Open Association of Landscape Industries
How to install floating pergola posts
Hi All,
I have a garden design on the go that includes a very simple pergola. It is designed to screen the view of the neighbour's house from the patio, rather than to be walked under, so is a bit narrow relative to it's height (before anyone pulls me up on that). If is a very basic structure of hardwood with 100mm square posts, 150 x 35mm beams (actually planks around the outside edges of the structure) and a trellis roof - illustration below. Dimensions approx 6m long x 1.7m wide x 2.2-2.5m high (client is undecided).
What started out as an aversion to having posts concreted in to the ground (for fear of rot) has turned in to a specific requirement for the structure to appear to float about 50mm above the ground.
Has anyone done this before?
I wondered about a drift pin approach (rebar perhaps), so there are no visible fixings, but don't know whether one pin per post would be sufficient. The posts are deliberately narrow to keep the whole thing looking light, so there's not much room, but could possibly use 4 pins per post. How easy would this be to fix?
Alternatively SIMPSON do elevated post bases. They would be easier to install but would they be sufficient, or does it depend on the bolts used to hold the post base to the post/concrete?
If any one has any views on how best to achieve the invisible, floating look (without the risk of it falling down) I would appreciate their thoughts.
Replies to This Discussion
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PRO MemberPermalink Reply by Neal Barker @ Etesia uk on -
what about monting the post's onto 3inc blocks of clear acrilic/pvc.
just a thought
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Permalink Reply by a design and build company on -
Hi Sue
For your pergola (which I presume is going in to a soiled area), I think you could probably drill in to the base of the posts say 150mm) and bash lengths of 800 mm long 10mm dia. galvanised rod/bzp threaded bar in so that they don't come out. You could then position the pergola at the right height etc. and then excavate down the side of the rods and concrete them in. As only 50mm of the bar is visible above ground level most of the force will acting straight down the post and therefore I reckon the bar will be unlikely to bend sideways etc.
On a similar note, I too am working on a project which involves a floating pergola:
I have changed the design of the galvanised shoe since doing this image to something a bit more rigid!
- Attachments:
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pergola.pdf, 68 KB
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Permalink Reply by Fenlandphil on -
We have made galvanised shoe type arrangements in the past to stop the posts from rotting out.
I wonder though if glass blocks or bricks could be used to acheive the floating effect, a pin going through the block and into the timber may be required to locate the posts. This pin could be cross drilled and bolted through a little above the bottom of the timber uprights and concreted into the ground below the glass block. It may even be possible with a fibre optic arrangement to light the glass blocks to give a really interesting effect after dark.
Just a thought.
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Permalink Reply by Sue Bell on -
Thank you everyone for your replies - nice drawings as ever David!
I like the clear block idea (thanks Neil & Phil) but it doesn't really suit the style of the garden (but I will remember it for next time). In any event, the posts will only be 50mm above ground, so I think the client is more concerned about raising the posts than in having everything completely invisible.
I prefer the idea of a foot plate on a pillar rather than a drift pin (if only for the ease of installation), and a footplate rather than a shoe because it's less obvious. My only concern is whether 4 bolts fixing the top plate to the bottom of the post and another 4 fixing the bottom plate to the concrete foundation is sufficient to take the load. What length bolt would you recommend? Is 100mm in to the wood / concrete enough?
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Permalink Reply by David Channon on -
There are two loads to consider: The compressive load from the weight of the structure and the torsion load from its windage.
Of the two, the compressive load puts the least demands on the metal used, but the higher the structure, the greater the sideways stresses might be.
I'd be tempted to make the whole structure out of RHS with the pins connecting the floating part welded to the rebar in the concrete foundation which acts as a cantilever. Then you can sleep through the next October hurricane without worrying that its all blown over.
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Permalink Reply by John on -
I don't think you will have any sideways wind problem with your design, it
is not the same as a fence, the oblong structure counteracts this force.
I have seen floating posts that have had a hole drilled into the end and then
simply slotted over the concreted in rods with a suitable glue added to the hole
first.
Admittedly the round post were 6" in diameter, but this was because it had to
support a high decking structure up against the house. I would think that 4" post
would be suitable for you job with a 1" rod.
Once the rods are fixed in at the same height, drilling equal depth holes in the posts
makes the job easy.
However I am only going on what I have seen, so you will have to check this out a bit
more thoroughly if you decide to do it this way.
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Permalink Reply by Rose Lennard on -
I'm sure you can just use one drift pin per post, as I've seen this done before and it's a lovely look. Personally I would run proposals past a structural engineer as I don't consider myself qualified to calculate the loads/ stresses and the strength required, and it's the sort of thing that would be a pain to rectify if it became damaged. You could overspecify it to limit the risk, but then it could end up being a heavier structure than it needs to be.
I'd be interested to see the end result!
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Permalink Reply by Sue Bell on
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Thank you everyone for your comments. Now to find a structural engineer....
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Permalink Reply by a design and build company on
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Hi Sue
I really think getting a structural engineer in for your project is totally un-necessary. If the pergola was made of huge sections of timber, steel or concrete then, yes, structural calculations/advice would be necessary. Granted, if your pergola is to be made of hardwood (very expensive in the timber sizes you have chosen) then it is going to be substantially heavier than a softwood one but at the same time it is also going to be a lot more rot proof. I used to work as a Junior Civil/Structural Engineering Technician and unless you know one as a friend, I think the cost of their advice may outweigh the risk in just doing what you think will work yourself. To be "safe", why don't you go for a driven in pin that has a square steel plate welded to it at the point it makes contact with the bottom of the post so that the majority of weight bears on the plate and the internal driven in pin holds it steady. I'd use twisted reinforcing bar for the pin as well so that it "bites"/"jams" in the lesser diameter hole that you drill. Anyway, it's your choice, but personally, I'd take the risk.
P.s make sure everything is galvanised also....
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Permalink Reply by John on
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I agree totally with this, a structural engineer is not required, and I think this suggestion
with a pin and a plate is the best option.
a design and build company said:Hi Sue
I really think getting a structural engineer in for your project is totally un-necessary. If the pergola was made of huge sections of timber, steel or concrete then, yes, structural calculations/advice would be necessary. Granted, if your pergola is to be made of hardwood (very expensive in the timber sizes you have chosen) then it is going to be substantially heavier than a softwood one but at the same time it is also going to be a lot more rot proof. I used to work as a Junior Civil/Structural Engineering Technician and unless you know one as a friend, I think the cost of their advice may outweigh the risk in just doing what you think will work yourself. To be "safe", why don't you go for a driven in pin that has a square steel plate welded to it at the point it makes contact with the bottom of the post so that the majority of weight bears on the plate and the internal driven in pin holds it steady. I'd use twisted reinforcing bar for the pin as well so that it "bites"/"jams" in the lesser diameter hole that you drill. Anyway, it's your choice, but personally, I'd take the risk.
P.s make sure everything is galvanised also....
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Permalink Reply by TrellisDirect on -
Hi Sue,
I've heard this product is pretty good from Estate Sawmills. If you want to keep the base looking sharp, i'm sure you could source some stainless tube to cover up the upright supports.
Best of luck, TrellisDirect
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