How to Build a Decking Subframe

A straight-talking guide from Wood Green Timber

Whether you're building a small seating area or a full garden transformation, your decking is only as strong as the frame it sits on. A solid subframe is what keeps your boards stable, level, and long-lasting — so it’s worth getting right from the start.

Here’s a step-by-step guide to help you build a strong, timber subframe for softwood decking.

What You’ll Need

From Us:

• Treated joists (typically 47x100mm or 47x150mm softwood)

• Weed membrane

• Coach screws or timber fixings

• Concrete blocks, slabs, or posts

• Joist hangers or brackets (if needed)

• Decking boards (sold separately)

• Postmix (if raising the frame)

Tools:

• Tape measure

• Drill/driver

• Circular saw or hand saw

• Level and string line

• Spade (if digging posts)

• Hammer or rubber mallet

Step 1: Plan Your Frame Layout

• Measure your decking area and sketch out your frame

• Decide which way your decking boards will run — your joists will run at 90° to this

• Plan joist spacing — we recommend 400mm centres for softwood decking

• Mark out your area using pegs and string

Step 2: Prepare the Ground

• Clear the area of turf, weeds and debris

• Lay down a weed membrane to reduce growth beneath the deck

• If building on soft ground, place concrete blocks or slabs to support the frame

• For raised decks, install 100x100mm posts in concrete using postmix

Step 3: Build the Outer Frame

• Use treated softwood joists to create a rectangle to the size of your deck

• Fix the corners securely using coach screws or brackets

• Check for square and level as you go

• If the deck is raised, fix the frame to your posts using bolts or heavy-duty fixings

Step 4: Add Internal Joists

• Fix joists inside the frame at 400mm centres, running the full width of the deck

• Add noggins (short horizontal pieces between joists) every 1.2m for extra stability

• Use joist hangers, brackets or skew fixings to keep everything secure

Step 5: Level & Check

• Use a long level and string line to ensure your subframe is flat and even

• Adjust supports or packing blocks as needed

• Make sure there's a slight fall away from buildings for water run-off (5–10mm over the deck)

Step 6: You’re Ready for Decking

• Once your frame is secure, level, and fully supported — you're ready to lay your decking boards

• Head over to our How to Lay Decking guide for your next steps

Need Materials? We’ve Got the Lot.

At Wood Green Timber, we stock:

• Treated softwood joists in a range of sizes and lengths

• UC2 treated timber for ground contact

• UC3 and UC4 joists available to order

• Posts, postmix, fixings and weed membrane

• A full range of decking boards and accessories

If you’re not sure how much timber you’ll need, just bring us your measurements — we’ll help you work it out.

Pop into our Wood Green or Potters Bar branch, or give us a ring — we’re here to help.