How to build a pergola?

[MamaCass]

Hi Guys! 

I am building a detached pergola. I’ve got a huge concrete slab in my backyard with no shade. 

I have bolted the 6 x steel U-shaped post supports to the concrete using dynabolts. These will hold the posts. 

I’ve got 6 x 90x90 posts arranged 3 a row. They span a total 8 meters. (4m span between post. And the depth of the pergola is 4.2m. 

I have used  2 x 9m SMART LVL 15 H3 240x42 Structural LVL Timber pieces to span as bearers. I wanted a 45cm over hang on either side of the furtherest posts. These LVL sit on the three posts affixed with bolts and set on a lap cut into to the posts. 

for the rafters I have chosen

Treated Pine H3 F7 Timber 140

X 45 - 4.8m lengths that I will space 55cm apart. 

my question is are the 140 x 45 suitable to span 4.8m without twisting and warping over time? It won’t carry a heavy load. I am fixing plastic roofing on top and that is very light. I plan to use battens 70 x 35 spaced 90cm apart. 

any advice?

I used the attached load sheet to confirm that the 140 x 45 H3 F7 

was the suitable timber to choose but now I am not sure. 

also, note that on my design drawing I had originally intended to set the rafters flush to the beam so birds couldn’t nest but now I have decided to sit them on top because I prefer the look. I have notched 6cm into the LVL per rafters for them to sit in that notch. What screw should I use to secure them to the LVL? 

thank you. 

 

 

 

[MitchellMc]

Hi @MamaCass,

If I'm reading the span table correctly, on a single span (between two posts at the ends), 140 x 45 F7 is capable of spanning 4700mm at 600mm centres. Your 500mm centres do help, but if it can span 4800mm, it would be right on the cusp of the maximum span, which typically isn't the best building practice. I'm not an engineer, of course, so you might need to speak to a builder or the like to get their opinion. When building a max span, it does open up possibilities of warping or sagging, though your battens will help with twisting and warping.

For the fixings from the rafter to the beam, you'd be looking at something like the Pryda Joist Hanger - To Suit 45 x 140mm and connector nails.

Please let me know if you have any questions.

Mitchell

[MamaCass]

I was looking at the “continuous span” section of the table and not the “single span”. Could you let me know the difference please?

When the 4.8m rafters are placed on the LVLs the rafters have an over hang of 30cm on the front and and 40cm on the back, so roughly 4.1m is the length that goes from LVL to LVL unsupported, Would that help? 

[MitchellMc]

Yes, that would help @MamaCass as it brings you considerably within the span capabilities of 140 x 45mm timber. A single span is one span between two supports. A continuous span is across three or more posts, sharing the load across them.

Mitchell

[MamaCass]

As usual the Bunnings guys have saved the day, quickly and clearly!

I couldn’t have done this project without the incredible support this forum offers! 

THANK YOU! 

[MamaCass]

Sorry Mitchell, I also just noted on my receipt that my rafters are H3 F7 MGP10 which if I read the span table correctly, adds even more to the span? Is that right? 

[MitchellMc]

Hi @MamaCass,

If it's our 140 x 45mm H3 treated pine, then it is MGP10-rated. It's slightly unusual that a product would be rated both MGP10 and F7, as they are two different rating systems. But, to answer your question, yes, MGP10 has higher span capabilities. They might have added the F7 notation for compatibility or reference with older span tables. You can safely use the MGP10 span values when working out your spans.

Mitchell