For a 20m span some kind of ‘engineered’ product would be needed since sawn timber is not available in such long lengths. Of course shorter spans could be supported with posts. For a clear span, LVL will be fine although for a higher quality appearance you could consider glued laminated timber (‘glulam’). LVL is produced with a structural grade face and is not generally used in appearance applications unless a rustic look is desired. While a 20m beam is theoretically possible to produce, it’s beyond the usual range of sizes used in domestic construction and likely to require a special order. We suggest you discuss lead times and transport considerations with an LVL producer. If the timber is shielded so it won’t get wet when it rains, preservative treatment is not necessary and the usual range of paints and exterior wood stains can be used. Maintenance will not be an issue if the timber is not exposed to the weather.
I am looking at designing a covered outdoor area using timber. I assume LVL is the correct product to span circa 20m.
The timber is external but undercover. It will not be exposed to the rain.
My client wants minimal maintentance.
What treatment is appropriate to the LVL and how long until first maintenance?
I am building a pergola that has no roof on it so the timber beams are open to the elements. The design also has beams that over hang so are not constrained. Because of the cost of recycled spotted gum, I am looking at Vic ash Glulam. My concern is whether glulam will bow or not in these type of application.
Glulam is a very stable product and is unlikely to bow even in an external application. However, we would be concerned about the durability of untreated Vic ash glulam in the long term. It’s not a highly durable timber under full weather exposure. Preservative treated Vic ash is now available, known as ‘IronAsh’, so if the glulam you are considering is made from preservative-treated material it should give satisfactory performance. Otherwise it would be a good idea to consider a more durable species.
Can you advise if weeds growing over decking will lead to the wood deteriorating?
Weeds growing over decking aren’t a hazard in themselves – they don’t contain any substances that attack wood. However, they are likely to hold water after rain, and may cause dirt to accumulate which will also retain water. Anything that keeps wood damp for long periods and prevents quick drying after rain will shorten its life. Damp patches may lead to fungal decay in the longer term. It seems a little unusual for weeds to grow over the top of a deck, but perhaps it’s a low-level deck. Anyway, we recommend that weed growth is removed from over (and under) decking.
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