Trading off rooflight size vs supporting steel

[RatFloofing]

Hi -

 

We recently moved into a house with a large extension (about 6.5x5.5m in the main area) that has a flat roof and a frankly enormous 'rooflight' (about 4.5x3m)

 

As we've now discovered the whole roof and rooflight needs replacing because

• the structure is totally insufficient for the rooflight, as well as a bunch of other problems like no ventilation at-all for the cold deck roof and no vapour barrier
• the 'rooflight' is actually just a lean-to roof panel that's been plonked on top and isn't really suitable for how it's been used

 

So, we've basically got free reign to replace it with something sensible, which includes

• Rebuilding the whole roof structure and probably making it warm deck
• Choosing how big / how many rooflights to put in (and associated steel work)
• Re-covering roof and finishing interior (e.g. board and skim, re-wire lighting)

 

(Tbh, we don't really need this room so it's become a bit of a burden. It's more like a conservatory in that it has electric heating and we've got it closed off from the rest of the house at the minute to avoid wasting money heating it. But we still want it to have a decent amount of roof opening to avoid having a dark corner of the house)

 

I have an architect / structural engineer who is coming soon to measure up for drawings. The problem is I'm struggling with all the different options for the roof and how much they might cost.

 

Firstly, I have no idea even a ballpark figure what the absolute cost is going to be, whether it might be £15k or £50k, so I don't know whether I need to try and economise or not.

It's a bit of a chicken and egg situation - I've talked to a couple of builders, but they won't give me a quote without a spec / drawing, and I don't know what to tell the SE to draw without some idea of whether / where to save costs.

 

For example, I could:

1. Try and recreate the size they've made but do it properly (We don't really need it to be this big and it's probably not a great idea - might be a challenge with U-values and the like?)
2. Go for 1 big roof lantern that's not as big as this (e.g. 4m x 2m - would still need steel)
3. Go for a couple of rooflights / lanterns (e.g. 3m x 1.5m - is that excessive? would still need steel)
4. Go for more small rooflights (e.g. 1.5m x 1m - probably wouldn't need steel)
5. Countless other options

 

But - is increasing the number of rooflights to save on the cost of support a false economy? Would it make any meaningful savings (or savings at-all) since I assume there's an overhead in cost for each rooflight that's put in. Is there any kind of rule of thumb for the rooflight / steel size trade off?

 

Or am I wasting my time thinking about it and I should just go for what I think looks best while meeting regs?

 

Any help with understanding this appreciated. Thanks!

 

(edit: located in Surrey if it's relevant)

 

Edited December 17, 2023 by RatFloofing

[Kelvin]

That is a substantial room and done well it could be a terrific room. It looks as if it’s pretty bloody cold on there too! 
 

Your starting point really needs to be what do you envisage using this room for. If money was no barrier and you turned into a wonderful entertaining space would you use it? Done well it would add real value to the house. Alternatively you say you don’t need the space so the other extreme is remove it completely and regain the garden space in what is potentially a sun trap. 
 

No matter what you do it won’t be cheap to do it well. 

[Andehh]

That's be closer to a £50k budget in my mind to take it all down and rebuild it.

 

Depending on where in the country you are on value for money. Central London yes, up north probably not!

 

I'd probably look to reroof, broadly similar, look to maybe dig down and retrofit UFH! Overboard with insulated pb....

Edited December 17, 2023 by Andehh

[Gus Potter]

1 hour ago, RatFloofing said:

Go for 1 big roof lantern that's not as big as this (e.g. 4m x 2m - would still need steel)

Agree with Kelvin, and as you know it could be a stunning space.  My first thought was an Orangery type roof using the existing structural opening. This type of option cf a lantern  can have particular features if done in timber that can't be so easily achieved with say aluminium profiles. Could fit well with the existing Architecture?

 

There are diferent schools of thought in terms of number of say roof lights. One valid point is if you have a few huge panes of glass and one fails = costly to replace. This is something on my mind for my own house. When you look carefuly many of the glass unit manufactur's only guarentee the units for 7 years.

 

If using the Orangery / pitched roof type lantern you have more scope to get opening roof vents in, not saying you can't get opening flat roof lights mind you. It looks a faily sheltered spot so you should reduce any risk of them leaking when windy after a few years of use.

 

You may find the existing structure is ok, maybe needs a bit of beafing up.

 

Probably the most challenging bit will be to get the flashing details sorted out as you have lots of awkward shapes around the edges of the flat roof. You'll probably be doing this anyway but look at doing the minimum and see if you can leave the existing flat roof as it is, just give it a bit of love. Then work you way up in terms of complexity vs return on appearance / improved insulation.

 

 

 

 

Edited December 17, 2023 by Gus Potter

[saveasteading]

Good question.

The Engineer will produce a fairly standard proposal using traditional techniques so I'd say let them get on with what they do.

However, I'd suggest you use 2, maybe 3, off the shelf rooflight units. That will provide plenty of light. 

They are much cheaper to buy and install if standard and small. Say 2m x 1 at most. £600 each.

They are also easy to support efficiently with secondary framing.

Think about that, then tell your Engineer your proposal and let them do their stuff.

Then it will be a straight forward job that builders will be happy with.

Edited December 17, 2023 by saveasteading

[RatFloofing]

Thanks all for input - have given me some things to think about.

 

In terms of what we want to use it for, currently there's an empty hot tub sunk into the floor but being realistic I doubt we'll ever actually use it (might even be worth getting it taken out). Probably just a 'nice place to be' during the warmer months (& my partner likes it as an art studio!) - we visited in May, July and September before buying and the temperature seemed ok all those times - even though it's south facing it does have shade from the trees.

 

When I think about it, in a few years time I'm probably more likely to regret making lots of tradeoffs just in the interest of saving money at the time, but it doesn't make it easier.

 

4 hours ago, Andehh said:

Depending on where in the country you are on value for money. Central London yes, up north probably not!

Surrey - so not far off central London for value 

 

4 hours ago, Andehh said:

 maybe dig down and retrofit UFH!

 

It has electric UFH right now. The rest of the house has wet UFH (which also has some really weird issues / mistakes with the installation) but I'm not sure it would be very easy to get it extended out to here as well

Edited December 17, 2023 by RatFloofing