9 Step Cedar Hot Tub Surround Decking Build with Millboard Over a 12-Square-Metre Frame

Locate the access hatch before the joists

The common failure point in a surround build appears when a pump or heater needs replacing and the equipment bay sits under fixed boards. Before any joist is cut, mark the tub panel that hides the pump, the control board, and the heater element; on a typical Canadian Spa or Jacuzzi J-series shell, that bay sits on a single corner roughly 600 mm wide.

Build the removable Millboard section directly over that service bay. Screw the boards in that area to a separate sub-frame that lifts out as one unit, kept separate from the main joists, and size it at least 700 mm by 700 mm so a service engineer can reach in with both arms. The surrounding deck can be fixed permanently, while the lift-out tray keeps future pump access available without cutting finished boards apart with a multi-tool.

Steps 1 and 2: sandstone slab base preparation

Set the hot tub on the ground or on a slab, with the deck wrapping around it. Excavate to 150 mm across the tub footprint. Lay MOT Type 1 as the sub-base, compact it in two passes with a plate compactor, then bed sandstone slabs on a full mortar bed mixed at a 1:4 ratio. Sandstone slab base preparation matters here because the point load under a filled six-person tub runs above 250 kg per square metre, and a rocking slab passes movement into the frame around it.

Check the slabs with a 1.2 m spirit level and aim for a flat, level bearing surface under the shell. Some manufacturers, including Hydropool, specify a 6 mm fall over 2 m for drainage off the base, so check the model plate before finishing the screed.

The deck frame footings sit apart from this slab. Pour or set concrete pad footings at 600 mm centres around the perimeter, each bearing onto firm subgrade below the local frost depth. In many UK details, a depth of about 450 mm is used as the frost-depth reference, so the footing layout should follow the site conditions rather than the shallower excavation used for the tub footprint.

Steps 3, 4 and 5: framing the 12-square-metre structure

Use C24 graded treated softwood for the frame, with 47 mm by 150 mm joists at 400 mm centres. At 12 square metres, a 4 m by 3 m layout can use two 4 m bearers carrying joists across the 3 m dimension, which keeps the span within the C24 span table before intermediate support beams become necessary.

Where the deck meets the tub shell, hold the frame back 15 mm from the acrylic. This lets the board overhang sit nearest the water line, while the timber frame stays back from splash and shell movement.

Fix joists into the bearers with galvanised joist hangers and 35 mm sheradised twist nails, using two nails per hanger flange. Run a perimeter band joist around the full frame to lock the joist ends. Across a deck this size the diagonal racking force is real, so add herringbone noggins or solid blocking at mid-span on every joist bay.

Set the frame on adjustable deck pedestals where the ground falls away, or on concrete footings where the bearing is firm. Ventilation under the boards is mandatory around a hot tub, because steam and splash-out keep the underside damp. Leave a continuous 25 mm air gap at the perimeter, keep the frame perimeter open until the final fascia stage, and include louvred sections at the base. A frame with poor drying can rot from the inside while the Millboard surface still appears sound.

Build the equipment-bay sub-frame at this stage as a separate lift-out tray. Use the same 47 mm by 150 mm stock, make it as a free-standing rectangle, drop it between two main joists, and let it rest on cleats. Keep hangers and main-frame fixings off the tray. Mark its position with a stainless screw head left slightly proud at each corner, giving fingertip markers once the boards are down.

Level the finished frame to within 3 mm across its diagonal. A long aluminium straightedge laid from corner to corner will show any twist. Correct the frame by packing the pedestals at this point, because once 36 boards are screwed down the geometry is fixed.

Step 6: laying the Millboard

Millboard is a polyurethane composite with zero wood fibre, which is why it resists water absorption compared with WPC boards made with rice husk or wood flour. The Enhanced Grain range comes in 3.6 m and 2.4 m lengths, at 32 mm thick by 176 mm wide. Lay the boards with the long edge running away from the tub so splash water sheds along the grain and drains cleanly through the deck layout.

The fixing system is the main difference from many hidden-clip composite boards. Millboard uses Durafix screws driven through the face into the joist, and the flexible top layer closes over the screw head, eliminating plugs and hidden clips. Use the Lossnay-style 4 mm spacer, or the branded gauge, to hold a consistent 4 mm to 6 mm gap between boards for thermal movement and drainage. Durafix screws can be driven into softwood without pre-drilling, but use a moderate driver speed so the self-healing surface closes cleanly.

Fix two screws at each joist crossing, set 25 mm in from each board edge. At the tub perimeter, run a square-edge board with a 15 mm overhang past the frame, oversailing the gap between deck and shell so water drips clear of the boards. Stagger board end joints by at least 300 mm across adjacent runs, keeping end joints separated across the joist lines.

Step 7: cedar surround skirt

Western Red Cedar takes the vertical faces and the raised bench backs around two sides of the tub. Cedar is chosen for its natural thujaplicin content, which resists rot in the constant humidity beside a hot tub more effectively than untreated pine.

Use 19 mm by 144 mm tongue-and-groove cedar cladding for the skirt fascia. Fix it to vertical cedar battens with A4 marine-grade stainless ringshank nails. Plain steel will bleed black tannin stains down the boards within a season.

Step 8: bench seating and the wisteria frame above

A raised bench along the long side gives somewhere to sit and step in from. Build it on its own cedar frame at a 450 mm finished seat height, then cap it with the same Millboard board run as the deck so the surfaces match. Keep a 50 mm gap behind the bench back for air movement against the cedar skirt.

If the surround sits under a pergola, an espalier pear or a wisteria can be trained over it for summer shade. Wisteria pruning technique matters where it overhangs a tub, because the species throws vigorous whippy growth that drops debris into the water. Cut the long summer shoots back to five or six leaves in July, then reduce those same spurs to two or three buds in February.

That two-stage cut on a frame above the tub keeps the canopy tight and the leaf fall manageable. A pair of Felco 2 secateurs handles the February spur cuts cleanly up to the 25 mm capacity of their blade; anything thicker moves to loppers.

Step 9: fascia, drainage and the final fit

Close the perimeter with a Millboard bullnose or square fascia, while keeping the louvred ventilation gaps at the base. Confirm that the slab fall sends water clear of the equipment bay. Lift the service tray once while fully assembled to confirm it clears the frame and that the board edges have free movement. Reseat it, then check that the proud stainless marker screws can be found by feel.

The final check is the space between the deck overhang and the tub shell. Set the visible gap at 15 mm to 20 mm. The gap should shed splash, allow loaded shell flex, and keep dropped items out of the bay below.