Pergola - best fixing location (#1, #2 or #3?)

[Molynoox]

Hi All,

I have started putting together a cantilever style pergola, made form Cedar.
The current design is as follows:

Posts: 100mm x 100mm
Spars: 2m long, 100 x 50mm
Braces: 140 x 50mm
The posts are fixed at the base on top of groundscrews, although I am not sure if I will use bolts or coach screws for that yet:

Here is a picture of the wood:

Because this is a cantilever I would like the spars on top to have a good strong fixing on the rear post because that will be in tension, and effectively holding the weight of 1.5m of cedar, which is 'pivoting' over the front post (the spacing between the posts is about 500mm). Not sure if that makes sense but you can probably get your head around it from the pictures.
I will be notching the spars to create two shoulders which should help resist the rotational movement a little, but it will still need a good fixing.

My question
What do people recommend as best method for fixing the spar in place on top from the 3 options below, red, green or blue?

GREEN - from the top

BLUE - from the rear

RED - from underneath

Screws
I will use strong exterior screws, either a spectre or timbertite, which are both green in colour, therefore visibility could be a concern against the nice cedar. Even the ones on top could be seen, if looking down from windows in the house.

Spectre: link
Timbertite: link

My thoughts

GREEN
From a purely mechanical / strength standpoint I feel that green is easily the best, but this could create a problem with water ingress. If it wasn't for water I wouldn't even be thinking about this, green is what I want to do, but worried about creating a weak point.

RED
This would need quite long screws and would be driven at an angle to get through the 6 x 2 and into the spar at the rear. This has the advantage that its quite shielded from the water. Screw head visible and could be a little unsightly.

BLUE
This is also quite shielded from water, but not sure how strong it would be and it might look unsightly for neighbors (not sure if I'm too bothered about that part, but prefer to keep it nice for them, although they are probably less sensitive than me to little details like a visible screw head)

thanks for any feedback and ideas,
Martin

 

[Richard_C]

Will you ever need to dismantle any of it? My pergola is nowhere near as fancy as yours, but I do need to take out a couple of cross pieces for ladder access to gutters and window frames so mine sit in metal "U" brackets on top of the frame.

I doubt the green method would allow any water ingress if the screws are decent quality and driven in tight. If you know you will never want to undo them you could do a shallow counterbore and glue a tight plug in with waterproof pva.

 

[Spectric]

Think of the forces involved and remember vectors, the weight of the spar would try to pivot on the inner support but the fixing on the outer member is preventing this. With the green fixing the threads are in tension, being pulled out but unless you are hanging heavy weights on the structure then would be fine.

If the blue went in parallel with the spar and into the frame the other side then it would be stronger, no need for the green.

I would not use either of those screws in this application, look for wafer head screws and Timco are a good bet. TIMco Timber Screws - Wafer - 6.7 x 95 - Exterior - Green (Box of 50).

If you don't like the finished look then counter bore and fit a plug of the same wood as the frame.

 

[Fitzroy]

Blue, as you need to snap the screw to have a failure. Green only needs to pull out, and red is equivalent to blue but with an annoying angle to secure at.

You could do blue, but with oak pegs/dowels, which would look nicer and be more resistant to corrosion/rot.

 

[Inspector]

I would not use blue or red as you are relying on the wood not splitting and I believe they will. Green is best but should be through bolts, carriage bolts (hot dipped galvanized not shiny electroplated) if you prefer. I would not notch either member as it will weaken them. The bolt going through will be enough to keep them from rolling over. Putting carriage bolts head down will be neatest to look at and the nut and penny/fender washers on top can always have sealant placed over them if you are that worried about water. It does make it more difficult to snug up in the future though.

Pete

 

[Fitzroy]

Pete, I was also thinking If I went for green it would be a bolted solution to mitigate the risk of pull out.

 

[Molynoox]

Will you ever need to dismantle any of it? My pergola is nowhere near as fancy as yours, but I do need to take out a couple of cross pieces for ladder access to gutters and window frames so mine sit in metal "U" brackets on top of the frame.

I doubt the green method would allow any water ingress if the screws are decent quality and driven in tight. If you know you will never want to undo them you could do a shallow counterbore and glue a tight plug in with waterproof pva.

I doubt I will need to dismantle. I made sure there was space to slide out a fence panel at the back.
Cool idea on the plug

 

[Molynoox]

Think of the forces involved and remember vectors, the weight of the spar would try to pivot on the inner support but the fixing on the outer member is preventing this. With the green fixing the threads are in tension, being pulled out but unless you are hanging heavy weights on the structure then would be fine.

If the blue went in parallel with the spar and into the frame the other side then it would be stronger, no need for the green.

I would not use either of those screws in this application, look for wafer head screws and Timco are a good bet. TIMco Timber Screws - Wafer - 6.7 x 95 - Exterior - Green (Box of 50).

If you don't like the finished look then counter bore and fit a plug of the same wood as the frame.

We are on the same page regarding the forces.
Thank you so much for that link to the screws, absolutely perfect! Wafer head seems like the way to go and those timco ones look really nice. Good prices on that website too, great stuff.
Martin

 

[Molynoox]

Blue, as you need to snap the screw to have a failure. Green only needs to pull out, and red is equivalent to blue but with an annoying angle to secure at.

You could do blue, but with oak pegs/dowels, which would look nicer and be more resistant to corrosion/rot.

Funny thing is I've seen a snapped screw loads of times but never seen a screw ripped out of its threads. That may well be a lack of experience but my instincts were telling me green was strongest so thanks for the alternative view on that one
Cool idea on the wood dowels... It feels very out of my comfort zone however . Hmmm, need to give that some more thought. Thanks for the ideas.
Martin

 

[Spectric]

Martin

what I like about the wafer head screws is that they apply a good compressive load rather than sinking into the timber. As for the recess to take a plug then look at TCT Parallel to make the plug and for the counter bore I use these Drill Counterbore and both in 16mm diameter.

 

[Molynoox]

I would not use blue or red as you are relying on the wood not splitting and I believe they will. Green is best but should be through bolts, carriage bolts (hot dipped galvanized not shiny electroplated) if you prefer. I would not notch either member as it will weaken them. The bolt going through will be enough to keep them from rolling over. Putting carriage bolts head down will be neatest to look at and the nut and penny/fender washers on top can always have sealant placed over them if you are that worried about water. It does make it more difficult to snug up in the future
though.

Pete

For blue I would think if I pre-drilled the hole in both bits of wood it would be ok and not split? I am quite liking blue option. But also green....
Red is clearly rubbish
I was hoping that good quality screws would be strong enough, rather than having to use bolts and have to either cover up the heads / nuts or have the heads / nuts showing. Good idea on the carriage bolts though they are a bit neater one one end at least.
Plenty of options.... Maybe too many
Martin

 

[Inspector]

Take a short piece of 2x4 and notch it like you plan to do. Stand it on end and tap it with a hammer and see how easy it is to break the notched bit off. It won't take much but it is your choice in the end.

Pete

 

[Molynoox]

Spectric, yes those wafer heads look great for the pulling power, I've just never actually tried them. If I go green it will be those or bolts if I need to.

Many thanks for the extra links.. plug cutter looks like reasonable value, counterbore not so much they look quality though.
Martin

 

[Molynoox]

Take a short piece of 2x4 and notch it like you plan to do. Stand it on end and tap it with a hammer and see how easy it is to break the notched bit off. It won't take much but it is your choice in the end.

Pete

You think it's weak because I have only a short overhang at the back? I will try your experiment tomorrow and report back.
I do see your point though, shearing along the grain is quite weak I suppose, I will try the test anyway, to get a feel for it.
M

 

[Cabinetman]

I would have the rear brace higher so that the rear of the spar rested underneath it, that way the fixings are just to stop it blowing in the wind, probably look more interesting as well. Ian

 

[owen]

What about green but start the screw further back and angle it into the piece below? Those proper wafer head screws take a lot of force to pull them out so I wouldn't worry about using bolts

 

[TRITON]

Green is best but should be through bolts, carriage bolts

My thoughts exactly.

 

[Molynoox]

I would have the rear brace higher so that the rear of the spar rested underneath it, that way the fixings are just to stop it blowing in the wind, probably look more interesting as well. Ian

I tried it in CAD
I'm not sure I like the look of it... but it's a clever idea and a nice solution to the fixing problem

 

[Molynoox]

What about green but start the screw further back and angle it into the piece below? Those proper wafer head screws take a lot of force to pull them out so I wouldn't worry about using bolts

thanks Owen - what would be the advantage to the angled approach, you thinking greater pull out force? I would be worried about accidentally screwing / breaking through the 6 x 2 if going that route because I would have to to get the angle just right.

Martin

 

[Inspector]

I see you are still showing the notches in the pergola. I mentioned in post 5 that it weakens the wood. You reduce the piece by the amount you take out ( 6 x 2 becomes a 4 x 2 etc) and add sharp corners in the notch creating stress points that weaken the structure. It will be easier to make, go up faster and be stronger.

Pete