Build a dormer window in an already reinforced roof

[Keymon]

Hello,

We are into this renovation of a Victorian house. It has a reinforced loft (poorly built, but it has 15cm joists), but without any dormer window, just pitched roof.

At first, our plan was to not touch the current loft, and focus on the rest. But due issues with the chimney (barely supported by timber), some old roof tiles in some areas, the need to retrofit membrane and add insulation, etc, we decided to just rebuild the roof and remove the chimney. And as we are doing that, we thought: Why not do now a dormer?

Now, if we do this it means: Architect will need to do drawings, SE wants to charge again, and then build might delay our works? Cost wise I was told by a roofer like ~10k

So my question is:

Providing that the loft is already reinforced, how difficult would be add a the box for a dormer window, like below? Would it usually require a lot of structural work (ie steel), or would be the existing 15cm Joists be enough? or maybe it just requires duplicate the joists?
 

[Temp]

I recommend you get input from an SE. The normal issue is the way the roof is triangulated. Without a dormer the rafters and floor joists form a triangle that stops the weight of the roof pushing the walls out. If you omit/cut a load of rafters for the dormer this removes the triangulation. One solution is to replace the ridge board with a stronger ridge beam. This beam typically transfers the load to the gable walls or is supported another way. There are other possible solutions. Whatever you do it needs to be designed and Building Control will probably want to see the design justufication/calculations. 

 

Other typical issues include:

 

Are the floor joists strong enough to convert the loft?

 

Will there be enough headroom if you have to fit a ridge beam and insulation in the roof? And strengthen the floor?

 

 

 

 

 

[PeterW]

As you’re going more than 30% of the roof width that would need steels most likely for ridge and below the dormer itself. Looks like multiple extensions already - have you checked you’ve still got PD rights..? 
 

The dormer to the left is a good example of how not to do it too …. 

[Keymon]

8 hours ago, Temp said:

Are the floor joists strong enough to convert the loft?

Thx for your answer!

Yes, that is my next question. I wonder if these joists are even installed correctly.

The joists seem just installed by the existing ceiling joists. No beam or anything. I guess resting on the walls?
I am not in the property, but IIRC they are 15cm spaced every 40cm.

Edited July 10, 2022 by Keymon

[Keymon]

7 hours ago, PeterW said:

checked you’ve still got PD rights..?

We had a previous bigger conceptual design (from other architect) with a loft as a L, which was meant to be under Permitted Development (<40m3). I am assuming that as it is much less, it should be fine. Anyway, I will ask the architect.

 

7 hours ago, PeterW said:

30% of the roof width that would need steels most likely for ridge and below the dormer itself

And of course this would require a Party Wall agreement, etc? 

Unfortunately we already served notice for the chimney removal and roof replacement. That is annoying, the surveyor would likely charge me again

Thx a lot for your response
 

Edited July 10, 2022 by Keymon

[Keymon]

One alternative we looked at is the Velux Dormer:  https://www.velux.co.uk/products/space-makers/dormer

What is your opinion on those? Do you reckon this could work without major changes?

[Temp]

14 hours ago, Keymon said:

Thx for your answer!

Yes, that is my next question. I wonder if these joists are even installed correctly.

The joists seem just installed by the existing ceiling joists. No beam or anything. I guess resting on the walls?
I am not in the property, but IIRC they are 15cm spaced every 40cm.

 

That might actually be correct but it depends on the span and the depth (150mm) and width of the new taller joists (eg the one on the left).

 

Ideally when those were installed they should be raised up slightly so there is a small gap between the bottom of the joist and the plasterboard below. They might have left a gap but the photo is a  bit out of focus so hard to tell.  This gap is so when the floor is loaded and the joists bend they don't bend the plasterboard. This reduces the possibility of plaster cracks appearing in the room below. For the same reason the new joists are not fixed to the old ones (except perhaps at the ends or where they are both supported on the same wall).

Edited July 11, 2022 by Temp

[Keymon]

Actually checked and it is 20x5cm joists with a 35cm between them (every 40cm). There is a gap as you described.

Edited July 12, 2022 by Keymon

[joe90]

I have built dormers just involving the BCO, no architect or SE. I know BCO,s are harder to get to visit nowadays but worth a try I guess. I get the point of gaps between new joists and ceilings (I have donethat) but the floor below will not have that and if the joists are structurally ok should not cause cracks in the ceiling!. 
Rather than replacing the ridge board (very intrusive) we have installed a ridge glulam or steel under the existing ridge board to carry the roof weight. Just make sure you insulate as much as possible to reduce house energy consumption.