Lean-To Roof - Window Well or Not?

[pete1980]

Hi all,

We are getting plans drawn up for a ground floor extension to our kitchen and dining room, with a lean-to roof. Because of the extension depth (4m) and roof angle this will clash with the bottom of the window on one of our upstairs bedrooms.

A few of our neighbours have similar extensions and have gone with cut-outs around the window - I believe these are called inverted dormers or roof window wells? However our architect has proposed avoiding doing this as in his view they are a pain in the long run, e.g. prone to leaks / maintenance etc. Instead he has proposed raising the base of the window in the affected bedroom by a foot or so to avoid clashing with the roof, i.e. take out the existing window, build up the wall underneath, replace with a less heigh window. This will then be level with the bathroom window next to it so shouldn't look odd or out of place.

I wanted to get a second opinion on this please - is the inverted dormer really that bad an idea? Is one option likely to be significantly cheaper in the shorter and longer term?

As I see it:

Raising the window:
+ Looks less fussy from outside
+ Keeps the roof design simpler
- Means doing additional building work in the bedroom which otherwise would not be affected (though it could do with decorating anyway)
- Replacing the existing uPVC window which is otherwise OK

Inverted Dormer:
+ No building work required on the existing bedroom (but will probably redecorate anyway)
+ Avoid replacing the existing window
- More 'fussy' looking roof design, possibly additional maintenance longer term?

The house is a fairly standard semi, circa 1960. The adjoining neighbour is also planning a similar extension in due course and his planning drawings show they have gone with the inverted dormer solution.

Opinions greatly appreciated!

Thanks,
Pete

 

[noseall]

No brainer - raise the window sill height.

 

[pete1980]

Thanks for the advice, though do you mind me asking what the rationale is behind this please?

Kind regards,
Pete

 

[noseall]

Thanks for the advice, though do you mind me asking what the rationale is behind this please?

Just had a thought (sometimes us builders occasionally do), raising the sill may contravene the escape window height restrictions, so I'd be inclined to look at this first.

Otherwise raising the sill will be less problematic and a more economical solution with little detriment to the view out of said window.

 

[tony1851]

Personally I think the idea of raising the cill is a cop-out on the part of your architect.

He is suggesting this because of the additional work he would need to do to detail a cut-out in the roof - it's easier for
him to write on the drawing " cill to be raised".

A competent builder/roofer should be perfectly capable of doing a workable, leak-free cut-out. Don't be fooled by the incompetent
nit who is doing your drawings.

 

[noseall]

Don't be fooled by the incompetent
nit who is doing your drawings.

Here's me thinking that part of an architects remit is to produce a building that is as economical to build as it can be.
It could be seen as a cop-out but I'm not convinced that the alternative is any better in terms of practical benefit to the customer.

 

[tony1851]

If he's going out 4m, then at 15 degrees slope (min for Velux) and allowing for - say- 8" joists plus 2-course flashing.
I think he will end up well-up the rear wall, and the cill when seen from in the bedroom might look oppressively high.

 

[tony1851]

Here's me thinking that part of an architects remit is to produce a building that is as economical to build as it can be.

In that case, he could bung a flat roof on it

 

[freddiemercurystwin]

Aye just what I was thinking, lazy architect.

 

[pete1980]

Thanks all for the replies so far.

Flat roof not an option I'm keen on as I'd like to keep it as a tiled lean-to in keeping with what some of the neighbouring houses have done, and what the adjoining neighbour already has planning permission to do (and might cast a shadow on a roof light on a flat roof).

I'll get the tape measure out and check regarding the exit regulations - thanks for the tip!

 

[pete1980]

If it's not a daft question, presumably if we were to have window wells set in to the roof then it will mean the inside ceiling of the room in the extension below will have to follow the same shape, with a 'stepped' appearance rather than a nice clean vaulted finish the full width? (i.e. you wouldn't expect to 'absorb' the flat section within the thickness of the roof cavity)

I've checked the window size and should be well clear of the minimum fire exit size for building regs (measured size of the opening section would be slightly over 75cmx 50cm, so 0.375m2, a quick google search indicates regs are >0.33m2 and minimum of any single dimension 45cm?). So it sounds like it will be down to weighing up the clean ceiling shape in the room below vs the loss of some window size in the room above, in which case it feels like raising the window is the way forwards if no strong cost argument...

 

[^woody^]

I would not have a horrible different height cill.

If designed and specified correctly, and built properly, the roof should not leak.

A mono "vaulted" ceiling would tend to have a flat section at the top in any case and not go up to a point

 

[tony1851]

@op; with the depth of rafters you would need (prob. 200mm) a competent designer can get the flat section in within the depth
of the joists themselves, particularly with a 15-17 degree pitch. When you factor in the depth of insulation below the rafters, it's
a fair bet you could do a flat which would not be seen internally.

With regard to the window and raising the cill; it's not just the opening casement size which matters, it's also the height of the opening
above floor level, which should not exceed 1100mm. And as for your preference in having the cill level with the bathroom window cill to
look uniform outside, that is a questionable approach. You would be sacrificing the internal 'feel' of the room for the sake of external appearance -
rarely a good approach to design.

 

[pete1980]

Thanks all for the feedback. It sounds like the vote is mainly towards maintaining the full height windows + window wells but this does lead to a couple more questions:

1) We plan to have Velux windows in the roof. How much of a gap would you expect to need between the flat area in the window wells, and a velux window further down the roof below it? One drawback I have realised is that having the window well would potentially push the velux windows further down the roof. If it only needs to be say 10-20cm then shouldn't be a problem, but if it had to be a lot more then we need to consider it.

2) From purely a cost perspective, do you see a significant difference between the two options? As I see it the window well option might drive additional cost and complexity in to building the roof, but the raised cill will trigger the cost of a new window and additional work to raise the cill - are these broadly comparable?

I have attached a couple of pictures / renderings to illustrate the first point, showing possible positions of the Velux roof with and without the window wells. The large upstairs window on the left is the existing bedroom (spare room / study) which the discussion is about. The middle window is the bathroom so already high enough. The right will be the extension we are building over the garage so will be starting from a blank sheet.

Thanks,
Pete

View media item 96821
View media item 96822

 

[noseall]

In terms of aesthetics the window well looks the better option. Financially speaking the window well will turn out the costlier option.
Looking at the image suggests that you would need to raise the sill by a fair amount probably contravening the minimum escape height. You measure from FFL (finished floor level including finishes) up to the openable part of the window and not just the sill.