Water and external systems · Decks & Paving
Timber Deck Framing and Board System
This entry explains how a timber deck behaves as a framed assembly, so that the frame, the void beneath it and the fixing method are read as the governing parts rather than the board profile on top.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What timber deck framing is
A timber deck is a small piece of structure standing in the weather. Ground supports transfer load into the earth or onto an existing hard surface, beams gather that load, joists span between the beams, and a spaced board surface sits on top. The visible boards are the last part of that chain and the least structural part of it.
What separates a deck from a paved terrace is that its surface is deliberately open. Rain passes between the boards rather than running off them, so the frame below is repeatedly wetted and has to be able to dry again between wettings. Everything about the void beneath, from its ventilation to whether anyone can ever reach into it, follows from that single decision.
The nearest sibling is the pedestal-supported terrace paving system, and the distinction is visible from the garden. A deck spans: joists carry the surface between supports the deck itself provides, so the framing is the entity being described. Pedestal paving does not span at all, because loose units rest on adjustable supports over a structural deck that already exists.
Post bases, ground plates and bearing pads appear here as a component role rather than as a foundation in their own right. How they are formed, and whether the ground beneath can carry the platform at all, is site-dependent and belongs with the people who assess the ground rather than with a description of the assembly.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- joisted timber deck
- framed deck substructure
- deck frame and boards
- garden deck assembly
- raised timber deck structure
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Carry occupancy loads on an external platform and transfer them into ground supports or an existing structure.
- Hold a usable, reasonably level walking surface at a chosen level above the ground beneath it.
- Let rainwater pass through the surface rather than pond on it, and let the frame beneath dry afterwards.
- Keep structural timber clear of continuous ground contact and out of trapped moisture wherever the design allows.
- Define an edge condition where the platform meets steps, planting, a wall or a change of level.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportGround supports, posts and beams
Posts, plinths and beams that receive the frame and pass load into the ground or onto an existing slab. Their arrangement sets the framing layout, and their detail decides whether timber stands clear of wet ground or sits directly in it.
- SubstrateJoists and trimmers
The members the boards actually bear on and fix into. They set the direction the surface runs, carry load back to the beams, and are trimmed around openings, steps and posts wherever the framing has to be interrupted.
- Cavity or voidUnder-deck ventilation void
The open space between the underside of the frame and the ground or hard surface below. It is the route by which air moves through the assembly and water leaves it, and it is also the only means of ever inspecting the frame.
- Finish surfaceSpaced deck boards
The trafficked layer, laid with deliberate gaps so water drains through rather than sitting on it. Board profile, orientation and moisture content at the time of laying govern how much the surface cups, twists or lifts once it is in service.
- AttachmentDeck fixings and connectors
Screws, concealed clips, joist hangers, post bases and strapping. Fixings hold the surface down while still allowing seasonal movement, and their corrosion behaviour in a permanently damp, timber-contact position is a design matter rather than an afterthought.
- Drainage planeBoard gaps and the path water takes below
Water that passes the surface has to keep going. Where the frame stands over a solid surface, the falls of that surface and the freedom of its outlets decide whether the void drains or holds standing water against the timber.
- Edge and terminationPerimeter, fascia and step junction
The exposed edge of the platform, where boards, fascia and any step or landing meet. It is the point at which the frame is most visible, most weather-exposed, and most often relied on to receive guarding fixings.
- ProtectionPreservative treatment and applied finish
Factory treatment of the timber and any site-applied coating are separate things doing separate jobs. Cut ends, notches and drilled holes expose untreated material, and coatings weather differently on horizontal faces than on vertical ones.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The gaps in the board surface are the reason the rest of the assembly has to be arranged the way it is. Because the surface is not a roof, the frame beneath it is a wetted zone, and its ability to dry depends entirely on the void. Close that void in with solid skirting, or set it over flat impervious ground with nowhere for water to go, and the frame stays damp after every rain while the boards act as a lid over it.
Fixing method and board behaviour are locked together. A board held rigidly at every crossing cannot move, so seasonal movement is expressed as splitting, cupping or fastener withdrawal instead. A board held by clips that permit movement transfers that movement into the clips and the joist edge. Neither is a free choice, because each moves the problem somewhere else, and where it lands has to be somewhere the assembly can tolerate it.
Ground contact is where decay starts, and it starts at the component least likely to be looked at. Posts and bearers set directly on soil, or embedded in a solid surround, are wetted continuously rather than intermittently, and no finish applied to the boards above changes that. Separation between timber and ground, together with drainage of the ground itself, does more for the frame than any coating applied later.
Where the deck runs up to a building it stops being a garden structure and becomes part of the wall's moisture problem. A raised external surface can bring the finished level up past the point at which the wall is protected against rising and splashing water, and a frame fixed back into that wall creates penetrations through whatever is protecting it. Deck design and wall design have to be resolved together at that junction rather than in sequence.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
These timber families are commonly encountered in deck posts, bearers and beams. Treatment, natural durability and exposure are product-specific and site-specific, so whether any of them belongs in a given frame is a matter for the manufacturer's documentation and the designer.
Substrate
Joisting in external decks is commonly seen in these solid and preservative-treated softwood families. Natural durability, how deeply a treatment reaches and how a section sheds and dries after rain differ between them, and what an exposed frame will take is set out in the product literature.
Finish surface
Board layers are commonly encountered in these families, each of which weathers, expands and holds surface water differently. None is inherently correct for an exposed deck, and what a given exposure and use will take is settled by the manufacturer's documentation and the designer.
Attachment
Deck fixings and connectors are commonly encountered in these metal families, because the environment is wet and, inside treated timber, chemically active. Compatibility between a fixing and the treatment it sits in is stated by the manufacturer rather than assumed here.
Protection
Applied external finishes for decks are commonly encountered in these categories. What each does for weathering, appearance and surface texture differs, and whether any of them is appropriate over a particular treated or modified timber is set out in the product documentation.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Guarding at the deck edge
Where the platform stands far enough above the ground to need a guard, the guard's loads end up in the deck frame. Post positions, trimmers and the route back into structure have to be settled while the frame is being designed, because a guard added to a finished deck can only be fixed to whatever happens to be there.
Read about Guarding System →Where the deck meets the house wall
A raised external surface changes the relationship between the ground outside and the moisture protection built into the wall. Whether the deck can abut the wall, stand clear of it, or need a separated frame is a question about the wall rather than about the deck.
Read about Damp-Proof Continuity →Threshold onto the deck
A deck that meets a door becomes part of the threshold detail, because the surface level, the drainage in front of the opening and the free path for water under the door all act on each other. A level or near-level transition makes each of those harder rather than easier.
Read about Door Threshold Interface →Ground beneath and around the deck
Water that passes the board gaps has to arrive somewhere. Where the platform sits over a hard surface or across a low point in the garden, collecting and disposing of that water is part of the deck's design and not a landscape item to be picked up afterwards.
Read about Surface Water Collection →Bearing at the ground
Load from posts and bearers eventually arrives on the ground, and the form that bearing takes is a ground question. Whether a simple bearing pad, a formed base or something more substantial is needed depends on the soil, the loading and an assessment of the site.
Read about Pad Foundation →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- The frame lives in an intermittently wetted environment, so its condition depends on drying rather than on exclusion. Trapped water at bearing points, in end grain and in closed voids is what shortens the life of a deck, and none of it is visible from above.
- Durability
- Exposure differs sharply across a single deck, because horizontal boards in full sun weather quite differently from shaded joists over damp ground. Judging the frame by the appearance of the surface is a common source of surprise.
- Movement
- External timber changes dimension with moisture and the assembly has to accommodate that somewhere. Fixings, board gaps and edge details are where movement is either allowed for or resisted, and resisting it does not make it stop.
- Maintenance and access
- Almost everything that governs how long a deck lasts sits under the boards. Whether the void can be reached, inspected and cleared of debris is decided at design stage, and a platform built tight to the ground is effectively sealed against future inspection.
- Interfaces
- Junctions with the building, with steps and with guarding carry more design risk than the field of the deck. Each involves another trade, another system and usually another point at which the frame is penetrated or supported by something it did not anticipate.
- Buildability
- Ground conditions, existing levels and the position of drains and services often determine what the framing layout can actually be. A layout drawn without those constraints tends to get redesigned on site, usually at the expense of the interfaces.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Is the deck a freestanding platform, or does it rely on the building for support at one edge?
- What happens to the water that passes between the boards once it has left the surface?
- Can the void beneath the frame be ventilated and reached again after the deck is finished?
- Is there a change of level that brings guarding into the design, and does the frame anticipate it?
- How is the timber separated from the ground, and what happens where it cannot be?
- Does the surface need to meet a door, and if so who is resolving the threshold detail?
- Will the board layer be fixed in a way that permits seasonal movement, and where does that movement go?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A deck is often treated as a surface product, when the part that fails is nearly always the frame or the ground contact beneath it.
- Gaps between boards are not a drainage system in themselves; they only move water on to the next problem, which is the void and the ground below.
- Treated timber is not the same thing as protected timber once it has been cut, notched and drilled on site.
- A deck that looks sound from above can be carrying decay at bearing points that nobody has ever seen.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is carrying this deck at each support point, and has the ground been assessed for it?
- How will the frame be kept out of standing water and continuous ground contact?
- Is the deck being fixed to the building, and what does that do to the wall's moisture protection?
- What is proposed for ventilating and later inspecting the space beneath the boards?
- Which fixing approach is intended, and how does it allow for movement in the board layer?
- If guarding is needed here, what supports it and where do its loads go?
What this page does not do
- Guarding, whether it is needed and what it must resist are safety-critical determinations for qualified professionals and the relevant authority, not for a reference page.
- Nothing here establishes whether a deck may be built at a given position, level or proximity to a boundary; that is a matter for the relevant authority.
- Decay in external timber is frequently concealed at connections and bearings, and assessing it needs physical inspection by a competent person.
- Structural adequacy of a deck frame, its supports and its connection to a building is a matter for a qualified engineer.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Decks, Paving and Hard Landscape Systems
External surfaces people stand on, plus the substructure, edge, support and guarding that make one usable, each decided by the layer beneath and the route its water takes.
Browse all decks & paving entries →