BuildingDeckAdvanced

Raised Deck: Posts, Beams and Joists

The bolted skeleton of a 3,000 x 2,400 mm raised deck: six posts, three beams over them, seven joists bolted across. Everything above the joists is a fixing outside this database.

Designthe dimensions everything else is derived from

Deck size

3000 x 2400 mm at 400 mm above ground

Posts

6 off 100 x 100 mm, 2 per beam line

Beams

3 off 2400 x 100 x 200 mm, running the 2400 direction

Joists

7 off 3000 x 45 x 120 mm at about 340 mm centres

Joist span

1500 mm between beams

Exposure

fully external and permanently damp

Cut list sizes are finished sizes. Add 5 mm to the width and 25 mm to the length of every solid-wood part for saw kerf and planing, and buy boards oversize by one thickness so a bowed board can be flattened rather than relegated.

Materialswhat each part is made from, and why

All materials

Fir (Douglas) · 6 posts, 3 beams, 7 joists

Structural softwood graded for the spans and used where it is never seen. It is also the part that is easiest to replace if a joist ever has to come out.

Ipe · the wearing surface, roughly 2400 mm of board width

Dense enough to resist rot and abrasion without a coating, which matters because a deck surface is re-sanded far too often to keep a finish on it.

Stainless Steel A4 (316) · every bolt, washer and nut

A deck is wet and often chlorides are involved. The fasteners are buried in joints that will never be inspected, so they have to be the version that does not need inspecting.

Cut listfinished sizes, worked back from the design

Part	Qty	Finished size	Stock
Post	6	100 x 100 x 400 mm	Fir (Douglas)
Beam	3	2400 x 100 x 200 mm	Fir (Douglas)
Joist	7	3000 x 45 x 120 mm	Fir (Douglas)

Fastenersquantities and lengths that follow from the joints

All standards

Item	Qty	Grip	Standard
M12 x 220 hex bolt, A4 stainless	12	200 mm	ISO 4014
M10 x 160 hex bolt, A4 stainless	21	145 mm	ISO 4014
M12 plain washer	24	—	DIN 125 A
M10 plain washer	42	—	DIN 125 A
M12 and M10 nylon-insert nut	33	—	DIN 985

Grip is the sum of the clamped thicknesses. A bolt has to clear the nut by two full threads, which is what sets the length that appears in each row — not a preference.

M12 x 220 hex bolt, A4 stainless · grip 200 mm

Two at each of the six post-to-beam crossings. Grip is the beam (100) plus the post (100). ISO 4014 →

M10 x 160 hex bolt, A4 stainless · grip 145 mm

One at each joist-to-beam crossing: seven joists over three beams. Grip is the beam (100) plus the joist (45). ISO 4014 →

M12 plain washer

One under every head and nut of the twelve post bolts. DIN 125 A →

M10 plain washer

One under every head and nut of the twenty-one joist bolts. DIN 125 A →

M12 and M10 nylon-insert nut

Twelve M12 and twenty-one M10, all A4. A deck flexes underfoot on every step, so every joint in it is a joint that will try to loosen. DIN 985 →

Toolsand what each one is for on this job

All tools

Drill

Boring the 13 mm and 11 mm clearance holes.

Twist Drill Bits

Long-series bits, because a 200 mm beam crossing plus a 100 mm post is not a hole a stub bit reaches.

Circular Saw

Cutting the 100 x 200 beams to length.

Socket Set

Thirty-three nuts, most of them below knee height.

Combination Wrench

Holding heads in the notch where a socket will not fit.

Tape Measure

Setting joist centres; a single error shows up as a board that will not land on a joist.

Torque Wrench

Thirty-three structural bolts brought to one tension.

Order of workthe sequence, and where it is not free to change

Set out the three beam lines and the six posts, level to each other.
Notch each beam where it crosses a post, or cut a half-lap if the beam has to stay at full depth.
Clamp, bore and bolt the posts to the beams, two bolts per crossing, snug only.
Check the three beams are level with each other before final torque.
Lay the seven joists across at about 340 mm centres and bolt each one at every beam it crosses.
Only then fix the wearing surface, with a fastener this database does not carry.

Where it goes wrongthe three failures worth designing out

Leaving the joists over a span the timber will not carry. Joist span is set by species, grade and spacing together, and this layout needs the middle beam precisely because the span would be too long without it.
Fixing the deck boards to a frame that is not square. The boards then have to be scribed one at a time and the job doubles.
Placing a post so its end grain sits in damp soil. Concrete or gravel around a post is not the problem; standing water against the end grain is.

Frequently asked questions

Why is the deck surface not covered here?

Because decking is fixed with screws or nails, and this database holds machine screws, bolts, nuts and washers under published dimensional standards. A wood screw has no such table here, so no size is quoted. The bolted structure below the boards is the part that can be specified from data.

Why three beams instead of two?

Span. At 45 x 120 the joist will not carry 2400 mm safely at 340 mm centres, so a middle beam shortens the span to 1500 mm. Two beams would need a much deeper joist, which costs more timber than the third beam does.

Do the bolts need to be countersunk into the beams?

No. A bolt head on the outside face of a beam is in a position nothing walks on and nothing sees. The joints that need counterboring are the ones in a wearing surface, and there are none here.