Advice sought for my solar mount foundations

[Moxi]

Hi Andy,

Would these be any better/ easier https://www.diy.com/departments/forest- ... %20Leisure other outlets available but seemingly about the same price point. then i found this

https://www.ebay.co.uk/itm/Heavy-Duty-C ... 8e20050cc7

or this kind of thing https://buildaresinshedvideo.blogspot.c ... ation.html

Moxi

[AGT]

Personally I think they are fine for decks most of the load is applied downwards but a solar array has uplift. So I would bury a post/unistrut in post mix to get more stability

[Stinsy]

I’m assuming we are talking an unheated space here?

*cough*. Yes because otherwise building regs need a ridiculous amount of insulation. I'll have some insulation and frost free heating.

We are fans of insulation in these parts. It pays for itself!

I'm not 100% on what your outbuilding will be used for. If it is a heated space (eg a garden office, or workshop, or whatever) then put 100mm of PIR on top of the MOT and beneath the slab. Then put cement board around the insulation and slab (to keep rodents out of your insulation and backfill with pea gravel. You can put land drain and a sock under the pea gravel if you feel it necessary.

For the frame it is worth thinking of upsizing to 6x2 so you can fit 100mm PIR between the studs and leave a ventilation gap between insulation and outer skin. But 75mm PIR and 4x2 will work too. You should also fit a DPM on the inside of the studs. And 100mm of PIR in the roof

The benefit of all this depends on what you're using the building for, how frequently you're using it, etc. You have to make your own decision.

Round my way “flat roofs” are all the rage. Primarily to avoid planning/building regs. However I can’t remember us ever getting more than 6” of snow. A 45° roof seems sensible if you get a lot of snow.

I've done the calcs and could make them work. And with the panels I already have, flat pitch would work as I really need cloudy weather generation.

You can do flat roof if you want. Just two things to consider: 1) You don't build it totally flat, you want a 1:10 or 1:20 slope. 2) If you're in Scotland and get a lot of snow then you should use 1.5kN/m² as the imposed load in your span calculations. This is probably going to end up being 8x2 C24 on 400mm centers. Just make sure you come under the critical 2.5m.

I personally wouldn’t overthink the solar panel mount on a shed! When I put them on my garden office I screwed 3x2s vertically through the felt and screwed the panels to the 3x2s.

Are you not worried about tiny amounts of moisture getting through and rotting from the inside? Or did you put a squirt of sealant around the penetration points?

Screwing through the roofing material is a bit terrifying. But I got over it, and a squidge of sealant does no harm. The panels on my garden office have been up there since 2021 and not a hint of dampness so far. 3x2 battnes screwed through the roofing felt into the OSB below and panels attached to battens with washer-head screws.

[Andy]

We are fans of insulation in these parts. It pays for itself!

I'm not 100% on what your outbuilding will be used for.

For the frame it is worth thinking of upsizing to 6x2 so you can fit 100mm PIR between the studs and leave a ventilation gap between insulation and outer skin. But 75mm PIR and 4x2 will work too. You should also fit a DPM on the inside of the studs. And 100mm of PIR in the roof

You can do flat roof if you want. Just two things to consider: 1) You don't build it totally flat, you want a 1:10 or 1:20 slope. 2) If you're in Scotland and get a lot of snow then you should use 1.5kN/m² as the imposed load in your span calculations. This is probably going to end up being 8x2 C24 on 400mm centers. Just make sure you come under the critical 2.5m.

Hi Stinsy, thanks so much. I've got a bit obsessed with bees as I wound down on the batteries, so I need a space for extracting. I also need a small space for a bit of exercise...So multi use. Its size is dictated by being a cover for the back of my ground mount. It was meant to be 20 degree slope but we squeezed it to 30º so it probably doesn't have the strength it was meant to. Which is why it is sheltered by lelylandii atm.

It can't be 'heated' or I'd need to hit quite low U values which is cost prohibitive given the occasional use. I was planning on 100mm king span between the studs. from the inside plastic sheet, studs with insulation, breather membrane, battons, either plastic coated metal sheathing or overlapping t+g wood. (which ever works out the cheapest. I was going to batten on the outside to allow it to breathe with a continuous space.I hadn't read about leaving a gap in the studs like you suggest. How do you get the airflow?

I'd worked out 8x2 as well but I've not heard about the 2.5m before. I see its to do with the struts.

As I'm typing this I've just found an amazing resource which specifies everything about framing timber buildings and more.
https://www.gov.scot/binaries/content/d ... %2B%2B.pdf

[Stinsy]

We are fans of insulation in these parts. It pays for itself!

I'm not 100% on what your outbuilding will be used for.

For the frame it is worth thinking of upsizing to 6x2 so you can fit 100mm PIR between the studs and leave a ventilation gap between insulation and outer skin. But 75mm PIR and 4x2 will work too. You should also fit a DPM on the inside of the studs. And 100mm of PIR in the roof

You can do flat roof if you want. Just two things to consider: 1) You don't build it totally flat, you want a 1:10 or 1:20 slope. 2) If you're in Scotland and get a lot of snow then you should use 1.5kN/m² as the imposed load in your span calculations. This is probably going to end up being 8x2 C24 on 400mm centers. Just make sure you come under the critical 2.5m.

Hi Stinsy, thanks so much. I've got a bit obsessed with bees as I wound down on the batteries, so I need a space for extracting. I also need a small space for a bit of exercise...So multi use. Its size is dictated by being a cover for the back of my ground mount. It was meant to be 20 degree slope but we squeezed it to 30º so it probably doesn't have the strength it was meant to. Which is why it is sheltered by lelylandii atm.

It can't be 'heated' or I'd need to hit quite low U values which is cost prohibitive given the occasional use. I was planning on 100mm king span between the studs. from the inside plastic sheet, studs with insulation, breather membrane, battons, either plastic coated metal sheathing or overlapping t+g wood. (which ever works out the cheapest. I was going to batten on the outside to allow it to breathe with a continuous space.I hadn't read about leaving a gap in the studs like you suggest. How do you get the airflow?

I'd worked out 8x2 as well but I've not heard about the 2.5m before. I see its to do with the struts.

As I'm typing this I've just found an amazing resource which specifies everything about framing timber buildings and more.
https://www.gov.scot/binaries/content/d ... %2B%2B.pdf

4x2 and 100mm PIR don't go together for several reasons. Firstly the actual finished size of 4x2 timber is 95mm so it just doesn't fit. Secondly, it is best practice to ventilate the cold side of all insulation, this is to avoid premature rot caused by condensation. Therefore 100mm PIR is usually paired with 6x2 framing. Covering the inside of your frame with DPM mitigates this need significantly, and some people argue it isn't required for seldom-used garden rooms, but the best-practice remains.

You need to make your own decisions. I've seen Youtube videos where so-called professionals try to justify their "hybrid roof" installations. You can do either a "warm roof" or a "cold roof" anything else is just ignorant of the principles of dew point. But the "professional" will be long gone in a few years when it starts to rot!

Have a look at composite cladding. I used to be against the concept of "plastic" decking, cladding, etc. But it looks good and lasts a very long time.

[Andy]

4x2 and 100mm PIR don't go together for several reasons. Firstly the actual finished size of 4x2 timber is 95mm so it just doesn't fit. Secondly, it is best practice to ventilate the cold side of all insulation, this is to avoid premature rot caused by condensation. Therefore 100mm PIR is usually paired with 6x2 framing. Covering the inside of your frame with DPM mitigates this need significantly, and some people argue it isn't required for seldom-used garden rooms, but the best-practice remains.

You need to make your own decisions. I've seen Youtube videos where so-called professionals try to justify their "hybrid roof" installations. You can do either a "warm roof" or a "cold roof" anything else is just ignorant of the principles of dew point. But the "professional" will be long gone in a few years when it starts to rot!

Have a look at composite cladding. I used to be against the concept of "plastic" decking, cladding, etc. But it looks good and lasts a very long time.

Funny you say that, I was watching the video below where he moans about exactly that. I've wondered about so many of the builds on YouTube for a while which is why I've spent so long procrastinating trying to work out the best way to do the build.

The image below is side view of a stud. sole plate and base plate top and bottom. Hatched area is PIR and green is external breathable membrane.
What I don't understand is why the need for the gap. Air can't flow into that cavity as it will be covered by the membrane. So is it not just giving an opportunity for moisture to condense on the studs? Ok, it is breathable so I guess it will recover at some point. Why not take the PIR all the way level or near as dammit with the outside of the stud.

[Stinsy]

4x2 and 100mm PIR don't go together for several reasons. Firstly the actual finished size of 4x2 timber is 95mm so it just doesn't fit. Secondly, it is best practice to ventilate the cold side of all insulation, this is to avoid premature rot caused by condensation. Therefore 100mm PIR is usually paired with 6x2 framing. Covering the inside of your frame with DPM mitigates this need significantly, and some people argue it isn't required for seldom-used garden rooms, but the best-practice remains.

You need to make your own decisions. I've seen Youtube videos where so-called professionals try to justify their "hybrid roof" installations. You can do either a "warm roof" or a "cold roof" anything else is just ignorant of the principles of dew point. But the "professional" will be long gone in a few years when it starts to rot!

Have a look at composite cladding. I used to be against the concept of "plastic" decking, cladding, etc. But it looks good and lasts a very long time.

Funny you say that, I was watching the video below where he moans about exactly that. I've wondered about so many of the builds on YouTube for a while which is why I've spent so long procrastinating trying to work out the best way to do the build.

The image below is side view of a stud. sole plate and base plate top and bottom. Hatched area is PIR and green is external breathable membrane.
What I don't understand is why the need for the gap. Air can't flow into that cavity as it will be covered by the membrane. So is it not just giving an opportunity for moisture to condense on the studs? Ok, it is breathable so I guess it will recover at some point. Why not take the PIR all the way level or near as dammit with the outside of the stud.

The cool moist air will circulate in the void and has a large surface area to seep through the OSB and breathable membrane on the outside. If it is trapped in one location (eg near a cold point such as a nail/screw) it causes rot.