Question 1

How do you join a granite door frame corner? Separate the mitre from the butt joint first

Accepted Answer

There are only two ways: a 45° mitre and a butt joint. The face of a door opening is a ∩ shape — two vertical legs and one horizontal head. Each corner carries two angles: the recessed one on the opening side is the internal corner, the protruding one on the outer edge of the surround is the external corner. A 45° mitre cuts the top of the leg and the end of the head at 45° each, and the diagonal seam carries the external corner and the internal corner of the same joint into one line. A butt joint cuts both at 90° square, one end face bearing on the face of the other piece, and the seam is a straight line. Example: clear opening 900mm, opening height 2100mm, surround width 100mm, slab thickness 30mm; cut as a mitre, the three pieces total 2×2200 + 1100 = 5500mm, or 5.5 linear metres. Choose the route first, then talk about material and adhesive.

Question 2

Mitre or butt joint — how much material does each take?

Accepted Answer

Only 0.02 ㎡ separates them. The finished area is identical for all three methods: 100 × (900 + 2100×2 + 100×2) = 100 × 5300 = 530,000 mm², or 0.53 ㎡. One square metre of 30mm slab ripped into 100mm strips yields 10 linear metres; the three pieces of this surround total 5.5 linear metres, and 5.5 ÷ 10 = 0.55 ㎡ — that is the mitre blank. The mitre cuts away four 45° triangles, 4 × (100 × 100 ÷ 2) = 20,000 mm² = 0.02 ㎡, so 0.55 − 0.02 = 0.53 ㎡. Both butt methods run at 0.53 ㎡ with no cut-off waste. 0.02 ÷ 0.53 ≈ 3.8%, so the mitre uses 3.8% more material. The money is not in the material.

Question 3

Where does a 45° mitre chip first?

Accepted Answer

At the point of the external corner. A mitre splits what would be one complete 90° arris on a single piece into two 45° points, and each piece tapers to zero across its width at the point while the 30mm thickness stays put. In head over leg and leg over head, the external corner is a complete square arris on one whole piece. Rework follows the same line: chip a mitre's external corner and both pieces must come off, because both need re-cutting at 45° to meet again; chip a butt joint and only that piece is replaced. The problem is not the glued face: the diagonal seam is 100 × 1.414 = 141.4mm long, so the glued face is 30 × 141.4 = 4,242 mm²; a butt seam is 100mm, giving 3,000 mm². The mitre has 41% more glued area. It is weak at the point, not at the seam. Shop practice: do not cut the mitre to a needle point, leave a 1~2mm flat — a rule of thumb, not a standard value — or it is likely to chip in transit or during installation.

Question 4

Which piece laps which — and how do you choose among the three?

Accepted Answer

By how long the head has to be. Head over leg needs a 1100mm head: the clear width plus one surround width at each end. Leg over head needs only 900mm. The difference is 200mm. The wider the opening, the longer that head becomes; once it exceeds the length this batch of slabs can yield in one continuous run, you either switch to leg over head or accept a splice — ask the factory how long this batch can run continuously, do not assume. The visual difference sits in the same place: head over leg leaves a horizontal seam running 100mm out from the internal corner along the top of the opening, reading as a head laid over the legs, and it is the quieter of the two. Leg over head leaves a vertical seam directly above the vertical edge of the opening, continuing 100mm upward, so the eye follows the door edge straight to it. To choose: head over leg when the seam should disappear; leg over head when the head runs longer than this batch can yield; a mitre only when the outer arris has to read as one unbroken line and you accept pulling two pieces if it chips.

Question 5

Can one corner be mitred on the outside and butted on the inside?

Accepted Answer

No. A joint has only two ends: one lands on the corner at the opening, the other on the outer edge of the surround. A mitre is a diagonal seam and a butt is a straight one, so the same seam cannot be diagonal at one end and straight at the other. The joint type is therefore decided for the corner as a whole; there is no compromise where the outside is mitred and the inside is butted. Window surrounds work the same way and cost more of it: a window surround is a four-sided frame, all four corners take the same decision, and choosing mitres means four pieces and eight ends cut at 45°, with one extra surround width of blank at every end. A door surround only has the two upper corners to decide; the bottom of each leg lands on the threshold or the floor and is not cut at 45°.

Question 6

When ordering, how do I state the blank lengths of the three pieces?

Accepted Answer

Give three numbers and three conditions. Example: head 1100×100 (head over leg) or 900×100 (leg over head); legs 2200×100 each (mitre or leg over head) or 2100×100 each (head over leg). State the method, state which end is cut at 45°, state whether the external corner keeps a small flat; miss one and the shop falls back to its default. The bottom of each leg bears on the threshold, and the threshold in turn runs under the side pieces of the surround, so the two lengths interlock: the blank length of the surround has to be fixed together with how far the threshold penetrates, and the opening height is measured from the finished face of the threshold. For scale: 100 ㎡ minimum order, roughly 180-plus door openings at 0.55 ㎡ each, 5 days to ship, and Maple Red G562 polished slabs at 30mm load 346 ㎡ per 20ft container.

Method	Head blank	Leg blank (each)	Total blank	Cut away	External corner
45° mitre	1100×100	2200×100	0.55 ㎡	Four 45° triangles, 0.02 ㎡	Two 45° points, one on each piece
Head over leg	1100×100	2100×100	0.53 ㎡	None	Whole square arris on the head
Leg over head	900×100	2200×100	0.53 ㎡	None	Whole square arris on the leg