Decks that move underfoot, pool water, or start to sag within a couple of seasons usually have the same problem: the subframe was rushed, undersized, or set out poorly. If you want a long-lasting deck, then installing the subframe properly is a high priority.
The subframe carries the load, keeps the deck level, and gives your boards the support they need over time. Get this part right, and the rest of the build is far more straightforward. Get it wrong, and even premium decking boards will struggle to look good for long.
Before you install a decking subframe
There are planning regulations on the size, height and positioning of decks. It is worth visiting the Planning Portal to check if you may need to apply for permission before going any further.
Start by deciding what the deck needs to do. A simple seating area in a sheltered garden is one thing. A large family deck that has to cope with heavy hardwood garden furniture, large planters full of soil and regular use by the whole family requires a lot more thought. The size of the deck, the ground conditions and the finished height from ground level all affect how the subframe should be built.
You also need to think about drainage from the outset. Decking should never trap standing water. In most gardens, that means allowing for a slight fall so rainwater runs away rather than sitting on the boards or around the frame. If the ground is soft, sloping or prone to holding water, that will influence your foundation choice as well.
Timber selection matters here. Treated structural timber is the usual choice for a timber deck subframe, and it is worth using timber sized for the span rather than trying to save money with smaller sections. Subframes are not the place to cut corners.
Setting out the area
Mark out the footprint with pegs and a string line. If your deck is square or rectangular, you can check that the corners are square by measuring the diagonals from corner to corner. If those diagonal measurements match, your layout is square. For decks that have some 90-degree corners, you can use 5-4-3. Measure out from the corner along one side three feet/metres (the units you use don't matter) and mark. Then measure out four of whatever units you use along the other side, and mark. If the corner is square, the distance between your two marks will be five units. Time spent ensuring your layout is spot-on at the start can save you a lot of extra work later on and improve the look of the finished decking.
At this stage, decide whether the deck will be ground-level or raised. A low-level deck may sit on pads or supports close to the ground, while a raised deck will need posts, bearers and more substantial bracing. The higher the deck, the more structural planning is required.
When you have established your finished deck height. From there, work backwards to allow for the thickness of the decking boards and the joists below. This indicates the height of the top of the subframe.
Choosing the right base and supports
There is no single answer that will suit every garden. How you install your decking subframe depends on what base it is sitting on.
For many low-level domestic decks, concrete pads or slabs provide a stable base for the frame to sit on. These work well on firm ground, keeping the timber off direct contact with the soil.
If the deck is raised or the ground is uneven, posts set into concrete are the better option. In that case, the posts support bearers and joists. Concrete support posts remove the worry of the posts rotting in a few years. Using posts allows more flexibility on sites with uneven terrain, but it also requires more attention to post spacing, frame layout, bracing, and the type of fixings and how they are used.
Whatever support system you use, your goal is the same: create solid, evenly spaced load-bearing points, leaving no opportunity for the frame to twist or settle unevenly later on.
Installing the decking subframe
Once you have marked out your deck base on the ground and installed and levelled your concrete pads or deck support posts, it is time to build the support framework. Remember when planning and installing posts or pads to allow for water drainage. There needs to be a small fall along the boards so that water runs off them.
It is usually best to begin with the outer frame. Cut your treated timber to length and build the perimeter first, checking for square and levels as you go. Use only exterior-grade structural timber screws and galvanised bolts. Using light-duty or cheap fixings that are not rated for outdoor use will come back to bite you in a few years' time. The perimeter frame sets the shape for everything that follows, so take your time, making sure corners are square and your levels are spot on.
Once the outer frame is in place, install the joists and noggins. Joist centres depend on the type of decking board you plan to use. As a general rule, most timber decking boards are supported at about 400mm centres, though some layouts benefit from closer spacing, particularly if boards are laid diagonally or the deck will be subject to heavier use. It is important that the joists are spaced no further apart than the maximum specified by the deck board manufacturer. Composite boards often have their own special manufacturer's requirements, so always check those before fixing the frame.
Keep the joists consistent and parallel. A tape measure alone is not enough - use a string line and level to make sure the frame remains true. Any inaccuracy in the subframe will show up quickly once decking boards are screwed down.
If your frame is large, fit noggins between joists to reduce twist and improve rigidity. These short cross pieces help maintain consistent spacing and give the structure a more solid feel underfoot. They are particularly useful on bigger decks or where the frame sits off the ground.
Where joists meet bearers or perimeter timbers, use suitable structural fixings or strong decking joist hangers if needed. The best fixing method depends on the design, but the basic principle is - joints should be tight, secure and able to withstand the loads that will be placed on the finished deck.
Levels, falls and ventilation
One of the most common mistakes in decking builds is chasing a perfectly level platform without thinking about water. In practice, decking needs a slight fall to encourage runoff. That fall is subtle, but it makes a difference to how well the finished deck performs in wet weather.
Ventilation is just as important. Timber subframes should not be boxed into damp, stagnant conditions. Air movement beneath the deck helps the structure dry out after rain and reduces the risk of premature decay. If your deck is very close to the ground, allow as much airflow as the design reasonably permits.
It is also good practice to keep timber clear of soil and standing water. Even treated timber lasts better when moisture exposure is reduced.
Getting spacing right
Spacing is not only about the decking boards. The support points under the frame need to be spaced correctly for the timber section being used, and the joists need to be set according to the board type and span. If support points are too far apart, the frame can flex. If joists are too widely spaced, the deck surface may bounce or feel weak.
This is where project specifics matter. A small domestic platform with standard timber boards may be relatively straightforward. A larger deck with steps, balustrades, outdoor kitchens or hot tubs is another level entirely. Extra weight means stronger framing, closer support and more careful design.
If in doubt, it is always better to build a little stronger than the bare minimum, provided the design remains properly ventilated and drained.
Protecting the timber subframe
Even pressure-treated timber benefits from sensible protection measures. Applying joist tape to the top of joists can help reduce water ingress around screw penetrations and standing moisture on horizontal surfaces. It is not essential on every build, but it can be a worthwhile addition if you want to improve long-term durability.
Make sure all cut ends are treated with a suitable end grain preservative. The treatment in structural timber does not fully protect fresh cuts made on site, and those cut ends are often where problems start.
Use fixings suitable for external timber construction. Outdoor conditions are hard on metalwork, especially where timber treatment chemicals are involved. The wrong screws and brackets can corrode faster than expected.
Common mistakes when installing a decking subframe
Most failures come down to a handful of avoidable issues. Building directly onto bare soil, skimping on support points, using undersized timber and failing to allow for drainage are all common problems. So is assuming that because the deck looks level on the day it will stay that way without a proper base.
Another mistake is forgetting the final deck layout while building the frame. Board direction, picture-framing details, step positions, and balustrade posts all affect where extra support may be needed. It is worth planning the surface finish before the subframe is started
For DIY customers, the big trade-off is usually time versus accuracy. A quick build can be tempting, but careful set-out and proper support save far more time than they cost.
When the job needs a more heavy-duty approach
Some decks need more than a basic garden frame. If the site is sloped, the deck is raised, or you are carrying significant weight, the structure may require input from a structural engineer or at least a more advanced specification. The same applies if the deck connects to a property or forms part of a larger landscaping scheme.
That does not mean every project is complicated, only that subframes should match the job. A simple platform deck and a raised entertaining space are not built to the same standard, and they should not be priced or planned as if they are.
For homeowners and trade installers alike, buying the right structural timber, fixings and support materials from the start makes the build far easier. The team at Crestala Fencing Centre believes that the practical side of the job matters just as much as the finished look and provides support for DIY and trade deck builders.
Take your time with the groundwork, check every level twice, and treat the subframe as the real structure rather than the hidden part nobody sees - because that is what gives a deck its strength for years to come.
