Advice on rafter size needed please (diagram attached)

[letterem]

Hi thanks for the answers,

firstly, as Tony 1851 Suspected , I was misreading the span tables . I took the span to be another measurement, but I am on track now thanks to following the olduns post

I apologise if this has led to posters doing unnecessary work for me, but I am a real novice (My mum has done all the planning on the extension to date but she is ill and is a little nonplussed by the roof hence I have taken on the plans)

oldun, I have drawn a diagram of the rafter you recommended and uploaded it to my album on here (beam and rafters/rafter) I think this should clarify whether I understand what you are saying.
The only thing I am not sure of is “450 down all pairs of rafters” I have googled it but not come up with anything

This is not set in stone, but in terms of insulation, I was thinking of 100mm between rafters and 50mm over. This is partly because I have a couple of packs of 50mm left over from building the walls, and I thought I may as well use it.

Tony 1851- I am not sure what a ceiling tie is – I am thinking of putting collar ties in, are they the same thing?
I thought that the length of the ridge and the fact that there is not going to be any rafter ties or ceiling joists to pull the bottom of the rafters together would mean that I would have to have a ridge beam rather than just a ridge board – of course I may be mistaken in this. The collar ties can be up to 1/3rd of the way down the rafter, but it would be good to have a little bit of flexibility to allow them to be higher as they will have to go above the top level of the velux windows. I am not sure yet exactly what height to place the windows. - I was thinking of seeing how they fit with the tiles before finalising the height.

Tony you are right in thinking that I am building this extension myself so labour costs are not an issue, though one of the reasons I do not want to over spec it is because if the beams and rafters get too big I wont be strong enough to lift them!

Ronny Raygun - I am quite happy to go with the simplest solution for the beam if indeed I have to have one at all – I only thought of a flitch beam or glulam because I thought I would have to have them, but if the 178x102 UB would suffice, I will go with that if it is cheaper.

I would like to put collar ties in as I intend to put plasterboard underneath these. Again, sorry if my description of the ceiling as vaulted was misleading, I suppose it is more 1/3rd vaulted as the hidden space above the collar ties will be useful to run cables for lighting in, and the beam will be hidden so doesn’t need to be pretty

Given all of the above am I right in thinking I can dispense with the ridge beam and just have a ridge board and 50x200 rafters at 600mm with collar ties on every rafter?
This would be nice and simple!

Thanks
emma

 

[tony1851]

Ceiling ties, collar ties - same animal.

While I respect the views of Ronny and the old fella, I'm still not convinced you need a ridge beam as such.
If your rafters are 200 deep (and imo they coud be less), and you have a good strong fixing between the ceiling ties and rafters, you would not need a ridge beam, just a thin ridge board.
Just my humble opinion.

 

[RonnyRaygun]

This is an instance where a glide shoe would be useful then; to take up the displacement as the rafters are loaded, and to prevent forces being transferred into the walls?

 

[tony1851]

letterem; just seen your sketch. Unless you had much deeper ties (to get more and better fixings to the rafters) and/or moved them further down the slope, you may be better with a ridge beam after all.

 

[chickenlips]

Or.... After also veiwing your sketch, why not just use raise tie trusses ? Surely costs of materials, ridge beam would not be less than pre manufactured trusses?

 

[theoldun]

Ronny, many thanks for steel sizes. To be honest we would have thrown in a 203x102x23, but then again as we are as thick as two short planks, our calculations are based on what we have done in the past, what way the wind is blowing and how much scotch we drank the night before.
You are about right on depth of top plumb cut collection.. On a 36° pitch it will be 247, but is then broke up again by the birds beak plumb cut 83mm in depth and your secondary plumb cut against steel 203 in depth.
Regards glide shoes, in our opinion, as much use as a chocolate kettle. Agree Simpsons have 15mm play other manufactures only have 12mm

Chickenlips, many thanks for information. Glulam beams. Remember you asked the question a while ago, why we do not use them? Only used them once. Took 2 days to find a supplier prepared to supply one 4m beam. With transport costs, beam cost more than a steel. We also had to have SE calculations for beam as supplier would not provide them.
After reading OP latest post you have got a hole in one with your raised tie. If the OP has any sense whatsoever that is what he will go for. We have just built two houses with raised ties and dropped plate and we got a metre rise on the ties. although that may be a little low for what he is looking for, but have seen them designed to give more rise.

Tony, no idea if the OP wants collars, but ask you a question, would you rather look up and see a collar every 1200 or look up and see a true vaulted ceiling? Know what we would rather have, but then again we would never have a vaulted ceiling, as we see no point in throwing money down the drain on energy bills.
Looked up some old notes. Collar ties, rafter ties and ceiling ties are all horizontal ties, each with a different purpose.
Collar ties are designed to tie together opposing spars to help brace the roof framing against uplift caused by wind. Collar ties to be placed in the top third of roof.
They may or may not be needed, depending how the spars are fixed to ridge.
Country to popular belief collar ties, do not prevent a roof from spreading.
Rafter ties are designed to tie together the bottoms of opposing rafters to stop roof from spreading.
Rafter ties should be placed in the lower third of the roof and as close to wall plate as possible.
RAFTER TIES ARE ALWAYS REAQUIRED UNLESS THE ROOF HAS A STRUCTURAL SELF SUPPORTING RIDGE
Ceiling joists all as rafter ties but sitting on wall plate.
Regards ridge beam, if you feel that a pair of commons with a collar two thirds down stood against a ridge board is almost akin to a truss, then by all means tell the OP that the way we would go about it is a load of old b*llocks. No skin of our nose, and we will take no offence, as we will still carry on doing it the way we were taught when we were a lad.
Regards oldun

 

[letterem]

Chickenlips, I have just looked at raised trusses- I can see they might be feasible, though I cant see how high the truss could be – it would have to go above the velux - would it add enough stability at that height? (the window is 780mm high + room for the flashing)
oldun- One of the reasons I want to put plasterboard under the collar tie is to save on the heating bills.

 

[catlad]

Well Emma the 450 the oldun is reffering too is builders band over your ridge beam and 450mm down each side of the rafters

 

[chickenlips]

Regards glide shoes, in our opinion, as much use as a chocolate kettle. Agree Simpsons have 15mm play other manufactures only have 12mm

Glide shoes on loose cut roofs, agreed, never seen them specified in that scenario or have supplied them my self.
Trussed rafter roofs, supply them on practically every raised ceiling job where calculation shows needing them.

Chickenlips, many thanks for information. Glulam beams. Remember you asked the question a while ago, why we do not use them? Only used them once. Took 2 days to find a supplier prepared to supply one 4m beam. With transport costs, beam cost more than a steel. We also had to have SE calculations for beam as supplier would not provide them.
After reading OP latest post you have got a hole in one with your raised tie. If the OP has any sense whatsoever that is what he will go for. We have just built two houses with raised ties and dropped plate and we got a metre rise on the ties. although that may be a little low for what he is looking for, but have seen them designed to give more rise.
Regards oldun

Can understand the reason you prefer steel then, cant believe someone would just supply glulam, without sizing it. We only specify steel when the application warrants it as we cant engineer it, if we can design it in house out of a item we stock then we will.

Raise Ties, IMO would be far easier, hardly any loose cut in work would be needed. Should even get cut trusses between veluxes supported on a purlin beams.


@ Ronny,
Dont know what Glulam you have seen, but GL28c is glued with cross laminated timbers, giving a far better strength class then standard C16/C24.
The other Laminated beams available are Timberstrand, Ultralam, Kerto, Parrallam.

 

[chickenlips]

Chickenlips, I have just looked at raised trusses- I can see they might be feasible, though I cant see how high the truss could be – it would have to go above the velux - would it add enough stability at that height? (the window is 780mm high + room for the flashing)
oldun- One of the reasons I want to put plasterboard under the collar tie is to save on the heating bills.

Yeah its not a massive span in terms of trusses and im certain the ceiling could be designed high enough to get the velux fully in the leg of the truss.

I can post a design up monday if you havent already made your mind up. Should give you a clearer picture

 

[theoldun]

You can fit velux into raised tie trusses with part of window in slope and part in the flat. you just stud out an insulated well hole in the flat section up each side that is in the flat. Bevel top and bottom of window at a 30 to 45 degree angle at ceiling line to gain maximum light value. There are two or three ways to do it. You may have to stretch your trusses out a tadge where your velux are going to drop in. For the future give a lot of thought to where you fit velux in relation to tile batten gauge.
Regards oldun

 

[RonnyRaygun]

Regards glide shoes, in our opinion, as much use as a chocolate kettle. Agree Simpsons have 15mm play other manufactures only have 12mm

Glide shoes on loose cut roofs, agreed, never seen them specified in that scenario or have supplied them my self.
Trussed rafter roofs, supply them on practically every raised ceiling job where calculation shows needing them.

Out of interest, why are glide shoes not suitable for a cut timber roof with raise ties? As I understand it, the glide shoe is designed to allow for lateral displacement of the roof as it settles under load, and thus prevent horizontal forces being transferred into the wall. If they work on trussed rafters with raised ceiling, why not on a raised collar cut roof?

@ Ronny,
Dont know what Glulam you have seen, but GL28c is glued with cross laminated timbers, giving a far better strength class then standard C16/C24.
The other Laminated beams available are Timberstrand, Ultralam, Kerto, Parrallam.

My mistake, can't remember where I read that, but agree Glulam is significantly stronger than C24.

 

[tony1851]

Oldun; I certainly wouldn't say that what you've written is b******s; I admire your knowledge, style and directness. But we are all entitled to our opinions and there is always more than one way to skin a cat.
It's easy for everybody to get bogged down in debate and none of this helps the OP.

For what it's worth, I've attached my own scale sketch of how I would do this. 8x2 spars at 600; ties of 8x2 offcuts about 1/3 down from the ridge. We are losing sight of the fact that this is not a very big roof and so subject to relatively small loads.

I did some quick timber calcs, and under maximum load, the 8x2s would deflect no more than 1mm. If the OP firmly screwed/bolted the timbers in the shaded area the whole assembly would be very rigid and any theoretical outward spread of no practical importance. On this scale, no need for glide shoes either.

Untitled

• tony1851
• 12 Oct 2012
• 1

(You might have to turn your laptop on its side!)

 

[chickenlips]

Regards glide shoes, in our opinion, as much use as a chocolate kettle. Agree Simpsons have 15mm play other manufactures only have 12mm

Glide shoes on loose cut roofs, agreed, never seen them specified in that scenario or have supplied them my self.
Trussed rafter roofs, supply them on practically every raised ceiling job where calculation shows needing them.

Out of interest, why are glide shoes not suitable for a cut timber roof with raise ties? As I understand it, the glide shoe is designed to allow for lateral displacement of the roof as it settles under load, and thus prevent horizontal forces being transferred into the wall. If they work on trussed rafters with raised ceiling, why not on a raised collar cut roof?

Correct and they are probably suitable for loose cut roofs in the raise tie scenario.

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

 

[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.

Tony, are you talking about the deflection in the member or the lateral displacement of the member at its base? I was expecting 5 or 6mm horizontal displacement, especially with the elongation of the tie.