Decking around a tree

Clearance, framing and trim for a tree opening, with an example deck whose hole you can drag to size.

A tree coming up through a deck is the detail everyone wants and almost nobody plans properly. It is not difficult, but it is unforgiving: get the clearance wrong and you will be cutting the deck apart in five years.

Leave more room than looks right

Two separate things need the gap, and they add up.

Growth. Trunks put on girth every year — very roughly 10–25 mm of diameter a year for a young amenity tree in good conditions, slower once mature. Over the twenty-year life of a deck that is easily 200–400 mm of extra diameter.

Movement. A tree in wind moves at the base more than you expect. A trunk that touches the deck will chafe, and the deck will lose.

A workable minimum is 300 mm of clear space all round the trunk at its current size, more for a young, fast-growing tree. For a 300 mm trunk that means an opening around 900 mm square — which is what the example below uses, and it looks generous until you think in decades.

Also leave the root flare alone. The widening at the base of the trunk is structural and it is where the tree breathes; do not bury it under a frame or a solid base.

Frame the opening, do not just cut a hole

An opening interrupts joists, and an interrupted joist carries nothing. The fix is the same one used for a stairwell in a floor:

1. Trimmers — short members across the opening at each end, fixed between the two full joists either side.
2. Doubled trimming joists — the joists flanking the opening pick up the load of every joist the hole cut short, so they are commonly doubled up.
3. Hangers — the cut joists hang off the trimmers on joist hangers rather than being end-nailed.

The configurator plans openings as part of the subframe: place a hole and the frame reorganises around it, and the joists, trimmers and hangers all land in the bill of materials with everything else.

Trimming the edge

The board ends around the opening are end grain and will drink water, so they want sealing on a timber deck and hiding on any deck. Options, roughly in order of effort:

• A simple mitred surround in the deck board itself.
• Edging or fascia profile returned around the opening — the same product you are using on the perimeter.
• A removable collar: the honest answer for a young tree, because in ten years you will want the opening bigger and a removable collar makes that a morning's work rather than a rebuild.

Whatever you use, do not fix anything to the tree.

An example to open

Try moving the opening off-centre. The frame changes noticeably depending on whether the hole falls between joists or across a bearer line, and the price follows — which is the sort of thing worth knowing before you dig the pads.

A note on the tree

None of this is arboricultural advice. If the tree is protected — a TPO, or anything in a conservation area — building over its root zone may need consent, and heavy excavation inside the root protection area can kill a tree years later without ever looking like it was your fault. Check first.

Plan your deck · Joist spans explained

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