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

[Molynoox]

Hi Pete,
Yes I wasn't ignoring the advice just didn't bother to model it because it was clear what you meant. I am back to my original design now anyway. Haven't decided if I will notch or not. Everybody on the planet is notching pergolas it seems, I guess it's fashionable, and I think I might prefer the look too, but reducing strength is not something I am too happy about so I may well go notchless. But then I don't really need that much strength in those spars... I will probably model it and see how it looks and go notchless if it doesn't look too weird and clunky.
Martin

 

[Jonm]

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.

I would not go for blue. The wood above the fixing is in tension across the grain, very likely to fail ie split.

 

[Jonm]

I have not read all of the comments so sorry if I am repeating what others have said.
If you go for green here is another suggestion for structural screws
https://www.strongtie.co.uk/en-UK/products/structural-wood-screw-sdwI have used them and they are very good.

I would not go for blue for the reasons given by inspector and others.

As for red, it depends where the screw goes, could easily be the same as blue.

For green you could always drill right the way down and use stainless steel threaded rod and plate washers but maybe you will not like the look.

 

[Jonm]

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

Spot on. Wood is not strong in tension across the grain. It is how chisels and planes work, they push the fibres apart, we all know that. Like the difference between using an axe to cut a tree down or split the logs, totally different.
Sorry if I sound facetious, I don’t mean to be.

 

[Molynoox]

I have not read all of the comments so sorry if I am repeating what others have said.
If you go for green here is another suggestion for structural screws
https://www.strongtie.co.uk/en-UK/products/structural-wood-screw-sdwI have used them and they are very good.

I would not go for blue for the reasons given by inspector and others.

As for red, it depends where the screw goes, could easily be the same as blue.

For green you could always drill right the way down and use stainless steel threaded rod and plate washers but maybe you will not like the look.

Thanks for the feedback Jon
Regarding the screws, they look great... except they are interior use only.

Martin

 

[Jonm]

they are interior use only.

They are service class 1 and 2 so inside and outside in protected areas. I looked up the definition.

Service Class 1 - indoor environment with low moisture content or no damp / condensation (fastener finish may be bright or galv nails with less than 12micron zinc, or a structural staple with min 12micron zinc)

Service Class 2 - area outside the structure's thermal barrier, but is a protected outdoor environment (e,g. fastener with a minimum 12 micron zinc coated galvanising).

Service Class 3 - Unprotected outdoor environment / exposed directly to atmosphere, high humidity, coastal areas (e.g. fasteners with a 50micron Hot Dip Galv coating or stainless steel)

Hope that helps, a lot if fixings on the market do not give the service class

 

[Terry - Somerset]

I would go for green with the wafer heads. They would largely eliminate the risk of water ingress and you could even put a rubber washer under the head to make it virtually water tight.

Using coach bolts could be ok but you could not fit them if the horizontal element was aligned with the vertical support (assuming I have understood the design properly).

 

[Molynoox]

They are service class 1 and 2 so inside and outside in protected areas. I looked up the definition.

Service Class 1 - indoor environment with low moisture content or no damp / condensation (fastener finish may be bright or galv nails with less than 12micron zinc, or a structural staple with min 12micron zinc)

Service Class 2 - area outside the structure's thermal barrier, but is a protected outdoor environment (e,g. fastener with a minimum 12 micron zinc coated galvanising).

Service Class 3 - Unprotected outdoor environment / exposed directly to atmosphere, high humidity, coastal areas (e.g. fasteners with a 50micron Hot Dip Galv coating or stainless steel)

Hope that helps, a lot if fixings on the market do not give the service class

thanks for the definitions, I would need class 3 by the sounds of it.

 

[Molynoox]

I would go for green with the wafer heads. They would largely eliminate the risk of water ingress and you could even put a rubber washer under the head to make it virtually water tight.

Using coach bolts could be ok but you could not fit them if the horizontal element was aligned with the vertical support (assuming I have understood the design properly).

I am leaning towards a high quality wafer head using green method.
Bolts would be nice but I don't think I need the added strength and the screw gives a much neater look.
Cedar is a soft wood and is pretty light, although it will get heavier when wet.

 

[Molynoox]

It actually looks fine without the notches so I will probably go that way:

I also had another idea, I could put some L brackets in locations shown below, in addition to the screws.

Martin

 

[Inspector]

I would skip the little brackets. Structural screws are more than strong enough and simpler.

One brand of structural screw here is GRK and they are rated for outdoor and treated wood. I have used some of the 8"/20cm long ones and shorter when building my decks. They are 5mm or 6mm in diameter and you will never break one. I drove most of them with my 3/8" air impact gun because I don't have one of the battery types. I have seen ones up to 2'/60cm long used by log home builders so if they are strong enough for them a couple will certainly be enough for the pergola.

Pete

 

[Molynoox]

I would skip the little brackets. Structural screws are more than strong enough and simpler.

One brand of structural screw here is GRK and they are rated for outdoor and treated wood. I have used some of the 8"/20cm long ones and shorter when building my decks. They are 5mm or 6mm in diameter and you will never break one. I drove most of them with my 3/8" air impact gun because I don't have one of the battery types. I have seen ones up to 2'/60cm long used by log home builders so if they are strong enough for them a couple will certainly be enough for the pergola.

Pete

Thanks Pete. GRK not really available in UK it seems. Shame because they look really strong. Timco looks like my best option.
I'm surprised to see you backing the screw option, thought you were into the bolts

 

[Inspector]

You made it clear you want to use screws so who am I to try and beat some sense in you from here.

The upside to carriage bolts is that they cost much less, here at least. I only mentioned the GRKs to illustrate the type. There are several other brands around here so the same must exist in your area.

Pete

 

[Molynoox]

Options for screws below. There will be more but these are the ones I stumbled across or had recommended on this thread.

Timbascrew
Reisser
Timco

Timco are more expensive but might be better, hard to tell.

Martin

 

[Jake]

Cedar is really acidic, so fastenings ought to be stainless. I wouldn't trust a coating long term (and galvanisation is no good at all). Prepare for some sticker shock though.

I'd bolt like Inspector suggests, I haven't priced it but I'd guess that stainless threaded rod will work out cheaper (and a better fixing). With a cap nut and washer on the underside (recessed into a hole if you care to) it would look neat enough.

 

[Jonm]

thanks for the definitions, I would need class 3 by the sounds of it.

I do not know what is available in class 3 and the price. You have some lovely wood there and you do not want a fixing rusting, causing stains and either the head rounding off or snapping when you try to change it because it has rusted. Water can get in obviously at the top but also at the joint between the two timbers.

You could cover the head with Vaseline/grease filling it flush. Could cover the top with something, lead would be easy.

I think any rust resistant screwed fixing with a large flat head, or stainless steel washer under the head would do.

EDIT, as others have said, stainless steel threaded rod with large washers would be fine as well.

 

[Molynoox]

Cedar is really acidic, so fastenings ought to be stainless. I wouldn't trust a coating long term (and galvanisation is no good at all). Prepare for some sticker shock though.

I'd bolt like Inspector suggests, I haven't priced it but I'd guess that stainless threaded rod will work out cheaper (and a better fixing). With a cap nut and washer on the underside (recessed into a hole if you care to) it would look neat enough.

I hadn't considered the acidity in the wood corroding the finish of the screw. I guess I just figured the screws would be tested for stuff like that but that's obviously an assumption
For stainless I found this one, and you were right about the price being higher; they are about double the cost.
EDIT: Found this screw also which has the wafer/washer head on - strange that in the pictures the screw looks rusty.....
EDIT2: found this one too which looks like stainless steel but isn't, not sure if that would be good enough on corrosion compared to stainless steel.

For the threaded bar option, you mention a cap nut for the underside, presumably I would use one for the top side too? Not being pedantic just trying to follow your thinking.

Martin

 

[Molynoox]

In case anybody thinks I am mad / obsessed with screws / both, my reason for pursuing them over bolts are as follows:

• some of the spars fall directly on top of posts meaning a bolt through solution is not possible for those ones. If I need to use a screw on some of the spars then I may as well use a screw on all of them (weakest link and all of that)
• the screw gives a neater look (it's effectively invisible)
• with bolts I would need to be pretty accurate drilling a long and thin (10mm x 250mm) hole through two bits of wood, and I cannot make a mistake as this is a decorative item that won't look so cool with my drill holes poking through the sides (I don't have a pillar drill), and I also don't have any spare wood (it was far too expensive to have a buffer just in case I screwed up)
• with bolts or threaded bar there is more faffing (drilling recesses for the bolt heads, drilling the long hole, cutting the threaded bar, buying and assembling nuts and washers, aligning the holes in the two pieces of wood to get the bolt through)
• and I just like structural screws, so there

Having said that I am still considering threaded bar, but I am swinging towards screws currently.

Martin

 

[Woody2Shoes]

I would try a re-design - which could eliminate a lot of (expensive) ironmongery:

One option would be to arrage for the back beam to be a few inches higher than the front one, with through mortices (roughly square I guess) cut into it. You then cut tenons in the ends of each of the 2m 'rafter' pieces. If you wanted to get fancy-pantsy/oriental-stylee you could make wedged tenons.

The advantages, besides eliminating ironmongery, would be no holes in the top surface of your timbers to collect rainwater.

Don't forget that the bases of your posts supporting the 'back row' will be trying to rise up and will need to be very securely attached to your base plates.

e.g. a bit like this:

 

[Zedgeezer]

Having just read all of this thread, and to go off at a slight tangent, a lot of very informative comment has been made. Most of that information will realistically give you at least 30 years of beauty. I suspect the primary failure will eventually be the base of the posts. My pergola lasted 30+ years with common sense fixings, but the eventual failure was the ground fixing. I soaked the base end of the posts for 2 days in creasote, treated the above ground stuff a couple of times, but when I decided to replace it last year, I was amazed how little strength was left in the support. How are you planning to fix / maintain your base support?