my new flat roof joists are going to sit on the purlin, but it seems wrong to have them sitting on the point at the top, so should I make pieces of wood to sit in between and spread the load, how to do this so that they'll last a hundred years and not split?
photo at:
Won't open. Says I have to join Facefart. Bollocks to that. Use a proper file sharer.
Bill
Seems to work OK for me, although it does ask for a log-in, which I ignored.
I'll admit to being totally ignorant on structural matters where roofs are concerned, but why rely only on the purlin for carrying the flat roof. Why not put a couple of decent sized bolts (with washers) through the flat-roof joists and the adjacent pitched roof rafter where they overlap, some wood glue (Cascamite or Gripfill) in between, and tighten up?
I know exactly what you mean. Been there; done that. But unless you can cut those angles to *precisely* match those of the rafters (presumably
90') and perfectly in line with them laterally then it's not going to make for any improvement. And that's not really a DIY job.
sorry i thought settings of facebook were open to everybody, and its easy to upload there from my phone... photo copied to here:
Or cut some wood off the top of the old oak purlin in line with new joists?
Or make 2 triangular wedge shape bits of wood for each side? (this wouldnt work on the left where I need the joists a bit higher)
Car body filler would crumble wouldnt it as the roof flexes over time?
[george]
Bolt them to the common rafters using timber connectors.
Might be a good idea; certainly better than attempting cut-outs.
No filler! Bad idea!!
A new inside photo of joists temporarily resting on purlin is now at:
I will also add a new support for the flying end of the purlin, which is at the moment nailed upwards.
When the joists are safely sitting on the purlin I will probably nail them to the sloping rafters.
I believe nails are better than screws as they can be removed if and when the roof is reslated in another hundred years.
[george]
I can't see that adding anything flat under the joists is doing any better than what the bottom of the joist is already load spreading wise.
That's what I would do.
The joist are all laid flat and fixed to the perlin. to form a slpe to drain the rainwater off you put fering strips on top of the joists
I was going to say that - well, screw then to the rafters, with a couple of corrosion resistant 6mm screws (self drill and/or pilot holes as required).
I would not just sit them on the purlin - you are loading it the reverse way to what it was designed to do - and your new rafters will try to pull it off the old rafters.
In fact I would consider running a new beam on the top of the old rafters to the load is going the *right* way.
Screw the new beam into every rafter.
Sorry, I dont understand that
At the level of the purlin? (Not below because the bottom of the rafters looks bodged, its not sitting on a wall plate.)
isnt it better to nail them than screw them?
Since we are talking about a flat roof, its presumably not carrying much load anyway?
I would simply fix them to the adjacent rafters (probably with a bolt through, and a timber "dog" between the timbers).
If the purlin is not level. then use different heights on the rafters to get your new joists level. You can use firrings of like slope, but increasing thickness to get your compound slope.
Purlin is under the old rafters.
Putting a downwards load on them is going to try to pull them off.
Normally, the purlin is in compression against the rafters (to support their mid span) - you proposal reverses this and I think that's dubuious
- unless you add some plates or other means to tie the purlin to the old rafters under tension.
Wherever height you want the support for the new rafters (which I assume is the same height as you were going to use the purlin as support, weren't you?
Right...
So where's the downward load going to go, when you hang the end of your new roof off the purlin?
If the wall plate and/or old rafter ends are knackered, is the purlin well and independently supported to accept a vertical load (normally purlins take load at 90 degrees to the rafters and usually have thrust poles running back to the centre wall)?
I think we need some pictures from inside the roof of the old rafter ends and wallplate and the purlin support poles.
I prefer thin self drilling screws - minimum disruption to the wood (like nails) but more subtle. Spax do a nice one, that I've been using for a not totally dissimilar job:
Available from Amazon.de (seems to be no problem shipping to the UK and took 2 days for me).
But it's not somethign to argue about - nails have served people for centuries in this role. In my case, I did not want a lot of banging disrupting the good bits of the roof.
>
I wouldn't underestimate snow load - I did the calcs on mine and dry, the roof weighs an estimated 1000kg (adding the per m2 mass of the roofing components). Adding worst case snow load to SE England standards that I found somewhere on the web took the total potential mass to 2400kg.
That's what I would have considered as an option, but the OP did say something about the old rafter ends and/or wallplate being dodgey - I think we need to know more :)
Tim Watts wrote: {I wouldnt unerestimate snow load]
I asked an architect who said 5x2 rafters for the span of 2.4 meters, but the span increased to 3.6m so i'm now using 7x2 rafters.
I'll ask hereford bulding control for another visit when the woodwork's done, before sticking down EDPM
Photos of today's joist ends o0n my facebook at
The spax screws someone recommended look great
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