Question 1

What does one granite tree surround set actually include?

Accepted Answer

Answer first: the price per set is not about the stone, it is about the word set. One supplier's set is four pieces, four rails mitred 45 degrees at both ends. Another's is eight: four short rails plus four separate corner blocks lapped at the corners. Same opening, exactly double the piece count, so the two set prices can never be lined up as they stand. Three more items belong in the same question: is an inner trim ring included, is a base plate included, and is a tree grate included. A grate is cast iron or resin, not stone; folded into a stone price it means the factory is buying it in for you. Write it on the order: one set equals how many pieces, each piece's length, width and thickness, and how the corners are made. A set that cannot be described that way is not a unit yet.

Question 2

I measured a 1200 mm opening, so why does the factory ask for 1500?

Accepted Answer

You measured the opening; the factory cuts to the outer edge. Outer edge = inner size + 2 x rail width. At 150 mm rails, a 1200 mm opening needs a 1500 mm outer edge, not 1200. Get this wrong and the perimeter, the piece count and the area all follow it, in a way that only shows up when the tape comes out on site and the surround is 300 mm too big. Say which line you measured, too: the bare tree pit or the finished paving edge. On many sites the two differ by tens of millimetres.

Question 3

With the same 1500 mm outer edge, how far apart are a square and a round surround?

Accepted Answer

Square: 4 x 1.5 = 6 m of outer perimeter. Round: 3.14 x 1.5, about 4.71 m, or 1.29 m less. The square is four 1500 mm rails; mitred at 45 degrees, each piece keeps 1500 mm on the outer face and only 1200 mm on the inner face at 150 mm width. A round surround split into 8 pieces gives an arc of about 589 mm each, and every arc has to be nested on the slab, which yields less than straight cutting. Quote them separately: one is straight-rail work, the other is curved nesting. The piece count can move as well; 6 pieces gives 785 mm arcs, two fewer joints but a tighter curve and more waste.

Question 4

Why does pricing by outline area come out 2.5 times the blank area?

Accepted Answer

Because the middle is empty. A 1.5 m square has an outline of 2.25 m2; the blanks are four strips of 1500 x 150, 0.90 m2 in all; the tree pit itself is 1.2 x 1.2 = 1.44 m2. 2.25 / 0.90 = 2.5, so an outline-area price charges you twice for the hole. Round, on the same basis: at 1500 mm outer diameter and 150 mm rails the ring holds about 0.64 m2 of net stone, but a ring cannot be bought as a ring. It comes from eight straight strips of 589 x 150, about 0.71 m2 of blank, which is 0.19 m2 less than the square's 0.90 m2. And 0.90 m2 is blank, not finished stone: mitring cuts a triangle off each end, leaving 0.81 m2, and that offcut is paid for, so against net stone the multiple rises to 2.78.

Question 5

Can a 30 mm slab retain soil, and how high should the surround sit?

Accepted Answer

A 30 mm slab is a trim, not a retaining wall. For tree surrounds we make one thickness only, 30 mm, laid flat on the tree pit edge, flush with the paving or slightly proud, to cap the joint and hold the line of the grate. To hold back more than 100 mm of soil you need masonry below or a separate thicker upstand, which is different money and a different trade; mixing it into a set price makes both numbers wrong. Weight is worth doing at the same time: a 150 mm rail at 30 mm is 0.15 x 0.03 x 2.7 = 12.15 kg per metre, so a four-rail 1500 square is about 73 kg, which two men can handle on site. Height has two more constraints: a mower deck has to clear the edge, so the stone usually sits slightly above grass level, and at accessible crossings the step has to be low enough for a wheelchair. Settle those with the landscape architect before fixing rail width and height.

Question 6

Per set, per linear metre, per piece, per square metre: how do I compare four bases?

Accepted Answer

Four bases cannot be compared as unit prices. Per set: split the piece count first. Per linear metre: fix the perimeter first, 6 m square against 4.71 m round. Per piece: fix the piece count and each piece's size. Per square metre: ask whether it is outline area or blank area, and that one question removes the 2.5 times gap. Only then compare numbers. So a per-set price has to be breakable: four pieces means 4 x piece price plus the 45 degree mitre charge; eight pieces means 8 x piece price plus four corner blocks. A supplier who quotes one set figure and will not break it down leaves you nothing to check, and that is not a question of price level but of whether the price can be verified at all. Sesame Black (G654) is held in 30 mm stock, 100 m2 minimum, 5 days to ship, eight finishes; for counting, order by the piece, not by the set.

Inner size (what you measured)	Outer edge (what the factory cuts)	Outer perimeter
1000 mm	1300 mm	5.20 m
1200 mm	1500 mm	6.00 m
1500 mm	1800 mm	7.20 m

Item	Square 1500x1500	Round, 1500 dia.
Outer perimeter	6.00 m	4.71 m
Pieces	4 rails, 45 degree mitre	8 pieces, 589 mm arc
Blank area	0.90 m2	0.71 m2
Area inside the outline	2.25 m2	1.77 m2
Outline / blank	2.5 times	2.5 times

Six lines on the order	How to write it for a 1500 square
1 Inner and outer size	1200 inner, 1500 outer
2 Rail width	150 mm
3 Thickness	30 mm
4 Pieces and corners	4 rails, 45 degree mitre both ends
5 Surface finish	Flamed, one of eight
6 Packing	Timber pallet and steel strap, state if trim ring included