Unbendy beam / the flexural modulus of oak

Posted 19th May
Any help appreciated.

i am looking to add a pergola/gazebo to the back of the house and need to work out the dimensions of an oak or similar hardwood beam that will not bend under its own weight when wet. It has an unsupported span of 5.5m. I could add corner supports to reduce that to 4.5m but prefer a clear span. I already have the 3m cross beams sorted but stepping up to 5m is proving a challenge. I have had lots of guessed help but the time has come for some science. Am willing to pay but can’t find someone who seems to know what they are talking about. Any thoughts out there

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So is it supporting its own weight or a lot of 3m cross beams? what else?

Look at online span tables and deflection tables. TRADA have various publications that will give you deflection or timberbeamcalculator.co.uk/cal…new

Why do you want hardwood?
Wood shouldn't be ablr to get wet enough for the extra weight to be an issue.

Standard 6x2 will do an unladen 5m span without difficulty, but any timber will have deflection over a 5m span - question is how much deflection is acceptable?

I'd chamfer the top surface to stop water sitting on it and possibly run a length of flashband over it.
very useful calculator thank you. comes out at 8x2 unless I laminate which is tempting for a better looking/interesting finish

hardwood for longevity. i know there are durable softwoods but with modern forestry growing methods, i feel its best to go back to traditional species and I as bought up on the maxim Expensive to buy/Cheap to keep

I was going to chamfer but hadn't considered banding. Thanks for the suggestion
Fair enough. I guess a lot depends on the design you have and what the cosmetic requirements are. I've used flashband along the top of gates with good results - as in mine is still good while next doors painted one has rotted along the top.

You could pick up some aluminium flashing and shape it to have a drip edge.
I built mine which is 4mx4m using oak posts and treated softwood at the top as the posts are the parts that would rot first and the softwood is lighter so wouldn't be an issue with the span.

About 9 years has passed and no sign of any rot at all and haven't retreated it.
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