Composite Decking Subframe Base

You’re not far off to be fair, just a couple things I’d change. 50mm of Type 1 is a bit light, I’d go more like 75mm minimum once compacted just so it doesn’t end up moving later on. Membrane wants to go under the hardcore as well, not on top. Slabs are a good shout but don’t just sit them loose, stick a bit of sharp sand or a dry mix under them so you can get them level and they stay put. For 21m² you’re roughly: about 2 bulk bags at 50mm about 3 bulk bags at 75mm Frame sounds fine. 300 centres is what you want for composite. Splicing joists is alright, just stagger the joints and if you can double them up where they meet even better. 600 spacing on slabs is fine, just make sure every joist is actually landing on something, especially ends and any joins. Might end up needing a couple extra slabs when you lay it out. Only other thing is make sure you’ve got a slight fall away from the house and don’t have it sitting too tight to the ground, you want a bit of airflow under it. Other than that you’re on the right track 👍

Your plan is broadly sound, but there are a few areas where I’d strengthen it so the decking stays level and doesn’t become springy or settle over time. 1. Ground preparation 50 mm MOT? For a composite deck, especially one around 21 m², I’d personally consider 50 mm compacted MOT a bit light unless the ground is already extremely firm and well-drained. A better target would be: * 75–100 mm compacted MOT Type 1 Reason: * composite decking systems are heavier than people think * slabs can settle individually if the sub-base is thin * timber frames hate movement and trapped moisture Your proposed build-up should ideally be: 1. Excavate 2. Compact soil/subgrade 3. Weed membrane 4. 75–100 mm MOT Type 1 5. Compact thoroughly 6. Small bed of sharp sand or mortar dabs for levelling slabs 7. Paving slabs 8. Timber frame That will feel much more solid long term. 2. How much MOT? Your area: * 3 m × 7.2 m = 21.6 m² At: * 50 mm compacted depth → roughly 1.1 m³ * 75 mm compacted depth → roughly 1.6 m³ * 100 mm compacted depth → roughly 2.2 m³ In tonnes: * MOT Type 1 is roughly 2 tonnes per m³ So: * 50 mm ≈ 2.2 tonnes * 75 mm ≈ 3.2 tonnes * 100 mm ≈ 4.3 tonnes Do not buy this in little bags if you can avoid it — bulk bags or loose tipped aggregate is massively cheaper. 3. Slabs under frame Your slab layout sounds sensible. 21 slabs at 600 centres That’s reasonable for: * 2x4 treated joists * composite decking * 300 mm joist centres But I’d suggest: * use full concrete paving slabs * ideally 50 mm thick minimum * ensure every slab is level with adjacent slabs A common mistake is having tiny height differences that telegraph through the deck. 4. Joist sizing / spans This is the area I’d check most carefully. 2x4 timber It can work, but only if: * spans between supports are kept short * timber is good quality and straight At 600 mm slab spacing: * 2x4s are probably acceptable * but I’d prefer: * supports closer to 500 mm * OR using deeper joists (6x2) Composite decking is flexible and bouncy compared with timber decking, so a stiffer frame is always worthwhile. 5. 300 mm centres Good choice. Many composite manufacturers actually require: * 300 mm centres * sometimes even 250 mm for diagonal laying So you’re right to be cautious there. 6. Things people often forget Airflow Leave: * at least 100 mm airflow under frame where possible Joist tape Strongly recommended. Use butyl joist tape on top of joists to stop water sitting on timber. Drainage fall Do not build perfectly level. Aim for: * about 1:80 fall away from house Frame separation from soil Do not let timber sit directly on soil or wet sand. Weed membrane Good idea, but: * use a proper woven geotextile * not thin plastic sheet membrane 7. About splicing joists Your staggering idea is correct. But ideally: * joins should occur directly over supports/slabs * use structural screws or coach bolts * avoid unsupported butt joints Overall Your plan is good and better thought out than many DIY decking builds. The only major change I’d recommend is: * increase MOT depth to 75–100 mm * consider slightly more support points or deeper joists if budget allows That will make the deck feel far more substantial and reduce future movement.

1. Sub-base (this is the biggest issue) 50 mm of compacted MOT Type 1 is too thin for what you’re planning. For a deck sub-base (especially one supporting slabs and a timber frame), typical guidance is: * Minimum 75 mm, but realistically * 100 mm compacted MOT Type 1 is the safe, standard approach Why: * Prevents future settlement and rocking slabs * Handles drainage better * Reduces movement in freeze/thaw cycles (important in the UK) 👉 Your proposed build-up should ideally be: * Dig out: ~120–150 mm total * 100 mm MOT Type 1 (compacted in layers if possible) * Weed membrane (under the MOT, not above) * Slabs on top (with a thin sharp sand bed if needed for levelling) Key correction: Put the weed membrane below the MOT, not above it — otherwise the aggregate can migrate and you lose stability. ⸻ كمية MOT Type 1 you’ll need Your area: * 3 m × 7.2 m = 21.6 m² At 100 mm depth (0.1 m): * Volume = 2.16 m³ Conversion: * 1 m³ ≈ 1.8–2 tonnes → You’ll need roughly 3.8–4.3 tonnes In bulk bags: * ~4 bulk bags (builders merchant dumpy bags) If you stuck with 50 mm (not recommended), you’d halve that — but it’s a false economy. ⸻ 2. Slab layout Your idea: * 600 × 600 slabs * 600 mm spacing * 3 rows of 7 (21 slabs) That’s generally sound, but: * 600 mm spacing is okay if joists are strong and well fixed * Make sure every joist end and splice is supported by a slab * Consider adding extra slabs under high-load areas (steps, edges, joins) 👉 One improvement: * Add a perimeter run of slabs under the outer frame for rigidity ⸻ 3. Timber frame design Joist spacing (important) * You said 300 mm centres → ✅ perfect for composite decking (many manufacturers actually require this) Joist count For 7.2 m length: * At 300 mm centres → ~24 spaces → 25 joists ✅ Your estimate is correct ⸻ Splicing joists Splicing is fine, but: * Always splice over a support (slab) * Use double noggins or plates to reinforce joints * Staggering front/back is a good idea ✅ ⸻ Missing / often overlooked Here are a few things people commonly miss: 1. Drainage fall * You need a slight slope: * ~1:80 fall away from the house * Prevents water pooling under the deck 2. Airflow * Don’t trap moisture: * Leave at least 50–100 mm clearance under joists * Avoid fully sealing the perimeter 3. Timber protection * Even treated timber benefits from: * Joist tape on top of joists (prevents rot) * Especially important with composite boards 4. Fixings * Use exterior-grade structural screws/bolts, not just basic wood screws 5. Ground contact * Try to avoid timber sitting directly on slabs: * Use DPC strip or plastic shims/pads ⸻ 4. Overall verdict Your plan is solid in concept, but: Must fix: * Increase MOT Type 1 to ~100 mm * Put membrane under, not above * Ensure all joins sit on slabs Good as-is: * 300 mm joist centres ✅ * Slab grid idea ✅ * Joist quantity estimate ✅ Optional upgrades: * Add more slabs for rigidity * Joist protection tape * Slight fall for drainage

​1. Ground Preparation & Sub-base ​The consensus is that 50mm is too shallow. To avoid the slabs sinking or shifting (which would make your deck wavy), you should aim for a more substantial base. ​Recommended Depth: 75mm to 100mm of compacted Type 1 MOT. ​Membrane Placement: Place the heavy-duty weed membrane underneath the MOT. This prevents the stone from sinking into the soil over time. ​Quantities for 21.6m²: ​At 75mm: ~3.2 tonnes (Approx. 3 to 4 bulk bags). ​At 100mm: ~4.3 tonnes (Approx. 4 to 5 bulk bags). ​Note: Bulk bags are significantly more cost-effective than small 25kg bags for this volume. ​2. Slab & Support Layout ​Using 600mm x 600mm slabs is a smart way to spread the load without digging dozens of post holes. ​Levelling: Do not lay slabs directly on the MOT. Use a sharp sand bed or a dry grit-sand/cement mix (6:1) under each slab. This allows you to tap them level. ​Support Points: Your plan for 21 slabs is good, but ensure every joist join (splice) and every joist end sits directly on a slab. You may find you need 3–5 extra slabs to support the perimeter and the specific locations of your staggered joins. ​3. The Timber Frame ​Your calculations for 25 joists at 300mm centres are correct and professional. ​Joist Protection: Since you are using composite boards, the frame will likely outlast the boards if not protected. Use butyl joist tape on top of the timber. This prevents water from sitting in the screw holes and rotting the joists from the top down. ​Airflow: Ensure the frame isn't "boxed in." Composite decking requires airflow underneath to prevent heat expansion issues and to keep the timber dry. ​Fall: Build in a slight slope (approx. 1:80 or 1:100) leading away from any buildings to ensure water runs off the boards and doesn't pool underneath. ​4. Splicing & Fixings ​Staggering: Your plan to stagger joins is correct for structural integrity. ​Splicing: When joining two 3m joists to get your 7.2m length, "sister" the joists (overlap them by at least 300mm–600mm and bolt them together) rather than just butting them end-to-end. Crucially, ensure this overlap sits directly over a paving slab. By increasing the MOT depth and ensuring your joins are supported, you'll have a deck that feels solid underfoot and won't require lifting in two years to re-level. Task Specification Dig Depth ~150mm (100mm MOT + Slab thickness) Membrane Woven Geotextile (Under the MOT) Sub-base 75mm-100mm Type 1 MOT, compacted in layers Joist Spacing 300mm Centres (Essential for Composite) Protection Joist Tape + DPC strips between timber and slabs Fall 10mm drop for every 1m of width

Your plan is broadly solid and much better thought-out than a lot of DIY decking builds. A few things I’d tweak or confirm: 1. Base depth / MOT Type 1 50mm compacted MOT is really the absolute minimum. For a long-term composite deck, especially at 3m x 7.2m, I’d personally go: 75–100mm compacted Type 1 Weed membrane underneath Compacted in layers if deeper than 75mm Composite decking is heavier than timber and movement is the enemy. The extra depth helps stop settlement over time. For roughly 21.6m²: At 50mm depth you need about 1.1m³ That’s roughly: 1.8–2 tonnes total Around 70–80 standard 25kg bags Honestly, bulk bags are far cheaper and easier. You’d probably want: 2 bulk bags at 50mm 3 bulk bags if going closer to 75–100mm 2. Slabs under frame Your slab spacing sounds reasonable. 600x600 slabs at ~600 centres is fine for a timber subframe with: 300mm joist centres Composite boards I’d definitely put: A small bed of sharp sand or mortar under each slab for levelling DPC/rubber pads between slab and timber to stop moisture sitting against joists 3. Timber sizing Treated 2x4 (47x100mm) is usually okay for low-level decking with short spans, but: Keep spans conservative Add noggins/blocking throughout Double up perimeter joists where boards picture-frame At 300mm centres you’re doing the correct thing for composite. 4. Splicing joists Good idea staggering joins. Also: Ensure joins occur directly over support slabs Use timber plates/fish plates or coach screws for strength 5. Important things people miss You may still need: Joist tape (very worthwhile on timber/composite builds) Expansion gaps for composite boards Adequate airflow under deck Slight fall/drainage away from house Edge trim/fascia planning Stainless or coated fixings only 6. One concern If the area already drains poorly or gets soft in winter, I’d strongly consider: Digging slightly deeper Increasing MOT depth Or using adjustable pedestals instead of slabs Because once composite starts moving or dipping, it’s annoying to correct later