Repairing a rotten pergola post

Timber pergola posts nearly always fail at the bottom. That is where water collects, where soil and mulch pile up and where the end grain soaks up moisture. The top of the post can look perfect while the base is soft. This guide covers how to check and what the realistic options are.

Where to look

• At ground level, or where the post meets a spike, shoe or pad.
• Just above a metal spike or shoe, where water collects in the socket.
• Behind climbers, planters or stacked items that keep the post damp.
• Joints and bolt holes higher up, where water can sit or run in.

How to check

• Prod it. Push a screwdriver or bradawl into the timber near the base. Sound timber resists. Soft, spongy or crumbly timber does not.
• Look at the colour. Dark patches, fungal growth or timber that looks fibrous or cracked into small cubes all need attention.
• Push the post. A post that moves at the base when you lean on the frame is a warning sign, whatever it looks like.
• Check the fixings. Rusty or loose bolts and screws near the base can mean the timber round them is going.

If you are not sure, compare it with a post you know is sound. Do this as part of a yearly check. The timber pergola maintenance guide sets out the rest of it.

Paint is not a repair

It is tempting to scrape off the worst, fill it and paint over the top. That makes it look better. It does not make the post stronger, and it can trap moisture in the timber and hide what is happening underneath. Wood fillers and resins have their place on small, non-structural damage. A post that holds up a roof is structural. If the base is soft, treat it as a structural problem.

The honest options

1. Small, early damage on a sound post. If only a little surface timber is soft and the post is firm, clear soil and debris away, let it dry, and treat it with a product meant for exterior timber, following its instructions. Then fix the cause, usually water sitting against the base.

2. Replace the post. If the base is soft, replacing the post is often the simplest sound fix. It means supporting the frame while the old post comes out. A replacement from the same kit maker makes the joints fit. Some makers sell spare parts.

3. Repair the base with a post repair. Some timber posts can be cut back above the rot and fitted to a new base, such as a bolt-down shoe or a post repair spike. Whether that is suitable depends on how much of the post is sound, the height you are left with, and the maker's design. We do not give a method here, because it depends on the frame.

4. Get a competent carpenter. If more than one post is affected, if the frame has a roof, or if the pergola has moved, ask someone who knows timber structures. They can tell you whether the frame is safe to leave, needs propping, or needs replacing.

Support the frame first

Never cut or remove a post without supporting what it holds up. A pergola frame can twist or drop when a post is removed, especially with a roof or a heavy climber on it.

Stop it happening again

• Keep soil, bark and gravel off the post base.
• Lift the end grain off the ground if the maker allows a spike, shoe or pad.
• Make sure roof water does not discharge against a timber post.
• Cut back climbers that keep the post damp.

The base and fixings guide covers how posts meet the ground. A weakened post also changes how the frame copes with wind, which the wind and snow guide explains. If you are tired of timber upkeep, the aluminium or timber guide compares the two.

Can I just fill a rotten post and paint it?

Not if the rot is in a post that holds up the frame. Filling and painting hides the damage. It does not restore the strength.

Is it safe to use a pergola with a soft post?

We cannot say from here. A post soft at the base can fail without warning in wind. Keep people out from under it and have it checked.

Will the guarantee cover a rotten post?

Possibly, if it is within the guarantee period and the terms were met. Read the terms, take photos and contact the maker before you cut anything.