Replacing / Enlarging a Velux Window

[mstizomad]

Hello,

We have a new roof, with new frames, new tiles and new velux windows (all 1 year old now).

We have done most things right on our project, however one decision that i am REALLY not happy with is the size of our Velux windows.

We currently have 550x1200 top hung Velux windows, which are quite small.

I want to rip out these windows are replace with 940 x 1400 or 940 x 1600 velux.

Our roof trusses are at 600mm centes and so no trusses were cur for the current windows, but a single top section of truss would need to be cut for the new windows.

Here is a plan for what i want to do:

(Diagram shows face on view of slopes of roof trusses with timbers of 47 x 175mm)

I would normally double up the trusses either side of the window. However, we have VERY heavy duty timbers of 47 x 175mm supporting the roof tiles and 47 x 225 across the bottom string (floor). Also, we are using light weight Roof tiles at a weight of 20kg per square metre, but our roof trusses were designed for tiles that are 60+ kg per square metre.

So overall our roof trusses are over engineered and so i do not think that i need to use a double either side of the new velux. As the TOTAL load of roof components (tiles + top of truss + lats) above each window is only 50kg.

However i have incorporated double horizontal timbers above and below the new velux to support this 50kg and spread the load side ways to the other trusses.

- My plan is to remove the current window and strip away a large amount of tiles etc.
- Remove the existing flashing and window frame.
- Put up acroprops and bolt in temp timber bracing across three timbers to support the one timber that is going to be cut.
- Cut the middle truss vertical.
- Then to install the new timber horizontal pieces (47 x 175) using structural bolts/screws.
- Then put in the new offset timber vertical pieces to the new windows size.
- Then install the new window frame and sort out the lats, felt, flashing and tiles.
- Then sort out the inside again including a bit of OSB and some insulation.
- Then slot the new window into it's frame.
- Remove temp supports and acroprops

But what do people think?

 

[chappers]

sounds about right but you won't really need to prop anything the battens will hold it all together, you might get a bit of sag in the cut rafters, but just push them back up be for nailing into your trimmers.

 

[mstizomad]

Hi,

Thanks for your reply. Yes the cut timber would be sandwhiched between batons on the outside every 225mm and OSB on the inside.

And do you agree that i don't need to double the vertical timbers that run up the side of the rood trusses?

Under normal circumstances i would double up. But these trusses are massively over-engineered for purpose.

We have 7" x 2" timbers on the slopes, but we we could have got away with either thinner timbers OR the same timbers but at much greater centres.

Our neighbours property has 6" x 2" timbers on the slopes at 1500mm centres, using a Heavier roof tile!!

Thanks

 

[chappers]

nah with 7x 2s I wouldn't bother, not much point if you can't fit it top to bottom either