Rooftop Deck

Dimensions

Cut list

Tools you'll need

• ✓

  Measuring Tape (5m or longer)

  Used for measuring each component

• ✓

  Pencil and Carpenter's Square

  For marking and checking right angles

• ✓

  Saw

  Using home center cutting services is recommended

• ✓

  Electric Screwdriver

  An impact driver improves work efficiency

• ✓

  Sandpaper

  Prepare #120 and #240 grit

• ✓

  Level

  Essential for checking levelness during assembly

• ✓

  Clamps (2 or more)

  Used for temporary fixing when joining

• ✓

  Brush and Paint Tray

  For applying wood preservative

• ✓

  Mason Line and Stakes

  Used for setting reference lines

• ✓

  Shovel and Crushed Stone

  Used for ground leveling and laying crushed stone

Materials

Build steps

1. 1

   Confirm Installation Location and Check Ground Conditions

   ≈ 30 min

   Measure the installation area for the roof deck (rooftop) (3600×3600×100mm). Check ground levelness, sunlight exposure, ground strength (whether it easily becomes muddy), and drainage paths.

   Measure

   W 3600 mm

   MeasureSTEP 1 / 10

   Tip

   • If the ground is soft, foundation preparation is necessary. Lay crushed stone thickly.
   • Maintain at least 100mm height for airflow under the deck to prevent moisture issues.
   • Confirm rainwater flow direction and arrange so that water does not pool.

   Watch out

   • If ground is weak, the deck will settle and tilt over time.
   • Water pooling causes early wood decay.
2. 2

   Prepare Materials for Cutting

   ≈ 30 min

   Prepare 36 pieces of Western Red Cedar deck boards (20×90×3600mm), 10 pieces of 2×4 lumber joists (38×89×3600mm), and 5 pieces of 4×4 lumber beams (89×89×3600mm). Also prepare 10 pier blocks 150mm square, and 720 stainless steel coarse-thread screws 90mm.

   Cut

   Use the in-store cut service

   CutSTEP 2 / 10

   Tip

   • Western Red Cedar is highly water resistant and a standard for outdoor decks.
   • Selecting boards with fewer knots results in a nicer finish. Although pricier, it lasts longer.
   • Stainless steel screws are mandatory outdoors. Iron screws cause black rust stains on wood.

   Watch out

   • Using SPF lumber substitutes leads to decay within 3-5 years. Use water-resistant species (e.g. Red Cedar, Ipe).
   • Using too short screws results in weak connections between joists and beams.
3. 3

   Install Pier Blocks and Level Them

   ≈ 60 min

   Place pier blocks (150mm square) at the four corners of the installation area. Dig and embed them, then align the top surfaces of the four blocks level using mason line and level. Measure and match the distances between blocks (approx. 450mm left-right, 450mm front-back).

   Build the frame

   Tighten joints firmly

   AssembleSTEP 3 / 10

   Tip

   • Leveling pier blocks is the most critical step determining overall levelness; take your time.
   • Using mason line allows checking all four points' level at once.
   • Install pier blocks slightly elevated from ground to improve drainage.

   Watch out

   • If blocks are uneven, the entire deck will tilt and objects on it may roll off.
   • Placing blocks where water pools causes moisture to rise into the wood.
4. 4

   Sand Beams and Joists

   ≈ 30 min

   Sand the 4×4 beams and 2×4 joists sequentially with #120 then #240 grit sandpaper. Since this is for outdoor use, preparing the surface before painting is crucial. Pay special attention to end grain as it is a path for water ingress.

   Sand

   #120

   SandSTEP 4 / 10

   Tip

   • Use the faces of beams without knots or cracks facing upward as they bear the most load.
   • Joists are placed perpendicular to beams. Level surface on both is essential.
   • Perform sanding outdoors and wear a mask to avoid inhaling dust.

   Watch out

   • Insufficient sanding prevents paint absorption, leading to early deterioration.
   • Leaving sanding dust causes uneven paint application.
5. 5

   First Coat Painting of Beams and Joists

   ≈ 60 min

   Before assembly, paint only the beams and joists. Painting joint areas before assembly, which cannot be painted later, is key. Apply outdoor wood preservative (e.g. Xyladecor) fully and let dry 24 hours.

   Paint

   Watco oil / varnish

   PaintSTEP 5 / 10

   Tip

   • Pre-painting joints greatly improves outdoor durability.
   • Apply plenty of preservative to end grain.
   • Members close to ground need thorough preservative treatment in this order.

   Watch out

   • Assembling before painting leaves joints unprotected, causing water ingress and decay.
   • Don't paint before rain to avoid paint runoff; check weather forecast.
6. 6

   Attach Beams

   ≈ 45 min

   Place five 89×89×3600mm beams parallel on the pier blocks spaced 450mm apart. Insert a waterproof sheet between beams and blocks. Fix from below with stainless steel screws or dedicated metal connectors. Confirm level of all four points with level.

   Build the frame

   Tighten joints firmly

   AssembleSTEP 6 / 10

   Tip

   • Place waterproof sheet (rubber sheet possible) between pier block and beam to prevent water capillary rise.
   • If available, use dedicated 'pier block fasteners' for stronger joints; obtainable at home centers.
   • Align beam lengthwise direction with water flow direction.

   Watch out

   • Omitting waterproof sheet allows water to rise causing beam decay.
   • Uneven beams cause the whole deck to tilt.
7. 7

   Attach Joists

   ≈ 45 min

   Place ten 38×89×3600mm joists perpendicular on two beams spaced 100mm apart. Drill 5mm pilot holes through beams then secure joists with two 90mm stainless steel coarse-thread screws per joint (total 8 screws).

   Build the frame

   Tighten joints firmly

   AssembleSTEP 7 / 10

   Tip

   • 300mm joist spacing is standard support span for 20mm thick decking; wider spacing causes the boards to bow.
   • Pilot holes prevent wood splitting; 4×4 lumber is thick but prone to cracking.
   • Confirm joist top surface level while fixing.

   Watch out

   • Joist spacing over 500mm causes decking to sag under weight.
   • Using screws that are too short prevents reaching beams and weakens strength.
8. 8

   Attach Deck Boards

   ≈ 60 min

   Lay 36 Western Red Cedar deck boards (20×90×3600mm) perpendicular to joists. Use 5mm thick spacers (thickness of a nail) between boards to maintain a 5mm gap. Drill 3mm pilot holes at joist joints and fix each board with two 50mm stainless steel screws (total 18 screws).

   Build the frame

   Tighten joints firmly

   AssembleSTEP 8 / 10

   Tip

   • A 5mm gap ensures drainage and ventilation; closing gaps causes water pooling.
   • Use 5mm spacers for consistent gap width.
   • Fix convex-curved boards with the concave side down (toward joists) to avoid discomfort when walking.

   Watch out

   • Skipping board gaps causes water to pool and decay the underside.
   • Screws too close to board edges cause end splitting.
9. 9

   Sand and Paint Deck Boards

   ≈ 90 min

   Sand the entire deck boards with #120 then #240 grit sandpaper. Sand surfaces, corners, and end grain carefully. Remove sanding dust, then apply two coats of outdoor wood preservative. Dry 24 hours after first coat, then second coat and dry another 24 hours.

   Paint

   Watco oil / varnish

   PaintSTEP 9 / 10

   Tip

   • Sanding opens wood pores improving paint absorption.
   • Apply more preservative on end grain.
   • Use stainless steel screws; iron screws will rust within a year.

   Watch out

   • Missing spots during painting allows water ingress and decay; paint carefully.
   • Don't expose fresh paint to rain before drying; check weather forecast.
10. 10

    Final Inspection and Maintenance Planning

    ≈ 30 min

    Inspect the whole deck. Check screw tightness, board levelness, uniformity of gaps, and paint coverage. Test drainage by pouring water. Avoid placing heavy objects on the first day to allow paint to fully cure. Annual repainting extends deck life beyond 10 years.

    Done!

    Done — nice work!

    DoneSTEP 10 / 10

    Tip

    • Lightly sand with #240 grit once a year and recoat with the same preservative to extend life beyond 10 years.
    • Monthly removal of leaves and mud under the deck cuts moisture sources.
    • In cold regions, painting before winter protects deck through spring.

    Watch out

    • Neglecting outdoor maintenance causes paint to peel and decay within 2-3 years; annual recoating is key.
    • Moisture sources under deck (leaves, soil buildup) left unattended cause decay from below.

Warnings

• ⚠️For outdoor use, applying wood preservative is essential (recoating recommended once per year).
• ⚠️Use stainless steel screws (iron screws will rust).
• ⚠️Drill pilot holes before screwing to prevent wood splitting.
• ⚠️Install pier blocks level (unevenness causes joists to warp).

Find more blueprints in this category or store