Glulam vs Oak for exterior uncovered pergola

[thefoxesmaltings]

As part of our design we intend to have a large 30 metre pergola adjoining the house. It has fairly chunky sized posts and sections. Approx 20x20. (Attached photo)

 

Having originally envisioned this in Oak, we've now been notified of some potential concerns of using Oak, for a pergola of this size:

 

1. It will shrink and twist over time, and given the large section sizes, shrinkage could be quite significant.
2. It will suffer from splits and cracks (‘shakes’), the extent of which will vary across the structure in terms of intensity and width/depth.
3. Engineering may require steel plates to hold connections together (as opposed to traditional joints); if this is the case it becomes even more expensive because they would need to be stainless steel (chemicals that naturally occur within the oak would corrode and eat away at mild steel)
4. It will stain any paving that’s underneath for a period of time (1-3 years) after rain due to the tannins leeching out of wet oak. It’ll disappear eventually, and it can be removed using oxalic acid – but just annoying.

 

Advice has been given to explore the possibility of using Glulam for the pergola instead. Yes we're aware that it will need to be treated every year with a UV protection/clear varnish, but we'd probably do this for the Oak anyway.

 

Anyone here with experience of using Glulam outdoors, in an uncovered area exposed to elements 24/7?

 

[markc]

You would not need stainless steel bracketry or fixings unless it was over a swimming pool. But getting oak in those sizes will be difficult and expensive and the shrinkage/splitting will be significant.

I would go with Glulam, will be cheaper, dimensionally stable and easier to clean/treat

[Ferdinand]

Looks tricky to get a ladder up there to treat it by the pool.

[thefoxesmaltings]

24 minutes ago, markc said:

You would not need stainless steel bracketry or fixings unless it was over a swimming pool. But getting oak in those sizes will be difficult and expensive and the shrinkage/splitting will be significant.

I would go with Glulam, will be cheaper, dimensionally stable and easier to clean/treat

 

Any issues with longevity of using glulam?

[JohnMo]

Hope not my glulam roof beams are exposed.  But I would think you would need to be on top of the preservation.

 

Have you considered doing the structure in box section steel?

[Conor]

Re steel. Just got a price from the company that are doing our gates and ballustrading, £1200 for a 7m long, 2m deep brise soleil with five supporting posts. Installed. Mix of 50x50mm box and 50x200mm, galvanised and powedercoated. Coming out cheaper than timber, and 4x cheaper than alu. Definitely worth pricing that option.

[markc]

Bognor Regis Waterworld, think it was 1988ish we built the glulam trusses, posts and purlins. Had to use stainless metalwork due to chlorine and ozone. Last time I saw it the timbers had discoloured but all still good. Horrible environment in the roof area.

[markc]

8 minutes ago, Conor said:

Re steel. Just got a price from the company that are doing our gates and ballustrading, £1200 for a 7m long, 2m deep brise soleil with five supporting posts. Installed. Mix of 50x50mm box and 50x200mm, galvanised and powedercoated. Coming out cheaper than timber, and 4x cheaper than alu. Definitely worth pricing that option.

Yes, I would reckon steel would be around half price of timber for a similar structure to the one pictured.

Galvanised would up the cost but reduce maintenance.

[thefoxesmaltings]

1 hour ago, Conor said:

Re steel. Just got a price from the company that are doing our gates and ballustrading, £1200 for a 7m long, 2m deep brise soleil with five supporting posts. Installed. Mix of 50x50mm box and 50x200mm, galvanised and powedercoated. Coming out cheaper than timber, and 4x cheaper than alu. Definitely worth pricing that option.

 

This is going to sound stupid of me asking.

 

What do you mean by box section steel? So basically it would be a steel pergola?

[JohnMo]

Square hollow section.  Large say 150-200mm square could have a thin wall thickness. Also rectangular could be an option.

[Conor]

2 minutes ago, thefoxesmaltings said:

 

This is going to sound stupid of me asking.

 

What do you mean by box section steel? So basically it would be a steel pergola?

Yes, but you can get box section steel in most dimensions, so you can replicate the chunky look of the timber. Obviously, painted steel looks nothing like oak. All comes down to the look you're after. You seem to be in quite a natural, wooded area, so steel maybe not the most sympathetic choice. 

 

An alternative to oak would be larch, but it's now expensive as well.

[thefoxesmaltings]

1 hour ago, Conor said:

Yes, but you can get box section steel in most dimensions, so you can replicate the chunky look of the timber. Obviously, painted steel looks nothing like oak. All comes down to the look you're after. You seem to be in quite a natural, wooded area, so steel maybe not the most sympathetic choice. 

 

An alternative to oak would be larch, but it's now expensive as well.

 

Appreciate the info. Having a bare steel (painted or not) pergola is a bit of a non starter for us. As you said it will look nothing like wood, and far too industrial.

 

I think perhaps as @markcsuggested Glulam is the way to go.

[Miek]

You can get treated glulams, might be worth looking into.. 

 

Larch heartwood is a very durable timber above ground , but you might need to get it cut by a sawmill to the sizes you want.

[SimonD]

13 hours ago, thefoxesmaltings said:

 

Any issues with longevity of using glulam?

 

It depends on the makeup of the glulam - you can order it in various different species of timber, including oak, many of which are perfectly fine and hardy outdoors. Have a chat with Buckland Timber. Their page on species: https://www.bucklandtimber.co.uk/timber-species/

 

I received an email recently where they were offering good prices on North American Douglas Fir glulam beams.

[SimonD]

Here's an example of one of their projects with exposed glulams - https://www.bucklandtimber.co.uk/project/bunavoneader-isle-of-harris-scotland/

[thefoxesmaltings]

10 minutes ago, SimonD said:

 

It depends on the makeup of the glulam - you can order it in various different species of timber, including oak, many of which are perfectly fine and hardy outdoors. Have a chat with Buckland Timber. Their page on species: https://www.bucklandtimber.co.uk/timber-species/

 

I received an email recently where they were offering good prices on North American Douglas Fir glulam beams.

Thanks Simon, have emailed them.

[Radian]

2 hours ago, SimonD said:

I received an email recently where they were offering good prices on North American Douglas Fir glulam beams.

My go-to timber for outdoor use. Victorians used it for the deck of the Clifton Suspension bridge and it was replaced once in 1958. Still there under the tarmac I think.

[thefoxesmaltings]

Generally, is Glulam Douglas Fir stronger than Solid beam Douglas Fir (non Glulam)?

[markc]

2 minutes ago, thefoxesmaltings said:

Generally, is Glulam Douglas Fir stronger than Solid beam Douglas Fir (non Glulam)?

Yes! Laminating greatly increases the structural strength, torsionally stiffer, bending greatly reduced and splits/shakes are limited to lamination sizes.