Thread: Pergola building (roofed)

Three quickies:

1. You'd normally put the rafters on top of the outside beam. I 'think' you can just skew nail but depends on the prevailing wind conditions in your area. I'd put triple grips on each rafter as well. Check with council.

2. I'm pretty sure you can get a 7 metre beam.

3. You'll need more than 3 posts as a timber beam will only span 3 metres max.

Originally posted by silentC
3. You'll need more than 3 posts as a timber beam will only span 3 metres max.

My tradac manual says you can span up to 3.6M. (Table 51-verandah plates)
You should be able to get timber up to 7.2M long, however at this length it's sometimes more expensive per metre than the same dimension timber in shorter lengths. Also at this length it's usually bowed badly which means you'll have to muck around trying to straighten it. I'd be more inclined to join 2 lengths with a stepped or cogged joint over the centre post.
Ashes, what do you mean by "stiffeners"? Are you reinforcing the rafters or the fascia? I'd be looking at (the very least) double blocking the ends of the rafters and then batten screwing your fascia to the rafters. Depending on wind loads etc. you may have to double each rafter instead. Be aware that most fascia is cosmetic only and you should be placing your hangers so as to coincide with the rafters.
Best practice is to put your new rafters (they're rafters not joists) on top of the plate (beam). However if you need the headheight or it's required aesthetically you can use hanging brackets. Just skew nailing is not good as you need to tie your posts/beam back to the house.

Mick

thanks Mick.

I'm new to this game so my terminology will confuse most people...

the stiffners I'm talking about would be around 900mm lengths nailed to the house rafters to reinforce the house roof/rafters to take the additional weight of the pergola. Yep, I would then ensure the facia is additionally strenghted by nailing/screwing into these stiffened rafters. I've seen some guidelines on this in regards to nail lengths/guage etc. I'll look to strenghten each rafter.

Is the stepped joint also called a scarf joint. ie, effectively a lap joint but for joining wood end on end?

Headroom will be an issue so looks like hangers will be the way to go on both ends of the rafters.

In your manual Mick, what size F7 Treated pine is spec'd for a 3.6m span?

thanks,
Brian

Mick,

Are verandah plates subject to a maximum roof load? I haven't got it in front of me but I think the NSW Framing Manual only specs up to 3m continuous span for beams and lintels under roof loads. I could be wrong.

I think this is a tiled roof and it's a 3.3 metre span.

I may not have been clear initially. The pergola will have a laserlight/colourbond roof. The house roof is tiled

Fair enuff. You'll probably be right anyway. I always tend to over-engineer things. For the sake of the cost of another post and footing, I'd rather have the extra support...

I am the same. I will overengineer my projects and accept the extra cost and durability. I am however making this for my sister and budget is a real consideration. Minimum spec is what I need to build to which invariably has contingency built into it anyway.

3 posts will be enough to span 7 metres even if the maximum span allowed is 3 metres.

Just move the end posts in 1/2 a metre so that there is 1/2 a metre overhang at each end.

This is not a huge amount and will still have good aesthetics

A good joint for joining long lengths is as per the attached sketch.

Silent C,
The manual I'm looking at is W41C (C for cyclonic, it's the lowest rated manual I have), it shows verandah plates/beams as max 3.6 wether single or continuous, sheet or tile roof. Lintels however are diiferent, much less depending on timber sizes. A lintel is a beam over an opening in a wall, whilst a verandah plate simply holds up a roof. You might be looking at the wrong part of your tables.

Brian,
I'm possibly wrong with my terminology here but what Bob has drawn I call a cogged joint. A straight version of it (ie: consists of only verticals and horizontals) is a stepped joint. A scarfed joint is when two members are tapered on their vertical axis. The taper is usually quite long, like 1:5 or more. This is not usually used in building work, more often in bent laminations and strip plank boat construction.
In my manual, which may be overboard for your possibly non cyclonic area , rafters @ 900 crs and 3.6 span, seasoned timber, F7 need to be 140x45.
I'd go with Bob on setting the end posts in from either end of the beam, this will actually look better than finishing right on the post.


Mick

Mick,

In the NSW manual (mine is at least 5 years old BTW), lintels and beams are spec'd in the same table. Basically, any member that spans an opening and holds up the roof or the floor above. If it's over a window or door they call it a lintel, otherwise they call it a beam. At the end of the day, they all do the same thing, which is to hold up the roof without bearing on anything that might be underneath apart from the supporting columns or walls.

Verandah plates - I'm not that familiar with. I just had an idea that a verandah plate would not be holding up a full roof load: ie. it would just be a short run from the top of the adjacent exterior wall. Regs could be different in QLD but they're all pretty much based on the same recommendations, so I'm most likely wrong about the 3m max.

Incidentally, an engineer once told me that verandah and other open roofed areas should be designed to hold the roof down, not up!

Silent C,
I would have thought that being in a cyclonic area my tables would be more conservative. The difference between a lintel in a wall and a beam on a verandah is that the beam merely needs to hold the roof down (and up also, but round here especially, down). A lintel in a wall, besides taking roof loads and cladding and lining weight also needs to resist sideways deflection due to wind loads and racking forces. In my tables the max span for a lintel in a single or upper storey with a tile roof varies from 3.6M (300 x 75 F8, roof span 2.4M, rafters @ 600crs) down to 2.4M (300 x 75 F8, roof span 8.6M, rafters @ 900crs) with even smaller openings for smaller lintels of lower grade. With a verandah most sideways forces are transferred back to the main structure via bracing, strapping and the diaphragm of the shheting itself.

Mick

Mick/Ashes,

Sorry fellas, I've had my head firmly planted where the sun doesn't shine. Just checked the NSW Framing Manual and of course it specifies lintels and verandah plates up to 3600.

Your half span is > 1500, so you'll need to go up to 2000. In my table it says that a verandah plate with a sheet roof spanning 3600 must be 200x75 unseasoned F4, F5 or F7 and 170x70 seasoned F5, F7 or F8.

Apologies for the red herring. Seems you can pick a span from the verandah plate table for rafter lengths up to 4000, after which you have to go to the lintel tables.