Bridge materials calculator — a lumber list from a bridge record

The bridge materials calculator turns a bridge's recorded dimensions into a lumber list — how many stringers, how many deck boards, what lengths, and roughly what fasteners. It is for the everyday question "what do I put on the truck to replace that", answered from the record you already have.

What it needs from you

Four dimensions off the bridge record:

• Length — the span.
• Width — the deck width.
• Decking thickness.
• Decking board width.

These are the standard dimension fields on a Bridge structure, so if the bridge is recorded properly the calculator has everything it needs. If a number is missing, fill it in on the structure rather than guessing here — the record is the thing worth having right.

Dimensions are stored in millimetres and displayed in your own preferred units, so it does not matter which you think in.

One choice is worth making deliberately: rough cut or dressed (S4S), set separately for stringers and for decking. A rough 2× board is a full 2 inches thick where a dressed one is 1½, and that difference feeds straight into the span table — so the same bridge sizes differently depending on which you pick. A build can mix the two, and often does.

Everything else on the sheet — stringer spacing, decking gap, stock lengths, whether center support posts are allowed — starts from a sensible default and can be overridden for this one calculation without touching the bridge record.

What it gives back

• Decking — how many boards, at what stock length.
• Stringers — how many, at what cross-section and length.
• Fasteners — a rough count of deck screws and joist hangers.
• A board-foot total, which is the number a supplier will actually price.

Each line also says whether you already hold it. If your lumber is tagged in Inventory with a matching lumber type and dimensions, the sheet shows what is in stock and what is left to buy; if nothing matches it says so and tells you what it was looking for, so an untagged yard reads as "not matched" rather than as "not owned".

Output is a materials list you can take to a supplier. Copy puts it on the clipboard for an email, and it exports to PDF for a quote request, a work order, or a grant application's cost breakdown.

The assumptions it makes

These are stated plainly because they decide whether the answer applies to your bridge:

• Deck boards run across the full width, butted end to end along the length. It picks the smallest stock length that covers the width and does not try to be clever about ripping boards or splicing offcuts. Trail bridges are short and single-length boards are the norm; if you build differently, the board count still holds but the lengths will not.
• Stringers are spaced 406 mm (16 inches) on center — one at each outside edge plus enough between to hold that spacing, rounded up. This follows residential deck guidance, which is what most trail crews work to. It is conservative: light foot bridges often carry wider spacing perfectly well.
• Stringer cross-section comes from a span table, sized for the length you entered.
• Fasteners are order-of-magnitude — two deck screws per board per stringer crossing, and two joist hangers per stringer. Exact hardware depends on a spec TrailsIQ does not track, so treat these as "how many boxes", not a bill of quantities.

Where it stops

Once a span is longer than the biggest beam in the span table, the calculator stops sizing stringers and says so.

With the standard dressed lumber list that happens at about 43 feet / 13.1 meters — the point where an 8×16 runs out. Rough-cut stock is stronger for the same nominal size, so its table reaches further. The warning names the size it ran out at, so you do not have to remember either figure.

That is the honest limit rather than a missing feature. Past it you are into engineered beams — LVL, PSL, glulam, steel — and picking one from four dimensions would be guessing at something with a load rating on it. It will still give you the deck, the layout and the fastener numbers; the beam spec has to come from an engineer.

Well before that hard stop you will meet a softer one. When the span is longer than the longest stock length you have set, the calculator keeps sizing the beam but flags that each run has to be spliced end to end over a mid-span support post. That is a note about how the bridge gets built, not a refusal — but a splice over a post is a structural detail, so it belongs in the same conversation as the beam itself.

More generally: this is a materials estimator, not a structural design. It does not know your species or grade of timber, your live-load requirement, your snow load, your abutment condition, or whether the bridge carries a machine. Anything load-bearing that people cross needs somebody qualified signing it off. Use this to plan the shopping and the budget, and to have a sensible conversation with whoever does the design.

Setting your stock lengths

Settings → Bridge materials calculator holds the stock lengths and stringer cross-sections the calculator is allowed to choose from.

Set these to what your supplier actually stocks. The default list is a reasonable general set, but a calculator that recommends a length nobody near you sells produces a materials list you cannot buy — and the substitution then happens in the yard, unrecorded.

Common gotchas

• Rubbish dimensions in, rubbish list out. The calculator is only as good as the four numbers on the structure record. Measure the bridge, do not eyeball it from a photo.
• It is not an engineered design. For anything spanning far, carrying a machine, or crossing water people could fall into, get it designed.
• Long spans intentionally return no stringer size. That warning is the answer, not a failure.
• A splice warning is not the same as a refusal. "Spliced over mid-span support posts" means the calculator sized the beam and is telling you it will not arrive in one piece. Check that a center post can actually land there — over water or a ravine, often it cannot, and the Center support posts allowed switch tells the calculator so.
• Fastener counts are approximate. Buy round numbers and expect leftovers.
• Set your stock lengths first, or the list will be right in principle and unbuyable in practice.
• Decking thickness matters more than people expect. It drives both the stringer spacing you can get away with and the deck weight the stringers carry.