Fixing rafters. Hangers and birds mouths.

[OldKettle]

Hi.
I need to run some rafters from a wall plate to a cross beam. All of the timbers are 2x6 and the run is 4000mm. This is part of construction of a lean-to/attached pergola.

The bottom of the wall plate has to be at 2340mm and the bottom of the cross beam can't be any lower than 2000mm.

If I do a birds mouth (diagram A) I think that will only give me a rise of 190mm (340mm drop - 150mm height of rafter) which will be a pitch of about 2.7 degress.

If I use joist hangers either end (diagram B) I can get a drop of almost the full 340mm (slightly less because a thin section of joist will still sit above the cross beam) which will get me to almost 5 degrees.

I will want to put some kind of lightweight clear roof on at some point, probably polycarbonate, so will need something to catch it past the beam. If I go with approach B, I can just add something on the far side of the cross beam to catch the roofing overhang.

Alternatively, I can do a variation of B with a very deep notch (diagram C), but I don't think that gives me any real benefit and just complicates the cutting.

I would like to ask whether you folks think that approach B is OK.

The bearing surface of the hangers is 70mm. Rather than use an angled hanger I plan to just notch the rafter to fit with the horizontal base of the hanger.

Any thoughts would be much appreciated.

 

[Mr Chibs]

Looking at your images, and the fact you are thinking of polycarbonate sheets, I think B would work, as most of the depth of the rafter will be in the hanger.

I'm about to be building something similar, but I was leaning to glazing, possibly polycarbonate as I'm not keen on twin wall roof sheets.

Personally I'd prefer design A, as I think it looks cleaner... is there no way you can increase the wall plate to give you more of a slope?

What in the way of glazing bars are you going to use?
Alukap look good, but seem expensive.

 

[OldKettle]

Thanks. To be honest I haven't looked into the roof covering too much at the moment, I'm just wanting to get a sufficient drop and leave space for flashing, so that I can have options for the future.

Because I don't want to spend a fortune, and because I may look to extend the house in a year or two, I might go with a corrugated poly or PVC roof.

The glazing bars that would be needed for flat panels look like they will cost the same as the 4.8m 2x6s that I am buying for the rafters, and that is with timber prices sky high.

 

[Mr Chibs]

I haven’t costed the parts up yet, but was thinking there will be no cheap way of doing it... I was quoted around £60 for glass per metre² Will need around 15m!

I was thinking of 6x2 Would suffice for woodwork, but will have to check the span tables.

I’m also going to investigate the possibility of using aluminium box section Instead of wooden rafters, I’m only going to build this once and it may look more modern this way.

Thanks.