How to fix rotten pergola beam?

amwhow

Hi all,

I've got a timber pergola attached to the house (approx. 20 years old, exact age unknown) with a wisteria trained over it. One of the main bearers that supports the rafters has rotted badly where it meets the post — the end grain has gone soft and dark, and when I pushed up on it, the whole beam moved. It's clearly not properly fixed to the post anymore. Photos attached showing the general pergola, the joint from a couple of angles, and a close-up of the rot.

A few questions for anyone who's dealt with this before:

1. Is this the kind of rot that can be cut back and repaired with a bracket, or does the whole beam need replacing?
2. If I need to sister a new piece of timber alongside the rotten section (rather than replacing the whole beam), what's a sensible overlap length and bolt spacing?
3. Can this be temporarily propped safely while I sort out a proper fix, or does it need to come down straight away?
4. If the beam does need to come out entirely for a section, will the rest of the pergola still hold its load, or does removing one bearer risk the whole frame?
5. The post and other beams nearby have similar staining/rot starting at the joints (not yet as bad) — is this something to fix one joint at a time or treat as a whole-structure issue?

Not looking to hire someone yet — trying to figure out if this is a DIY weekend job or something that needs a carpenter to look at properly first. Any advice on brackets/hardware, or things I should check before I touch it, would be appreciated.

Thanks!

Mark

MitchellMc

Welcome to the Bunnings Workshop community @amwhow. It’s great to have you with us and many thanks for your question on timber decay.

Unfortunately, with the amount of decay visible at the end of that bearer, I don't think there's enough sound timber remaining to make a conventional repair with a bracket. Sistering or otherwise repairing the damaged section might be possible, but the connection and required overlap would need to be appropriately engineered to ensure it was structurally sound. It's not something I'd recommend estimating bolt spacings for.

I'd suggest having a carpenter or other suitably qualified professional inspect it, with the expectation that the affected bearer will likely need to be replaced.

More importantly, there doesn't appear to be much material left supporting that connection. I'd isolate the area and prevent anyone from walking or spending time underneath the pergola until it has been assessed and made safe. I also wouldn't start removing or propping individual members without understanding how the loads are being transferred through the rest of the structure.

While you're having this beam assessed, I'd have the professional check the entire pergola, particularly every post and beam connection. If one timber has deteriorated this extensively after around 20 years, there's a reasonable possibility of concealed decay at other joints as well. It would be better to establish the overall condition of the structure before repairing individual sections.

Please let me know if you have any questions.

Mitchell