Advice on rafter size needed please (diagram attached)

[chickenlips]

However, since loose cut raise ceiling roofs generally have some sort of SE calcs backing up the sizes specified its generally assumed that horizontal thrust will also of been included in the calc and designed accordingly. Kind of like what tony has included above

The point is that no SE wants to put anything other than the most nominal horizontal forces into the top of a masonry wall, as masonry doesn't like bending and tends to crack...
Hence the reason most SEs will specify a ridge beam. It just looks like a nice alternative to a ridge beam to me, to allow for a bit of horizontal movement, without having to build the wall out of reinforced concrete! I think 5 or 6mm would not be unreasonable.

So using collars etc.. doesnt help to reduce the horizontal force ?

 

[RonnyRaygun]

However, since loose cut raise ceiling roofs generally have some sort of SE calcs backing up the sizes specified its generally assumed that horizontal thrust will also of been included in the calc and designed accordingly. Kind of like what tony has included above

The point is that no SE wants to put anything other than the most nominal horizontal forces into the top of a masonry wall, as masonry doesn't like bending and tends to crack...
Hence the reason most SEs will specify a ridge beam. It just looks like a nice alternative to a ridge beam to me, to allow for a bit of horizontal movement, without having to build the wall out of reinforced concrete! I think 5 or 6mm would not be unreasonable.

So using collars etc.. doesnt help to reduce the horizontal force ?

Collars will reduce horizontal forces, but they won't eliminate them. Obviously the lower the collar, the lower the force.
Tony's design will definitely impart horizontal forces into the wall, but if a sliding support is used those horizontal forces will be eliminated.

It's up to the individual SE to use their discretion as to whether the forces are significant, but it certainly offers the SE an alternative to a ridge beam and perhaps allow slightly smaller section sizes than would otherwise be used to reduce deflections to nominal amounts.

I've never worked anywhere where I would be allowed to specify Tony's design, but perhaps if the horizontal forces could be eliminated more SEs would consider such a design.

 

[theoldun]

Letterem, Just to confuse the issue a little more for you, instead of raised tie trusses you could consider either a scissor truss or coffer truss. Have used scissors, but never used a coffer.

OR, RISC have a design for a raised tie, stick and throw roof with standard ridge board, for both a 5metre and 6metre span. Couple of problems though, spars are 75x195 at 400 centres, ceiling joists are 38x175 bolted to spars. We would make ceiling joists 47x175 for ease of fixing both insulation and P/B. Fixing ceiling joists like this are a right pig to get right unless you know the wriggles how to do them. One other thing is they have made a balls up with their pitch figures and you may not get the height above plate you require.

Google midsussex.gov.uk/media/RaisedTieRoofRICS.PDF and read your fill.

Tony Ron Chick lips, you may find design of interest.
Regards oldun

.

 

[RonnyRaygun]

Thanks Old Un, good read and useful information.

Direct link to file here

 

[tony1851]

I shouldn't be surprised if the OP is more confused now than when this thread was started; I know I am!

 

[letterem]

Hi
thanks again for the interesting discussion. I am not more confused than ever, though it is clear that there are several options which would suffice for my roof. I am really grateful for everyones suggestions and opinions

With regard to the raised and scissor trusses, whilst I can see the sense of the ready made trusses, I think I am going to stick with the cut roof. The access to my house is quite difficult ( I am in a terrace with a narrow path as access to the back) I think it will be more manageable for me to manoeuver the individual rafters around instead of the trusses, as I am doing the work mostly on my own.
I think that even though it is feasible that I could get away without putting in a ridge beam, I will put one in anyway because I can get a couple of friends to help me lift it into place, and that will give me a firm base for me to to fiddle around and hang the the rafters from. I think that getting a ridge board to stay in place might be a bit trickier for someone with only minor carpentry skills. I also think that the building inspector might be happier with a ridge beam given that I am not having a structural engineer do the calculations.
If I go with the ub or usb, for a beam I am presuming that the wood which slots into the flanges has to be fixed to the steel – can anyone advise me how to attach the steel and wood - do I bolt it through at intervals and if so, how wide should the intervals be?
thanks
emma

 

[tony1851]

If I remember correctly, Ronny suggested a 7"x4" UB. If so, a couple of 6"x2"s will just about fit against the web. Fix them with short lengths of 1/2" threaded bar at about 3ft centres - no need to go overboard on it You can buy 1m lengths of threaed bar and nuts/washers at B+Q and cut them to the appropriate length (say approx. 6").
(Don't forget the padstsones each end of the beam).

 

[catlad]

When putting your rafters in check the widths of the timber, even though you get them of the same pile, you can get discrepancies of 5mm or more and can be a pain especially on a vaulted roof when fitting plaster board.

 

[letterem]

thanks to everyone,
I'm ordering my steel today!