roof joist spacing

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Established Member
2 Dec 2013
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Waltham Cross
Hi all, I am currently making some plans for a single story extension I want to do at the side of my house. The roof is really a lean to type of affair and im going for a 20 degree pitch so I get the head room I want on the lower side. We plan to have velux windows ( the 980 by 780 odd ones) my question is what do the joists have to be centred on there will be about 3 -4 windows in total. Is it a case of having 780mm inbetween the joists to be enouhh? I was going to use 2 x 6 for the joists or does it need to be bigger? The span would be about 8 and a half foot so the joist will be like 3m in length.

Any help appreciated as I'm new to all this actual building work.



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Fitting Velux windows will mean trimming out the "gaps" to accept the windows.
As the table CHJ has kindly posted shows 150mm x 47mm @400 cntrs is acceptable, when you cut out for the windows, you will cause more weight and stress on the remainder, having to take the weight.
I think you would need to consult someone qualified to calculate the rafter sizes whilst taking into consideration, the roof covering layers, insulation, air gap, location of the velux windows etc.
You will possibly be required to double up the rafter around the window area, but as this in itself is causing more weight on the entirety, you may well have to increase the rafter sizes.
For these reasons, and more, I recommend using truss roofing when possible as the system works everything out for you, cheap in manufacture fast and easy fixing and acceptable by the jolly old building inspector.
One stop shopping and loads of manufacturer's out there!
As an aside have you checked you're house insurers and flat roofs, may well affect you're premium!
HTH Regards Rodders
OK so from the looks of it I could get away with 600mm centres but I think I'd go with 450 just to be safe. May will double check the span to ensure though.

I've seen what needs to be done to spread the load from the cut joist to the 2 surrounding joists (a header and footer on the cut joist with two jack studs on the sides to pass the load). The roof won't be flat it is a about 3.4m at the top to about 2.2m at the bottom. So it has a 20 degree slope in all (the minimum needed for velux windows according to there documentation)

The clear span for common rafters is pretty similar to the flat roof joist table above.

A clear span of 2.69m is possible using 6 x 2 at 600centres, for a slate tiled roof. Whilst you could go wider in theory with deeper rafters, the spacing is limited by tiling batten span.

you could set out the joists at normal centres and cut in trimmers to create the width opening you need, then pack out the sides. Or I suppose, set out with rafters set at 780mm apart where you want the veluxs, fit trimmers and then add an extra rafter half between.
I did a similar style of roof a few years back.
We went with 400 centres over a 3.7 metre span at 20 something degrees. We had two velux windows in it, the building inspector told me to double up the rafters either side of the windows, then one rafter from the centre of window trimmer to abutment and one centre of trimmer to wall plate for each of the velux. This was to give the tile battens something for nailing too. We used C16 grade timber 6"x 2".
Between the windows we diagonal braced with 4"x1". Tyvex membrane then 2"x1" battens.
One bit of advice, try to space the windows full tile/slates apart. It avoids a lot of cutting and looks better.
Hi robin,
Ill probably be going for concrete or clay tiles would that make a difference.

Nolegs, sounds about what I was thinking. What where the 4x1's for though not sure what you mean with that. And yes I will be wanting to plan that out to avoid the cutting.

20 degrees is pretty low angle and not all tiles will work at that pitch. Check the makers data for the tiles you want before you cut any timber.
Hi Mark,

I assumed slate, as very few other tiles work at such a low pitch, at least without a sub roof of onduline.
Yeah been looking at the tiles that can cope with it so I have found a few options as long as the planning office says they are OK to use if not back to the drawing board.
markblue777":6kbjv7gc said:
Hi robin,

What where the 4x1's for though not sure what you mean with that. .


Just to provide some bracing to the underside of the rafters, I guess it's to stop them twisting as they're retained on the tile side by the battens. Or the building inspector likes to create more work for you :lol:
Actually my inspector was a cool as .... :D
Don't know what style you want but Marley Modern go down below 20 degrees, you have to alter the headlap with some to achieve the lower pitches.
Myfordman":2okeqbmz said:
20 degrees is pretty low angle and not all tiles will work at that pitch. Check the makers data for the tiles you want before you cut any timber.

As above, you will need to find tiles, acceptable in type and colour by you're local planning office and specifying the minimum angle required.
I think the problems start at 35 degrees.
The 4"x 1"is for wind bracing, ridge to wall plate in a "w"and also wall plate restraining straps, running down the block wall, minimum of 900mm long, and every 3 mtres.
make Your wall 600mm c/c then make Your roof 600 mm c/c cut out the opening you need, if you want to be safe double up on the rafters that take the extra load but usually you dont need too when you only cut one.
I would go 2x8 for the roof and keep everything 600 c/c saves you both time and Money
Yeah been looking at them. I'm hoping I can get planning office to say it's fine to use them as we have red tiles on the house. I want to make the extension so it is seen as a "modern" extension and I like the way black tiles pop.
What's do you mean problems start at 35 degrees Rod? What for?

Yeah I think I may go for 2x8s bill. Could be better PFF with that really.