right choice depends on the timber treatment, location and degree of weather exposure. Mixing unsuitable metal hardware with treated timber can lead to premature corrosion, staining or connector failure. For coastal sites, exposed pool areas and high-moisture locations, stainless steel hardware may be the better specification.
Plan the framing and surface together
A reliable deck starts with matching the subframe to the decking system. Composite boards have their own joist-spacing, fixing and ventilation requirements. Natural timber decking has different movement, fixing and finishing needs. Confirm board direction, joist centres, fascia details, stair framing and drainage before ordering timber.
It is also worth allowing for the whole material package at the start: structural H3 and H4 treated timber, joist hangers, post supports, bolts, screws, protection tape, decking fixings and finishing materials. Ordering only the boards and a rough estimate of framing is how small delays become expensive return trips.
For owner-builders, a clear plan showing dimensions, support locations and finished deck height makes timber selection much easier. For trade jobs, verify the specified treatment class rather than assuming that all treated pine is interchangeable. Green Forest Timber supplies structural treated timber alongside decking, framing hardware and accessories, making it easier to assemble the materials required for a complete outdoor build.
Make the exposure decision first
The most useful question is not whether H3 or H4 is better. Ask where each piece of timber will sit, how often it will stay wet and whether it touches the ground. Use H4 where soil contact demands it, use H3 for properly drained above-ground framing, and give both the ventilation and detailing needed to do their job for years.