How to secure pergola posts?

Question

Hi there,

I bought a louvred pergola but have a question on where and how to secure the posts for stability and longevity.

The issue I have is the pergola will be installed over a semi recessed spa area which consists of a concrete slab bordered by a single course of large (600x300x300) limestone blocks cemented together. Unfortunately the spacing between the pergola legs is just wide enough that they will sit off the concrete slab.

So I can either mount the posts to the top of the limestone blocks or try and remove some blocks where the legs will sit, pour concrete footings and mount the posts to the footings once they cure.

Mounting to the top of the blocks with chemset and threaded rod is obviously quicker and easier, plus keeps the steel feet and posts out of an area where water may pool and thus reducing chances of rust. In terms of aesthetics the pergola will be 300mm higher and block less of the view of the parkland and large gumtrees through the back fence. Though one thing I am not sure of is the ability of the blocks to withstand the wind loading when the louvres are closed. My gut tells me this would be fine as the blocks are huge and are cemented together, plus I believe any gusts of wind strong enough to move those blocks would deform the louvres before causing any movement of the structure itself.

Removing blocks and pouring footings to mount to is much harder to do but the more by the book method. I would have to find some way to cut and move the blocks, which google estimates may each weigh 100-130kg. Then I would have to retain the soil that the blocks were retaining. Then pour the footings and then mount the posts to the footings. This would obviously leave an unsightly gap in the blocks, and be an area where water may pool. Plus would put the pergola roof at a lower height.

I have already checked with my local council, no permits required to install this pergola. So my question to the brains trust is this, do I trust my gut and go with simpler installation method? Or do I spend the considerable effort into the more complex method?

Thanks for any advice

Nailbag · Answer

Hi @bigkid

Definitely go the harder approach and remove the blocks and create the pergolas own footings. Unlike the block limestone wall which could move over time, dedicated footings will provide an dependant long term solution.

Nailbag

Dave-1 · Answer

Afternoon @bigkid

:) You already know the answer….. lol And I would dislike it as well. Been in that type of scenerio too many times myself. You need to remove the blocks and do the correct footings :/ Sucks but as you have spent the $$$$ on a louvered pagolla (nice choice) there is no way you want to go for an easy option and chance fate.

Having the pagolla mounted on the block (they look like bessa blocks to me, filled with concrete and capped) of limestone and securing it without cracking the block, might be doable. Mounting the oppisite legs on something higher is were I hesitate. I would rather mount them all on a level surface without raising it up.

The distortian you mention if you mount it high and the wind blows says it perfectly. The legs on the ground, really shouldnt have a chance to sit in water otherwise you should factor in drainage around that recessed spa. (Im installing a spa and really should be out there working on the drainage solition right now)…. But its hot and muggy so it can wait :)

A crow bar and leverage for the limestone blocks plus a square nosed shovel to slip under it once you lift it a tint bit will help you twist and move the block, dig out soil behind the block that needs to be moved. Keep the soil the same level as the ground it is on. Leverage the block backwards to form a wall behind where the post will be might be an easier way to install it instead of carting the blocks away.

Dave

JacobZ · Answer

Hi @bigkid,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

The first place to start is always the manufacturer’s installation guide for your specific louvred pergola. The guide should outline the required footing type, minimum fixing requirements, wind rating assumptions and any notes about acceptable mounting surfaces. It is important to follow those instructions closely, as installing outside of the manufacturer’s specifications can affect both performance and warranty.

In terms of mounting to the limestone blocks versus installing dedicated concrete footings, we are unfortunately not able to make a determination on which option is structurally suitable for your situation. While the blocks are large and cemented together, wind loading, especially with closed louvres, can create significant uplift and lateral forces. Assessing whether the blocks and proposed fixings can safely resist those forces requires consideration of site-specific wind classification, footing depth, soil conditions and the structural capacity of the limestone and anchoring system.

For that reason, we strongly recommend speaking with a suitably licensed builder or structural engineer who can assess your exact conditions and confirm the most appropriate and compliant fixing method. They will be able to advise whether mounting to the existing blocks is adequate or if engineered footings are required.

Allow me to tag our knowledgeable members @Dave-1 and @Nailbag for their thoughts.

Jacob