How to Build a Decking Subframe

Getting the subframe right is the foundation of a long-lasting deck. Whether you’re planning a compact seating nook or a generous entertaining area, the structure beneath your boards carries the load, keeps everything level, and protects the build from moisture and movement.

This guide explains how to build a decking subframe step by step, covers the materials and tools you’ll need, and highlights best practice for decking subframe spacing, support methods, and final checks—so your deck feels solid underfoot and stays that way.

Founded in 1921 and now the largest independent Timber, Building Materials & Fencing merchant in London and the South East, Lawsons supports both Trade Professionals and DIY Enthusiasts with dependable stock, technical guidance, and delivery options to keep projects moving.

Important: This is general guidance. Always follow your deck board manufacturer’s specification for joist centres, fixings, and expansion gaps—especially for composite systems. If your deck is raised, attached to a building, or carrying heavy loads (e.g., hot tubs), seek competent professional advice.

A decking subframe is the structural base beneath your deck boards. Typically built from pressure-treated structural timber (or composite/recycled plastic joists), it distributes weight across supports, keeps the deck stable, and helps resist movement over time.

If you’re learning how to build a decking frame, the job is essentially:

1. plan and set out accurately
2. prepare the ground/support points
3. build the perimeter
4. install internal joists at correct decking subframe spacing
5. stiffen and protect the structure
6. check levels and secure everything before boards go down

Built well, the subframe reduces bounce, sheds water effectively, and improves the finished look and feel.

Choosing the right materials makes the build safer, faster, and more reliable—especially outdoors.

Materials

Structural timber (common choice)

Use kiln-dried, pressure-treated structural joists (often C24 grade where specified) for consistency and strength. Common sizes include 47×100mm, 47×125mm, 47×150mm, selected based on span, load, and deck height.

Composite / recycled plastic joists (low-maintenance option)

High moisture resistance and dimensional stability. Useful for damp sites and low-height builds where airflow can be limited.

Fixings and connectors

• Exterior-grade decking screws
• Galvanised or stainless steel joist hangers and angle brackets
• Structural coach screws/bolts where required (e.g., ledger/bearers)
• Packers/shims for levelling
• Joist protection tape (helps reduce water ingress at the top face)
• End-grain preservative for cut ends on treated timber

Base/support materials

• Weed control membrane
• MOT Type 1 for sub-bases (soft ground)
• Sharp sand (where appropriate)
• Concrete slabs/pads, adjustable pedestals/risers, or ground screws
• DPC strips/spacers for interfaces where moisture could be trapped

Tools

Tape measure, string lines and stakes, spirit level or laser level, square, saw (circular or mitre), drill/driver + impact driver, lump hammer, spade, PPE (gloves, goggles, ear protection). For raised decks, a post hole digger/auger helps. A rubber mallet is handy for seating slabs and adjusting shims.

Ground conditions determine support method and how you achieve level. Start by checking whether you’re building on a hard surface (concrete/paving) or soft ground (soil/lawn/gravel).

On hard surfaces

• Ensure the surface is sound, clean, and drains well
• Support the frame on rubber pads, risers, pedestals, or slabs to spread loads
• Use packers/shims to level
• Maintain ventilation beneath the frame (don’t trap moisture)
• Joist protection tape on timber helps shed water from the top face

On soft ground

• Strip turf/vegetation and remove soft organic material
• Lay weed control membrane
• Build a compacted sub-base using MOT Type 1 in layers (compact thoroughly)
• Place slabs/pads/footings at positions that match your joist layout
• For raised decks: set posts into stable ground with concrete, or use ground screws as an alternative

Levels and fall: Check heights as you install support points. A slight fall away from buildings is commonly recommended (often in the region of 1:80 to 1:100), but keep it consistent and don’t “fight” your site drainage.

Good planning saves time and reduces rework.

Board direction and joist direction

Decide the direction of deck boards early. Joists generally run perpendicular to boards to provide the right support.

Decking subframe spacing

Always follow your board manufacturer’s guidance. As a typical reference:

• Timber decking is often supported around 400–450mm centres
• Many composite boards specify 300–400mm centres
• Diagonal board layouts usually need tighter centres due to increased effective span

Span and load planning

Sketch your footprint and mark:

• ledger board (if attaching to a building)
• perimeter frame
• internal joists
• support points/pads/slabs/ground screws
• doubled joists or extra beams under higher loads (steps, planters, pergola posts, hot tubs)

Ventilation and drainage

Avoid sealing the frame hard against walls. Use spacers/standoffs to maintain airflow and drainage paths.

Build the outer frame first, then square and level it before committing to internal joists.

If using a ledger board

Fix into sound masonry with appropriate anchors and spacers to maintain drainage/airflow behind the timber. Use DPC strips where specified and ensure moisture detailing is correct.

Perimeter assembly

• Cut ends square and pre-drill near edges to reduce splitting
• Secure corners with suitable brackets or structural fixings
• Check squareness by measuring diagonals (they should match)
• Set the perimeter on supports (slabs, risers, pedestals, ground screws, or posts)
• Confirm level and fall before final tightening

For post builds, keep posts plumb and brace while curing (if using concrete). Fix bearers with appropriate brackets/bolts and avoid crushing timber fibres at point-load connections.

Install internal joists to the centres specified by your decking board manufacturer. Keep members straight, parallel, and level so your decking subframe spacing stays consistent.

1. Fixing joists
   • Use galvanised joist hangers or appropriate direct fixings. Use the hanger manufacturer’s specified fasteners (nails/screws) so the connection performs as intended.
2. Support alignment
   • Make sure support points beneath joists line up so joists do not exceed allowable spans. Add intermediate beams or additional supports if needed.
3. Noggins (blocking)
   • Fit noggins to reduce twisting and lateral movement. Stagger them to encourage airflow and avoid moisture traps.
4. Double joists
   • Add double joists where board joints need a fixing point between supports, and under high-load areas where you need extra stiffness.
5. Protect cut ends
   • Reseal all cut ends of treated timber with end-grain preservative to maintain protection.

Correct spacing and stiffness are what make a deck feel “right.”

• Too wide spacing can lead to bounce/deflection and may void board warranties
• Tighter spacing costs more in timber but improves rigidity and finish quality
• In high-traffic zones or under heavy features, add supports or increase joist depth rather than accepting bounce

Best practice details that pay off:

• Use corrosion-resistant fixings rated for outdoors
• Add joist protection tape to top faces to reduce water ingress and help around screw penetrations
• Maintain ventilation under the frame and keep perimeter gaps clear so moisture dissipates

1. Set out: mark the footprint with string lines, establish finished level, confirm fall. Check diagonals for squareness.
2. Prepare supports: build sub-base (soft ground) and place slabs/pads, or install ground screws/pedestals/posts. Set consistent heights with a laser/level.
3. Install ledger (if used): fix to sound masonry with correct anchors + spacers; align to planned joist height.
4. Assemble perimeter: cut and fix perimeter timbers; square and level; seat on supports and shim to final height/fall.
5. Fit internal joists: install at specified centres; fix with hangers or approved methods; ensure full bearing at supports.
6. Add noggins: reduce twist; stagger for airflow.
7. Reinforce high-load areas: double joists, add beams, add supports under steps/planters/pergola posts/hot tubs.
8. Protect timber: reseal cut ends; apply joist protection tape to top faces.
9. Dry-fit check: run string lines; recheck straightness, centres, levels, and fall before final tightening.
10. Final fix: tighten structural fixings; confirm hanger fasteners are correct; ensure the frame resists lateral movement.

Before boards go down, do a proper final inspection.

Check for movement

Walk the frame. If you feel bounce or deflection:

• add supports/pads
• adjust risers/shims
• reduce spans with a beam
• increase stiffness under high-use routes

Bracing and racking resistance

• Attached decks: confirm the ledger is properly anchored into sound masonry and protected against moisture
• Freestanding decks: use diagonal bracing or triangulation (especially on posts) to prevent racking

Drainage and airflow

Keep pathways open for air circulation beneath the deck and for water to escape. Avoid backfilling against the frame and keep soil/mulch away from timber edges.

Once the subframe is secure, proceed to install deck boards using the specified fixings and spacing. Pre-drill near edges on timber, maintain consistent gaps for drainage/expansion, and stagger joints for a clean finish.

Lawsons makes deck projects simpler by supplying proven materials and offering practical support in one place—structural timber, composite joists, ground screws, slabs, membranes, fixings, hangers, and protection tapes.

Trade Professionals benefit from specification support and compatible systems that reduce rework and call-backs. DIY Enthusiasts get clear, practical guidance and confidence that the materials will work together.

If you’re planning a materials list or want to sanity-check spans, centres, and decking subframe spacing, speak to your local Lawsons team—we’ll help you build a subframe that’s strong, stable, and ready for a high-quality finish.