Advice on rafter size needed please (diagram attached)

[letterem]

Hello,
I am building an extension. I have no experience so am doing it from library books in my spare time. I would like some advice on the roof, as I would like to know which size rafters and ridge beam to use. I am on a shoe string budget so I would like to know the most suitable spec without going oversize.
I am asking advice because after consulting the trada tables, I was surprised that the clear spans seem quite short – according to the table I would have to have 47 x 195 rafters at 400mm centres (second edition page 39) but after looking at this forum, the most common size is 50x150mm rafters for extensions that can be bigger than mine.

The extension is single story with a dual pitch vaulted cut roof
the roof will have 4 velux windows (2 either side) width 550mm height 780mm
The roof pitch is 36 degrees
The span is 3360
The run is 1680
The tiles are concrete marley double roman
I live in suffolk (for snow loading)

The length from the existing outside wall to the far edge of the inner wall of the extension is 5040mm
Ideally I would like the roof to be open, with no rafter ties. I will put collar ties in 1/3rd down the rafters measured from the top of the ridge beam.

Originally I was thinking of placing the rafters at 600mm centres in order to fit the windows in without cutting the rafters, however after reading that it is more difficult to fit the tiles with the rafters at this width, I am considering either 400mm or 450 mm.

I think that these are the things I would like to know (though there may be other unknown unknowns):

Do I put the collar ties on every 2nd rafter?
Is it best to put the rafters at 400mm or 450mm, is it worth trying to put the rafters round the windows at 600mm, or is it better to keep the spacing completely even all the way along the roof. Do I have to double up the rafters round the windows, as I have read that this is unnecessary as the weight of the windows is less than the tiles anyway.

Which is the best value for money - a glulam beam or a flitch beam? (or is there another sort of beam that I have not considered).

The end wall of the existing building is not in great condition, but I have been told that I can bolt wall plates to the existing wall and fill drilled holes with epoxy resin. I know it is hard to tell without seeing the wall but does this sound like a satisfactory method to attach the ridge beam.

The ridge beam will rest at the other end on a steel plate on the new block wall . I am presuming that this steel plate just has to be a few centimetres wider than the ridge beam.

Thank you.
Any help would be appreciated,
Emma

 

[tony1851]

I think we could have built it in the time it takes to read your post!

Have you been looking at the correct table (or reading the correct table incorrectly)? 2x8 seems a big section for your span.

 

[theoldun]

Will try and help you out..
Confirm 36degree pitch, overall span of plates 3360, run 1680. As you will not be having ridge board, this will make hypotenuse, length of pitch line, slope, rake, call it what you like 2077. i.e. length of rafter from top plumb cut to back edge of heel cut plus overhang of say 450, will make total rafter length 2700.
Both dead load of roof timbers and imposed snow load are calculated into the Trada span tables.
Marley Double Roman, felt. battens, insulation and plasterboard will weigh in at 0.70kN/m2, so you will scratch in at above 0.50, and below 0.75kN/M2
Insulation. To achieve U value of 0.18W/m2K, you will require either Celotex GA4000, 165mm thick between rafters or 100mm between rafters and 55mm below rafters or 50mm between and 75 below. Will come back to this.
If you look again at your tables page 39 you will see that C16 47x100 at 600 centres would suffice for a pitch line length of 2077, but makes hard work for your insulation, so tell you the way we would go
As we are old school and like to see a bit if meat on the spars for a vaulted ceiling, strength to top beak and ease of insuation we would go C16 47x200 regulised to 195mm at 600 centres. Make sure you have spars regulised.
Fit 165 celotex between rafters and run 100mm wide foil tape down each rafter so that 25mm overlaps insulation each side, then plasterboard.
We never use glulam beams or flitch beams as ridge beam,. We always use a USB. You will need SE for calculations. We know what size you will need, but not qualified to do calcs. Ronny Shogun may help you out, if he reads this? Cut three course deep pocket in old wall and lay three brick wide three course Staff Blues under end of beam.
Your top plumb cut will be 54 and seat cut will be 36 degrees. Your rise will be727mm per metre run and your secant is 1.236mm. Set your pitch line for your birds mouth at 2/3 down on a 90 degree line from top edge of spar and 1/3 down ditto on top bird beak.
Your velux will drop in between your 600 centre rafters. No need to double up rafters to velux.
Providing you make a good job of fixing top birds beak and nail builders band over the top of the apex and 450 down all pairs of rafters then you will not need collars. Also consider a 9mm gusset plate at top of all rafter pairs.
Any thing you do not understand,come back.
Regards oldun

 

[tony1851]

Marley Double Roman, felt. battens, insulation and plasterboard will weigh in at 70kN/m2,

Eeek!

 

[tony1851]

we would go C16 47x200 regulised to 195mm at 600 centres. Make sure you have spars regulised.

oldun; with respect that's far too big, even allowing for the insulation.
With 6x2,she could put 100 or 125 thick board between the rafters and, say, 50 below

Also, if she's putting ceiling ties in, why should she use a steel beam at the ridge (and pay for calcs as well)? This was supposed to be an extension on a budget

 

[theoldun]

Marley Double Roman, felt. battens, insulation and plasterboard will weigh in at 70kN/m2,

Eeek!

Thank you.
Sincere apologies to the OP for our typing error.
Post now corrected.
Regards oldun

 

[RonnyRaygun]

Old 'Un, I thought you were making your insulation out of neutrons

As for the ridge beam, you could just use a steel, with timber in between the flanges to secure your rafters to. You will be looking at something like a 178x102 or possibly a 152x89 UB.
If you want to opt for a flitch beam, you will need something like a 200x12 plate, with a couple of 50x225s each side. You may well get away with 10x200; I've based the calc on the steel alone for simplicity and lack of time.
I've always wondered how much restraint the timbers actually give to a flitch beam, but it's a standard detail and seems to work...
Not sure about cost comparisons, but if you did want to opt for a Glulam beam, I do know that they are no stronger than standard timber. The advantages are that they can be made to deeper section sizes, and the stronger timber can be concentrated at the outer edges of the section, where the stresses are higher.
Basing the calc on C24, you would need a section something like 125mmx300mm deep.
There is also a similar product (can't remember what it's called) in which the plies are orientated vertically rather than horizontally. Very deep sections available, but might not look as nice as a Glulam. Probably can't leave a ridge beam exposed due to fire regs anyway...I'm sure someone will confirm or refute this.

As for the rafters, I agree with Tony, they aren't spanning very far at all and 150 deep (or less) would be deep enough.

 

[theoldun]

Tony, Have read your post, and appreciate your initial thoughts. Will now explain our reasoning.
Due to the amount of timber that we buy, we get very. very good rates, however the OP should be able to buy C16 47x150 regulised at £2.80 metre VAT inc. C16 47x200 regulised at £3.50 metre inc. Difference 70p metre inc. Roof requires 54metres by 70p metre difference equals say £38.00 dearer using 47x200 than 47x150.
Now lets look at insulation. Roof would need approx 7 sheets of celotex to underside of rafters if using 50x150 rafters. Additional hours work for two lads, that uses up your £38.00 saving, so basically no difference in timber prices.
Plus we have specified 600 centres instead of 400, so saving a few bob. Velux will drop into rafters at 600 centres, labour cost saving, collars not required now, labour and material saving, deeper rafters give much better fixing for birds beak.
SE fees for USB. Who is going to do the calcs for glulam or flitch beam? Out of interest what would you use as a beam?
Conclusion we only suggested the way we would do it. Lets hope someone comes along with better suggestions.
Regards oldun

 

[RonnyRaygun]

Just checked and unlikely a 152x89 UB would work.
So minimum size would be 178x102 UB.

Old Un, if using 200 deep rafters, would you want a deeper beam anyway, as you will have an edge of around 240mm to join to the ridge?

 

[tony1851]

[quote="RonnyRaygun";p="2539149". Probably can't leave a ridge beam exposed due to fire regs anyway....[/quote]

RR; I think elements of a roof structure do not have to be fire resistant.

 

[tony1851]

Am I missing something?!
Why does this extension roof need a ridge beam rather than just a ridge board?
If there are to be ties one-third the way down, its almost akin to a truss.

 

[tony1851]

Tony, Have read your post, and appreciate your initial thoughts. Will now explain our reasoning.
Due to the amount of timber that we buy, we get very. very good rates, however the OP should be able to buy C16 47x150 regulised at £2.80 metre VAT inc. C16 47x200 regulised at £3.50 metre inc. Difference 70p metre inc. Roof requires 54metres by 70p metre difference equals say £38.00 dearer using 47x200 than 47x150.
Now lets look at insulation. Roof would need approx 7 sheets of celotex to underside of rafters if using 50x150 rafters. Additional hours work for two lads, that uses up your £38.00 saving, so basically no difference in timber prices.

Regards oldun

I take your points regarding the economics. However, I half-suspect by the way the OPs post was written that they intend doing the roof themselves. If this is so, then perhaps any labour costs won't be an issue?

 

[RonnyRaygun]

Am I missing something?!
Why does this extension roof need a ridge beam rather than just a ridge board?
If there are to be ties one-third the way down, its almost akin to a truss.

Does the OP definitely want ties, or did she just believe they were necessary?
Using 50x200 timbers you certainly aren't going to get much deflection over the span, so a ridge beam probably isn't necessary if ties are used.

 

[tony1851]

Does the OP definitely want ties, or did she just believe they were necessary?
.

good point; hadn't thought of that.

 

[chickenlips]

If the op wants to use glulam for a ridge without any collar ties, then 90 x 360 GL28C would work based on 5.040 run for ridge, carrying 2x6 rafters at 600mm centres with rafters doubled up for 2 veluxes either side.