Pergola Advice

[DavidRa]

Been tasked with replacing this 3.6m 47 x 150 joist. The sagging has occurred over 20 years ( not my work!) and I was considering attaching a metal bar to the inside face to add some strength along it’s length.
Questions
How long should that bar be on 3.6m span ?
Is screwing it to the face enough or should it be routed in?
Easy enough to get 1m bars from Screwfix or similar or should I source just 1 bar of the recommended length if the view is that the bar needs to be longer than 1m?
Thanks

 

[Jacob]

Bar idea no good even if full length. Cable + tensioner would make more sense.
I'd either leave it alone - it's not going to collapse without warning, or stick another post in the middle.

 

[Phill05]

What about adding a brace to the uprights? that should counter some of the sag.

 

[Bingy man]

I’d be inclined to agree with @Phill05 and add side braces . I’d also add another joist or make it bigger as 3.6 mtrs is quite a run with no support in the middle. Maybe even consider a double joist mortised into the 2 end posts ..

Edit what is that on top of the pergola ( honey suckle, clematis) fhat plant can add many kilos especially if overgrown and wet . It will benefit from trimming it back but difficult to see fully from your pic .. it

 

[paulm]

Turn it up the other way, in another twenty years it will sag enough in the other direction to be straight !

 

[Bingy man]

Turn it up the other way, in another twenty years it will sag enough in the other direction to be straight !

Tbh I thought this but don’t think the customer would approve

 

[ChaiLatte]

Bar idea no good even if full length.

The Young's modulus of steel is roughly 30 times that of redwood. That means if the steel plate was the same depth as the existing wooden beam, it could be 1/30 the thickness and have the same bending stiffness as the wood.

That means adding a 150 x 1.5mm full depth plate would double the bending resistance of the structure.

The bending stiffness of a rectangle is proportional to the cube of its depth. Hence, a 100mm x 6mm steel bar screwed on would also double the bending resistance.

The numbers above are to give the OP an idea of just how much steel is needed to be equivalent to the wood that is there right now. Keeping the steel rust-free for the next how many years is a different issue.

 

[ChaiLatte]

Maybe even consider a double joist mortised into the 2 end posts ..

Anything sistered to the existing does not need to sit on the supports. There is zero bending at the supports, so it is extra work for no improvement. The beefing up is only needed midspan where the bending is at its maximum. He might get away with something 2/3 the existing span fixed centrally.

 

[duke]

I would support either side of centre of front face . Make a cut dead centre then jack up either side till level and slip another header or beam behind it to support the load. Also use a mechanical fastener/bracket to support the ends to either post. Leave the existing beam in place fasten it to the new. That's if client is ok with it.

 

[duke]

And add in diagonal braces as mentioned.

 

[Inspector]

From the end closest to the camera it looks like the beam was notched to take the rafters (?) reducing it to more of a 47 x 100 beam. I not an engineer but I doubt doubling the beam would work long term because of the notches unless you can fit a deeper board. Maybe cutting a pocket in the posts to accommodate a 47x200 or deeper board. If using steel angle iron would be stiffer than a flat plate. Get it powder coated after drilling all the holes etc.

Pete

If the deeper beam or angle iron runs longer than the posts the droop of the pieces past the post will be taken out too.

 

[dizjasta]

Hi after 20 years in place and perhaps without any maintenance a renewal seems the best solution to restore the visual aspects for your customer. Good luck.

 

[wsb1207]

Been tasked with replacing this 3.6m 47 x 150 joist. The sagging has occurred over 20 years ( not my work!) and I was considering attaching a metal bar to the inside face to add some strength along it’s length.
Questions
How long should that bar be on 3.6m span ?
Is screwing it to the face enough or should it be routed in?
Easy enough to get 1m bars from Screwfix or similar or should I source just 1 bar of the recommended length if the view is that the bar needs to be longer than 1m?
Thanks

Much easier and cost effective to prop up the joists and replace with a new timber

 

[TRITON]

I'd replace the Entire structure - its a project innit. Tell the customer its rotten* and best make the whole thing anew.

Adding side supports isnt going to magically make the beam straight again, though it might prevent it from snapping as summer plant growth adds weight.


*Looks rotten to be honest.

 

[Bingy man]

Anything sistered to the existing does not need to sit on the supports. There is zero bending at the supports, so it is extra work for no improvement. The beefing up is only needed midspan where the bending is at its maximum. He might get away with something 2/3 the existing span fixed centrally.

My assumption was the op wanted to maintain the existing span hence my advice . If he chose to fit a central support post then the span would be reduced to 1.8 mtrs so naturally he could use a smaller beam /joist ..

 

[dizjasta]

The distress shown by the front beam shows it is sagging by failure of it's structure and not by a problem of over loading. It needs to be replaced.

 

[mc281]

Been tasked with replacing this 3.6m 47 x 150 joist. The sagging has occurred over 20 years ( not my work!) and I was considering attaching a metal bar to the inside face to add some strength along it’s length.
Questions
How long should that bar be on 3.6m span ?
Is screwing it to the face enough or should it be routed in?
Easy enough to get 1m bars from Screwfix or similar or should I source just 1 bar of the recommended length if the view is that the bar needs to be longer than 1m?
Thanks

Better, as lighter weight, is a thin flat strip of steel underneath, screwed every 100mm in zig zag pattern, to avoid splitting. If you jack it up in the middle, before fixing, you will amazed how well this works. It costs much less than angle and may transported rolled up.

An angle will flex over time as the timber will continue trying to bow along the opposite side but a strip will not stretch, prevent any twisting by evening out the bending force. Good length and well gripping screws are required.

 

[Jameshow]

Unless the metal bar is deep 100mm l+ and preferably a L section it's not going to help sagging.

Tbh for the price of £20 for some timber I'd be replacing it!

 

[DavidRa]

Thanks for all the comments, very helpful. To clarify , I was always going to replace the joist with the same sizing due to aesthetics and will now look to add end brackets attached to the posts and also a steel bar across the middle , 1.5- 2m long and possibly 6mm thick. It may not last forever but seems like a longer lasting and cost effective solution than doing nothing

Thanks All

 

[Sgian Dubh]

Been tasked with replacing this 3.6m 47 x 150 joist. The sagging has occurred over 20 years ( not my work!) and I was considering attaching a metal bar to the inside face to add some strength along it’s length.

Well, you seem to have some creep deformation going on there, a phenomenon studied by rheologists. Relatively small loads, which is basically what you have with what seems to be only those front to back crossbeams to carry, within the elastic limit of the 3.6m (wooden) beam has turned into permanent plastic deformation because of the load's twenty year duration.

As others have suggested, and you're contemplating, adding a steel plate will help provide additional beam load carrying capacity, but then there is the rust thing to think about. It also won't hurt to include end brackets.

Here are a couple of other things you might take into account. You say the original beam is 150 X 47 mm. Doubling it to 150 X 94 mm doubles the load carrying capacity. This could easily be achieved by fixing two pieces 47 mm thick together (or simply buy 150 X ~100 mm stuff), and you'd need to tweak the existing crossbeams to suit. You say you're not going to increase the width (depth) of the beam, but if you could do so by ~40% you would again almost double the load carrying capacity; in your case that means adding ~60 mm of depth taking the beam's depth to ~210 mm.

Shame you can't stick a post at the mid-point though. Slainte.