How broken link building works
Websites rot. Companies close, blogs are abandoned, sections are deleted in a redesign, and the articles that linked to them are never revisited. The result is a page on somebody else’s site pointing at a URL that now returns a 404. Broken link building is the practice of finding those dead links, having something equivalent on your own site, and telling the owner.
In practice you start from a page or a topic, crawl the sites that cover it, and list the outbound links that fail. Ahrefs and Semrush both report broken outbound and broken inbound links, and a desktop crawler such as Screaming Frog will do the same for any page you point it at. You then check what the dead page originally said — an archive copy usually shows you — and decide whether you already have a better answer or need to write one.
The email is short and does the editor a favour: here is the link that is broken, here is where it sat on the page, and here is something that covers the same ground if it is useful.
Why it works better than a cold ask
Most outreach asks somebody to do work for your benefit. This one starts by reporting a fault on their page, which is genuinely useful whether or not they take your suggestion. That changes the tone of the reply, and it is why the tactic survives long after other link tactics have been squeezed out.
It also tends to find relevant pages by accident. A dead link on a page about your subject means the site once thought that subject was worth citing, so the topical fit is proven before you write.
Where it goes wrong
The commonest failure is having nothing worth substituting. Publishing a thin page purely to fill a gap you spotted produces a page nobody would link to on merit, and editors can tell. If the dead resource was a detailed study and your replacement is a short blog post, the honest answer is that you are not a replacement.
The second is that many editors simply delete the broken link rather than replace it, which is a legitimate outcome and not a reason to chase them. The third is drifting into buying expired domains that once held the dead resource: search engines treat that as expired domain abuse, and it is a different activity wearing the same clothes.
How to act on it
Run the same check on your own site first. Links pointing at your deleted pages are easier to recover than links you have never had, and fixing them with redirects is link reclamation rather than link building. Then pick one narrow subject where you already publish something strong, crawl the pages that rank for it, and work through the failures you find.
Keep the volume honest. This is careful, slow work that suits a few good pages, not a monthly quota, and it belongs inside a broader link building plan rather than standing alone.