Hi all,
Looking for some advice on my extension proposal before I progress with an architect.
Currently have a small conservatory with polycarb roof and the usual summer too hot / winter too cold issues. My plan is to replace this with a flat-roof, timber clad extension that we would use for dining, utility and a small office. There would also be an unheated, small porch. Picture windows are to make the most of solar gain and panoramic S-W facing views. See images attached.

The main house is stone-built with about 500mm sandstone and 120mm internal mineral wool insulation between studs - so not the warmest by any means. A wall would be removed to create a new open plan living / dining space.
Armed with a thermal camera, I have a general plan to upgrade cold spots and air tightness in the existing house as I go. Oh, and I'm in sunny, but cold Aberdeenshire.

I had been thinking SIPS panels (244mm EPS) would be a safe bet - good insulation, relatively easy to design & install and would suit a flat roof extension very well. However, on searching the forum I read some reservations about connecting this to an existing stone building, which I hadn't really considered. Is the general consensus that block build with external insulation would better suit? And are there good resources showing make up of well insulated flat roofs?

The existing house has an oil combi and a log burner. My hope was if the extension is suitably insulated then very little 'extra' heat input will be required and it may even be of net benefit.
I've been looking into single room air-air ASHPs as an option for the main open plan space - with the hope that additional heat input may only be needed on the colder winter days. Thoughts on air-air ASHPs? The rads through the house are too small I think for a full-blown ASHP system - although, once all the cold spots are dealt with?....

I also like the idea of using the new footprint to house some form of interseasonal store via PV or solar thermal - this one needs a bit more research to calculate size and feasibility! Any links or resources would be greatly appreciated. I have plenty garden space to site South facing panels.

And one from above. The red outline being the existing house.
The extension is around 45sqm internal.

Posted By: tonyCan you design out the flat roof? Lots of downsides. If you do go flat roof min fall 1:40

Potentially yes. My initial assumption was that a flat roof would keep the costs down and maybe keep it simple enough that I could do a lot of the work myself, along with tradespersons.
I was also assuming that I need to keep the connection to the existing house along the existing 'gutter-line'. Even if that is the case, I could possibly pitch it up away from the house.

What are the main concerns? I think EPDM covering or the like can be warrantied for up to 30 years?

Posted By: tonyWhat u-values have you asked architect to aim at and beware of west facing glass as it can lead to overheating

I'm not quite at architect stage yet. I've layed out the extension as best as I can for how it will work for my family. Next phase was to get my head around structure, heating, etc, do a bit more research and then speak with an architect or architectural technician armed with as much info as I can.
I can work AutoCAD and create detail drawings, so my plan was to use an architect for as little as I can get away with.

U-values of the components I've selected so far are:
SIPS walls & roof 244mm 0.12
Windows 1.2
Floor, inc 100mm Kooltherm 0.18

The existing sunroom has chronic overheating in the summer. My thinking of the new space is:
- there is less window area (e.g. no roof windows) compared with existing smaller sunroom
- it is connected to a MUCH bigger space - I.e. connected to living area, which has open stairs to upper floor. And will be connected to utility and small office.
- I'll ensure there will be windows that can be opened and secured either end of the space.
- Blinds / curtains could help in extreme occasions.

Also bear in mind that I'm in Aberdeenshire. But, yes, that will be one of my questions for an architect.

Posted By: neelpeelWhat are the main concerns? I think EPDM covering or the like can be warrantied for up to 30 years?

It's very easy to get puddles of water forming on the roof, but these shouldn't cause a problem of themselves (we have one that forms on our sun room roof when 'stuff' gathers around the drain outlet) Ours is EPDM and yes I'm hoping for at least 30 years from it. The main worries are leaks if the job isn't done right, or if the roof is damaged. Some people put gravel or a green roof on top of the EPDM which helps to protect it from damage, UV, ice etc. Another thing that might be worth considering is a skylight. We have a large PH rated one (4 layers of plastic) which helps a lot with ventilation in summer.

External blinds or shutters are most effective for preventing overheating, but they need to be wind and weather proof.

Posted By: Dominic Cooney

I could possibly pitch it up away from the house.

I would avoid sloping the roof so that the rainwater runs towards the house (if that is what you mean here?)

That is always the advice, but I wouldn't be afraid to do it, with open eyes, if it makes sense in other ways. Certainly preferable to an affordable flat roof - has to be hi-spec to be reliable - and if it's flashed into the roof slope, not much different, risk-wise, to a well-engineered back-gutter.

The most straightforward thing would be a corrugated tin roof, extra-thick spec, colour coated, of minimal say 12o pitch, sloping back to the house, with a capacious back gutter with good upstands, perhaps bent up from single length of heavy-gauge galvanised or even stainless sheet by the corrugated supplier, or fibreglass or similar, which could be systematically cleared once a year.

With your overhang to eave and verge, nicely detailed, exposed rafter/purlin ends, could look really handsome, in traditional 3" x 3/4" 'corrugated iron', which may be locally traditional for cottage outbuildings, and I use a lot of, rather than more 'modern' factory/barn profiles which look cheap/utilitarian.

Posted By: neelpeelI don't have much headroom to play with and I don't want a step down within the house.

wouldn't it just mean starting the roofslope a bit higher up the roof of the main house?

excuse the bad sketching to exaggerate the point!

Posted By: fostertomcapacious back gutter with good upstands, perhaps bent up from single length of heavy-gauge galvanised or even stainless sheet by the corrugated supplier, or fibreglass or similar, which could be systematically cleared once a year.

Ordinarily I would not have been put off a good design solution with a good engineering solution, but I now have a valley gutter between 2 farm buildings made a bit like this (although it's not one single length, it has a couple of joins) and it leaks a bit (at the joins). Not a lot, but it does. It doesn't really matter for this application, and one of the roofs will be taken down soon anyway so it will be gone completely, but it does put me off having a valley gutter anywhere that really does matter. And this reminds me I need to systematically clear all the leaves out of it again!

Posted By: Dominic Cooney
wouldn't it just mean starting the roofslope a bit higher up the roof of the main house?

Er, yes, that would work wouldn't it. Certainly preferable to a valley I think.
Did I mention that I have no building experience.

My assumption was that keeping to the gutterline, would keep the connection to the existing roof simpler, but I guess I would just need to remove a few rows of slates. Food for thought.

Posted By: neelpeelthe connection to the existing roof

The connection will need to be really well flashed-in anyway so I can't see it being any different if it is on the bottom edge of the tiled roof or a couple of rows up. But I am not an expert, only that I am also interested in this junction because I intend to do it on our own house as well (replace a flat roof with a shallow pitch joining to the main tiled roof of the house)

Posted By: neelpeelI had been thinking SIPS panels (244mm EPS) would be a safe bet - good insulation, relatively easy to design & install and would suit a flat roof extension very well. However, on searching the forum I read some reservations about connecting this to an existing stone building, which I hadn't really considered.

I think either will work. I'd guess SIPS will give better insulation for a given thickness and don't involve wet trades. Blocks and EWI are a more traditional approach. SIPS will move differently than a stone building (thermal expansion and contraction, wind movement, perhaps initial drying or absorbing of moisture etc) so they'd need more thought about how to seal them long-term to the stone walls. Unless the walls are completely flat you'll need to consider how to deal with the undulations as well.