I am proposing to install a Velux type window in my roof and understand that I should be able to complete the whole process from inside. However my roof is fully boarded with diagonally laid 24mm planks under a slate (composite) tile. Can anyone please advise how best to go about this and whether it can be achieved from inside?
It can be done from inside - although it is simpler if you have outside access.
The windows are designed to be a multiple of standard rafter spacings in width, so your first task is to choose the location between two (or three) rafters where you will fit it. If your rafters are at the wrong spacing then you will need to cut the next size hole up and then add some extra carpentry to reduce the gap to fit the frame.
Cutting away the boards will probably be the hardest job since these will probably need to be cut flush with the inside edges of the rafters in question. Do you know what is above the boards? (i.e. felt, tile batons etc)
An electric reciprocating saw with a long rough cutting blade may do the job. You obviously want to avoid hitting the slates with the blade while doing it - so a shallow angle of cut and slow speed is in order!
Once you have a suitable hole you can de-tile/slate the area and fit the flashing kit. Then remove the sash from the window and fit the frame - they usually screw into the rafters using brackets supplied. Finally you can refit the sash to the frame and adjust for square etc.
Warning ... one cuts around the perimeter of the device, this'll liberate a rectangle of your 24mm (wide?) boards plus their attached felt(?) battens(?) and tiles ! It'll be heavy ... and require some support as the saw cuts leave less and less of the remaining roof attached. Don't allow it to drop onto (and through?) the ceiling of the bedroom(?) below. A 'workmate' was positione below it -itself on planks straddling joists- to provide somewhere to place the liberated bit of roof. { you might consider screwing some scrap battens across the roof's boards within the perimeter to be cut to hold the thing together ).
I fully endorse what John has said and add the following ...;-
The Velux 'device' will separate into the glazing unit and the outer frame. You haven't mentioned the size of unit that you intend to fix but you'll have to get it into/onto the roof. Check the size of the loft-space hatch(?) opening! In my case, the frame was hoisted into the opening 'externally'; a rope was dangled down through the hole over the gutters and handing onto the frame, then the 'heavy' frame was lifted up a ladder while the line was handed in - ( I'm trying to say; the rope didn't 'lift' the weight) this was a three man job -one inside the loft with the rope; one footing the ladder which had been staked to the ground; and a third climbing up the ladder supporting/guiding the frame.
You may have a 'huge' loft-access hatch and be installing 'tiny' Velux lights - but I hadn't thought of this constraint. [Blindingly obvious with hindsight!]
Apart from the 'how do we get the thing up there?'; the job was executed from the inside - although obviously one's body is protruding through the roof line - from about waist height - you'll need a good support/firm footing.
For the carpentry work a nailgun was invaluable - the 'kerphump' as yet another nail goes home is less disturbing to the existing roof than the bang-bang-bang as one tries to hammer in another nail.
BTW, the dust/crap was unbelievable; I can only repeat what Norm says; 'There is no more important rule ... than safety glasses', a dust mask is pretty important too; my wife insisted on laying down dust-sheets on the landing and stairs -well worth the effort.
The amount of light up in the loft-space was reward enough.
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