Lean to roof Timber sizes and dead load

[Adrian80]

Hi Guys,

Wanted to check i am not going crazy, and that my interpretations are correct.

I am looking at a lean to exension,
Clear span from existing wall to the begining of the wall plate is 295cm.

Pitch is 23 Degrees

600 centres for the rafters ( so i can fit some veluxs between them.)

not sure of the dead load, i am in bristol so risk of large snow fall is quite low. so was thinking 0.5 to 0.75

So looking at some trada tables on the web.

http://www.broxbourne.gov.uk/PDF/BC_TRADA_SPAN_TABLES _2008.PDF

table 6.1 page 31 grade c16 timbers would suggest 47mm x 150mm rafters
table 6.9 page 35 also agrees

I was intending to use 50x100mm for a wall plate and bolt a rafter to the wall, and birds mouth at each end.

Have i forgoton anything? I don't think i need purlins? Perhaps a few noggins to stop any twist.

Thanks in advance...

Adrian

 

[tony1851]

That size/span seems OK. Don't forget to double-up the rafters each side of the Velux.
Aim for about 125mm Kingspan or equivalent ins, leaving 25 air gap under the felt (assumed breather felt).
Ensure there is a good firm fixing at the upper ends of the rsfters.

 

[catlad]

I would put 175mm joists on if you are loading it up with concrete tiles.

 

[Adrian80]

Thankks for the advice, i will use some truss clips, and big old screws to hold it all together.

If i go for 600 centers, you can get velux that would fit between the rafters, would there be any need to double up as i am not cutting the rafter about.

I see the need if you had rafters at 400 Centres.

What do you all think?

A

 

[catlad]

Your building inspector will insist on you doubling then up I'm sure. 600ctrs can be a pain when nailing slates/ tiles on.

 

[tony1851]

Catlad has a point; 600 is the usual for trusses, but for traditional cut rafters, on reflection a closer spacing would be better.
If you are bothered about leaving a suitable gap(s) for the Velux, why not just have the 600 gap at the Velux position, and bring all the other rafters in a little closer?
In terms of bending stress etc, 6" rafters would be OK, but TRADA increased the sizes on all spans when they took over publishing the tables - presumably only to sell more timber.

 

[AronSearle]

but TRADA increased the sizes on all spans when they took over publishing the tables - presumably only to sell more timber.

No, the regulations changed requiring more safety margins, as they have again for the 3rd edition (Eurocode rather than British standards).

 

[theoldun]

Sorry but post is a bit long.
We do not feel that you may be quite right with your rafter sizes.
The span of a roof is measured overall the plates and half the span is the run. The run is the measure from outer edge of wall plate to centre of ridge. In your instance it would appear that you have taken the run from inside of wall plate to existing wall and called it 2950, therefore, add 100mm and run is 3050 Your pitch is 23degrees, so your secant is 1086x3050 = length of rafter from top plumb cut of 67 degrees to ditto heel cut of birds mouth is 3313 plus what ever overhang you require.
You have not stated what roof covering you will be using, therefore. With imposed snow load of 0.75kN.m2
Slate will just scratch in with C16 47x150 at 600 centres
Concrete interlocking tiles and clay plain tiles will require C16 47x175 at 600 centres.
Concrete plain tiles will require C16 47x200 at 600 centres.

For ease and economy only ever work either 400 or 600 centres, occasionally 450

If you reduce centres to 400mm then C16 47x150 will be sufficient for all above roof coverings.

If you wish to do a vaulted ceiling then length of rafter from plumb to plumb cut as stated will be 3313 plus overhang as required. Once again at 400 centres C16 47x150 will suffice on all above roof coverings. If you vault the ceiling, then take your pitch line on bottom birds mouth two thirds down rafter and half way down rafter on top birds beak. This will then give you 50% depth of rafter against house wall and 50% against top wall plate and 47mm seat cut hanging on top plate,. Fix top with standard truss clip, 2no 75 nails skewed into plate and 2no 75 screws skewed through birds beak into top of plate.

Our figures are based on 2nd edition Trada and not on the 3rd edition Euro code 5 which came into force end March 2010. We have compared the 3rd edition with the 2nd edition and the difference is so small that it is not worth worrying about unless you are on the bread line of your spans.

Insulation. To achieve 18Wm2/K you need 165mm celotex GA4000 between rafters, which means that if you are using 150mmm deep rafters, then you are f*cked. Leaves you three choices, increase size of rafters, batten down rafters as required or 100mm celotex GA4000 between rafters and 52.5mm celotex PL4000 under rafters. PL4000 is 40mm insulation stuck to 12.5 plasterboard.

Velux roof windows. On the 550 wide range and 600 centre rafters we do not double up rafters as the kN/m2 weight of window is less than the loading of the roof covering. Suggest you check it out yourself. The total weight in kg is on side of box. Convert this to kN/ms and providing it is under 0,50kN/ms then it is less than any of your roof covering.
Anything you do not understand, come back.
oldun

 

[tony1851]

Velux roof windows. On the 550 wide range and 600 centre rafters we do not double up rafters as the kN/m2 weight of window is less than the loading of the roof covering.

Precisely, and if I was doing my own roof with 6" rafters, I wouldn't double them up at the velux. But most building Inspectors I've come across ask for this. The alternative is to provide them with structural calcs, but its easier/cheaper just to put another rafter in, if only to shut them up.

 

[noseall]

Have i forgoton anything?

Gable restraint straps (mid-span) on the underside of the rafters. Don't forget to chop them in to the rafter if you are using the 5mm variety.

Set your velux (horizontal) trimmer to the nearest tile, probably ideal at four courses up. You will need to set the tile gauge on the rafter prior to any roof tiling, as a dry run, remembering to set the velux bottom batten 80mm up from the last roof tile top edge, whilst the top velux batten needs to be the length of the window plus 45mm from the bottom batten.

Don't forget to set the trimmers vertical (low level) and horizontal, or near as (high level). This will admit maximum light from the window.

 

[Adrian80]

Thanks for all you replies.

Not sure i understand oldun's calcs? possibly me being a bit dim.

The outer edge of the wall plate is 280cm from the far edge of the ridge (timber bolted to side of house), on the horizontal, the veritcal rise is 120cm.
therefore the run will be less than: (just noticed trada calls the run, clear span of the rafter)

=SQRT(120^2+280^2) = 304.cm

Assuming birds mouths are same top and bottom, the angle should just translate up.
=DEGREES(ATAN(120/280))= 23.2 degrees

concreted smooth double roman tiles as covering.

Indeed 400 and 600s work for the plaster board underneath.

I am also looking a 2nd edition table, can't be that much different to the 3rd.

Insulation is 100mm celotex between, and 50mm under.

Good point about the weight of the veluxs being less than the tiles, will check it out.

Noseall, yes will remeber the bloody straps!

Excellent point about setting the veluxs to nearest tile depth, will make the laying out a bit easier.

Interesting point about angling the trimers differently top and bottom, will do..

I will speak to the BCO check the he is okay with the timbers and spans, etc..

cheers for the advice and help..

Adrian

 

[AronSearle]

I am also looking a 2nd edition table, can't be that much different to the 3rd.

The Euro-codes essentially demanded further safety margins, so most spans require slightly bigger timbers.

However both the 2nd and 3rd edition are still current, as the regulations still allow both BS5268-2(2nd ed) design and Eurocode 5(3rd ed).

I only mention this as sometimes building control mistakenly insist that the 3rd edition over-rules, and you can't use the 2nd edition spans.