Small Atmos Studio

Hello all, I've already learned so much from reading this forum and have already changed my plans based on a few responses, so I figure I should be sharing from the beginning as I muddle my way through designing and building a studio. Here's a brief rundown of the project: Where: UK What: A small 7.1.4 mix room for one operator and possible occasional client attendance. A small booth for VO/SFX recording. A Toilet. A Kitchen/Sitting area. Budget: £30,000 max - would prefer it to come way under if possible Starting point: Flat grass - no previous structure Isolation Requirements Will be next to a road, so is more about keeping sound out than keeping sound in. I will measure properly what kind of levels the road is presenting to see what kind of reduction I need. Building: Self build as much as possible. I should state that I read Rod Gervais' book around a year ago, but it is in a location I can't access due to the COVID19 lockdown - as soon as I'm able to, I'll retrieve it and re-read as I'm going a lot from memory at the moment. --- My current plan (I'll call Plan #1 for ease, and for keeping track of in future posts) is to lay three separate concrete slabs - one for the outer walls and kitchen/toilet, one for the studio, and one for the booth to reduce the chance of flanking. They're only different colours in the Sketchup diagram to show the separation. I'm proposing the outer leaf to be of regular design - the mass on the outside of the stud and the air gap inside. I was then planning on the inner leaves being an 'inside out' design to maximise space inside the rooms, and to give me a really dead starting point for acoustic treatment. The gap I currently have between the two leaves is ~20cm between the booth and studio, and ~30cm between the inner leaves and outer leaf. The sound proofing (or adding mass to the structure) will stop between the Studio/Booth and the Kitchen/Toilet as the kitchen/toilet do not need to be sound proofed. The Sketchup design attached is very rough in terms of stud placement, and the room/building sizes are relatively fluid as I'm starting this from scratch and chopping and changing as I go. Just trying to get a gauge of size and space before delving into the intricacies - I've read far too many threads already that start off with a finished plan that then have to get unpicked to make work. --- Questions: 1. Assuming that the internal height of the sound proofed rooms will be 2.4m - do the ratios look ok for the space? 2. Is my idea of the outer leaf being of regular construction, and the inner leaves being inside out a sound idea? 3. Is stopping the soundproofing/mass between the studio and kitchen/toilet ok? The outer leaf in these areas will not be soundproofed, so it won't be creating a third leaf right? 4. Any suggestions at this early stage of things to improve or try out?

Hello Fred, Welcome aboard the roller-coaster! It seems to me that your budget is realistic enough and I know it's definitely possible to save money on materials by bulk ordering.

I will measure properly what kind of levels the road is presenting to see what kind of reduction I need.

:thu:

I should state that I read Rod Gervais' book around a year ago . . . I'm going a lot from memory at the moment.

P.M. if you need me to reference something urgently - might not be able to get straight back to you as I'm spending a lot of time on my own build at the moment. Also, check out The Master Handbook Of Acoustics by F. Alton Everest - here: http://www.roletech.net/books/HandbookAcoustics.pdf

The gap I currently have between the two leaves is ~20cm between the booth and studio, and ~30cm between the inner leaves and outer leaf.

Greg built a great tool for calculating mass, cavity depth and their effect on T/L (apols if I'm preaching to the choir). http://johnlsayers.com/phpBB2/viewtopic.php?f=1&t=21770

I'm starting this from scratch and chopping and changing as I go.

You clearly know yourself how important it is to make, revise and update your plan - I've lost count how many times I did this. I look around on the forum for projects similar to mine, adapt them to my building, then re-draw according to suggestions offered by other members or when I make an advance in understanding a particular concept or technique.

Assuming that the internal height of the sound proofed rooms will be 2.4m - do the ratios look ok for the space?

Not my specialist subject I'm afraid - I'm sure some other members may have some thoughts. There's a room mode calculator here: https://amcoustics.com/tools/amroc?l=434&w=372&h=234&r60=0.23

Is my idea of the outer leaf being of regular construction, and the inner leaves being inside out a sound idea?

Are we taking about cladding a stud frame? I don't see any reason why this wouldn't work given the right density and construction.

Is stopping the soundproofing/mass between the studio and kitchen/toilet ok? The outer leaf in these areas will not be soundproofed, so it won't be creating a third leaf right?

Again, as described, I don't see why this can't work. You'll need to build a air supply and exhaust path (on opposite sides of the structure ideally) and I'm guessing you'll be installing a mini-split AC system as well.

Any suggestions at this early stage of things to improve or try out?

Keep reading, learning, work the numbers and your layout. Looking forward to seeing how your plans develop Fred. ATB John.

Fred Pearson wrote:

Hello all, I've already learned so much from reading this forum and have already changed my plans based on a few responses, so I figure I should be sharing from the beginning as I muddle my way through designing and building a studio. Here's a brief rundown of the project: Where: UK What: A small 7.1.4 mix room for one operator and possible occasional client attendance. A small booth for VO/SFX recording. A Toilet. A Kitchen/Sitting area. Budget: £30,000 max - would prefer it to come way under if possible Starting point: Flat grass - no previous structure Isolation Requirements Will be next to a road, so is more about keeping sound out than keeping sound in. I will measure properly what kind of levels the road is presenting to see what kind of reduction I need. Building: Self build as much as possible. I should state that I read Rod Gervais' book around a year ago, but it is in a location I can't access due to the COVID19 lockdown - as soon as I'm able to, I'll retrieve it and re-read as I'm going a lot from memory at the moment. --- My current plan (I'll call Plan #1 for ease, and for keeping track of in future posts) is to lay three separate concrete slabs - one for the outer walls and kitchen/toilet, one for the studio, and one for the booth to reduce the chance of flanking. They're only different colours in the Sketchup diagram to show the separation. I'm proposing the outer leaf to be of regular design - the mass on the outside of the stud and the air gap inside. I was then planning on the inner leaves being an 'inside out' design to maximise space inside the rooms, and to give me a really dead starting point for acoustic treatment. The gap I currently have between the two leaves is ~20cm between the booth and studio, and ~30cm between the inner leaves and outer leaf. The sound proofing (or adding mass to the structure) will stop between the Studio/Booth and the Kitchen/Toilet as the kitchen/toilet do not need to be sound proofed. The Sketchup design attached is very rough in terms of stud placement, and the room/building sizes are relatively fluid as I'm starting this from scratch and chopping and changing as I go. Just trying to get a gauge of size and space before delving into the intricacies - I've read far too many threads already that start off with a finished plan that then have to get unpicked to make work. --- Questions: 1. Assuming that the internal height of the sound proofed rooms will be 2.4m - do the ratios look ok for the space? 2. Is my idea of the outer leaf being of regular construction, and the inner leaves being inside out a sound idea? 3. Is stopping the soundproofing/mass between the studio and kitchen/toilet ok? The outer leaf in these areas will not be soundproofed, so it won't be creating a third leaf right? 4. Any suggestions at this early stage of things to improve or try out?

Hello fellow brit, Atmos is a very ambitious design acoustically, especially since I am assuming this will be your first ever studio build? Props to you... I am not one to be a party pooper and hopefully we can help you achieve your dream. Might be worth researching the Atmos room criteria - the biggest challenge will be your limited size, especially a height of 2.4m, not ideal for a professional control room let alone an Atmos mix room, but you may know more than me on the topic. You can use bobgolds.com room mode calculator to get a very good analysis of your proposed plan, make sure to check all the details in the list at the bottom as well as the bonello chart. Things I would definitely aim for is for your room to pass the 3 critical listening tests, no "near" or "same" modal incidences, a volume of at least 1500 cubic feet (more if possible, bigger is definitely much better) and a smooth bonello criterion chart. Inside out inner leaf is fine - just be aware that in order to get acceptable isolation the gap between the two sets of framing will need to be substantially bigger since you'll not have the depth of the studs from the inner leaf to count towards your air gap. At least 100mm but ideally more if you want excellent low frequency isolation. I see you have 300mm - that's pretty close to what you'll need depending on how much mass you have on each leaf - have you used Gregwor's MSM calculator to figure out how much isolation you can expect to achieve with your current plan? The kitchen and toilet are not a third leaf since the space between them and your isolated studio is substantial, in fact it will increase your overall isolation. Looking forward to seeing this project unfold! Paul

John Steel wrote:

Hello Fred, It seems to me that your budget is realistic enough and I know it's definitely possible to save money on materials by bulk ordering.

Good to know I'm in the rough ballpark!

John Steel wrote:

P.M. if you need me to reference something urgently - might not be able to get straight back to you as I'm spending a lot of time on my own build at the moment. Also, check out The Master Handbook Of Acoustics by F. Alton Everest - here: http://www.roletech.net/books/HandbookAcoustics.pdf

Appreciated - thanks John!

John Steel wrote:

Greg built a great tool for calculating mass, cavity depth and their effect on T/L (apols if I'm preaching to the choir). http://johnlsayers.com/phpBB2/viewtopic.php?f=1&t=21770

Yep, I had a quick whirl at this and ended up with the attached graph. I'm going to keep fiddling with different materials and thicknesses to see what I can achieve. Being close to the road, I think that sub 100 Hz is really important for any passing lorries/tractors etc. I appreciate this will be a challenge!

John Steel wrote:

Not my specialist subject I'm afraid - I'm sure some other members may have some thoughts. There's a room mode calculator here: https://amcoustics.com/tools/amroc?l=434&w=372&h=234&r60=0.23

I gave this a whirl too, but wasn't entirely sure what I was looking at. More research on room modes and what I want to achieve is needed here!

John Steel wrote:

Again, as described, I don't see why this can't work. You'll need to build a air supply and exhaust path (on opposite sides of the structure ideally) and I'm guessing you'll be installing a mini-split AC system as well.

Yep, looking at mini-splits at the moment. This is definitely an area I need to do more research on. I'm confused about what I actually need - I've read several posts in the forum that imply that an AC system doesn't supply fresh air, just cools the air that's in the room. If I've understood correctly then obviously this is a massive issue as air tight room = death, but I've struggled to find info on a fresh air inlet model. On my list to keep exploring!

Paulus87 wrote:

Hello fellow brit, Atmos is a very ambitious design acoustically, especially since I am assuming this will be your first ever studio build? Props to you... I am not one to be a party pooper and hopefully we can help you achieve your dream. Might be worth researching the Atmos room criteria - the biggest challenge will be your limited size, especially a height of 2.4m, not ideal for a professional control room let alone an Atmos mix room, but you may know more than me on the topic.

Hi Paulus87! Thanks for your response. I should definitely clarify here. This won't be a Dolby certified Atmos room for theatrical use, but one for monitoring and creating mainly Atmos for Home premixes with the intention of hiring a Dolby Certified stage for final sign off and rendering as and when needed.

Paulus87 wrote:

You can use bobgolds.com room mode calculator to get a very good analysis of your proposed plan, make sure to check all the details in the list at the bottom as well as the bonello chart. Things I would definitely aim for is for your room to pass the 3 critical listening tests, no "near" or "same" modal incidences, a volume of at least 1500 cubic feet (more if possible, bigger is definitely much better) and a smooth bonello criterion chart.

Thanks for the heads up on this. I hadn't come across this yet but what a useful tool. I moved my length from 3.8m to 3.9 and got rid of the only 'pink warning' mode and evening out the Bonello curve. I'm currently coming in at just over 1000 cubic ft, so will struggle to get close to 1500 cubic ft but will see how close I can get.

Paulus87 wrote:

...have you used Gregwor's MSM calculator to figure out how much isolation you can expect to achieve with your current plan?

I have, but need to play with it a bit more to trial and error the best combination.

Paulus87 wrote:

The kitchen and toilet are not a third leaf since the space between them and your isolated studio is substantial, in fact it will increase your overall isolation.

Phew - good to know. Thanks!

Just a suggestion - do you really need a booth? Small booths never usually sound very good and are only useful for isolating an amp or a guide vocal that will be scrapped later and re-recorded in the live room or control room. Many many singers record their vocals in the control room as it means instant communication with the producer/engineer and the control room environment is very suited to vocals. There's no reason this couldn't work for voice over and special FX too, and just use headphones while tracking if you need to. You could even use the sitting room for this, you don't need extreme isolation and a sitting room is a very comfortable environment for clients. This means you'd be able to have a much larger control room, which would be overall much more useful than a small booth that 90% of the time will be used as a storage cupboard. Something to think about? Paul

Paulus87 wrote:

Just a suggestion - do you really need a booth? Small booths never usually sound very good and are only useful for isolating an amp or a guide vocal that will be scrapped later and re-recorded in the live room or control room. Many many singers record their vocals in the control room as it means instant communication with the producer/engineer and the control room environment is very suited to vocals. There's no reason this couldn't work for voice over and special FX too, and just use headphones while tracking if you need to. You could even use the sitting room for this, you don't need extreme isolation and a sitting room is a very comfortable environment for clients. This means you'd be able to have a much larger control room, which would be overall much more useful than a small booth that 90% of the time will be used as a storage cupboard. Something to think about? Paul

Thanks for the suggestion Paul but a booth is essential for me. Mainly for voice overs, but I need an extra space in case I ever need another pair of hands on site. Nice idea though!

I've come on a little bit since my last post, including re-sizing my rooms slightly thanks to Bob Gold's room mode calculator - here is the design in its current form: Starting to get the studding in including the booth window: Also starting to look at the outer leaf and cladding - I thought originally that I could have all 2 x gypsum and 1 x OSB on the outside of the studs to create a slightly bigger air gap. I can't however figure out how to attach this configuration to the outer frame studs without compromising the isolation (i.e. screwing through the gypsum). This was the original idea: Now I'm thinking it would be best to just have OSB on the outside (attached to the studs), with two layers of gypsum between the studs fixed in with green glue and cleats (there will of course be visually appealing cladding on the outside of this OSB - but undecided what yet): I've recently retrieved my copy of Rod Gervais' book - so going to have another re-read of that to make sure I'm on the right track. --- Questions: 1. If I had the 2 x layers of gypsum and the 1 x OSB on the outside of the outer studs, could I screw through all three layers (still with green glue in between) into the studs without isolation suffering? Or would I be better off having an outer layer of OSB screwed into the studs and then the 2 x gypsum boards cleated in? 2/ Does the booth window look right? Currently designed with 2x4s with the 4cm being the shelf/sill depth, but all the studios I've seen with a window seem to have a larger shelf (to cater for the bigger angle for the glass?). 3. Anything I should be paying more attention to, or anything I should re-read as it doesn't look quite right in my mockups?

Greetings Fred,

Now I'm thinking it would be best to just have OSB on the outside (attached to the studs), with two layers of gypsum between the studs fixed in with green glue and cleats (there will of course be visually appealing cladding on the outside of this OSB - but undecided what yet):

Yes, except no green glue and each layer caulked like your life depended on it (as per Rod's book). You want everything air tight.

1. Or would I be better off having an outer layer of OSB screwed into the studs and then the 2 x gypsum boards cleated in?

Yes. (Reiterated from above) Don't forget the caulk. All the best, Paul

Very very helpful - thanks Paul! I am in need of reading Rod's book again so will get on that straight away - but shouldn't there be a layer of green glue compound between the layers? Or is that just in the inner leaf? Cheers,

Greetings Fred, Just the inner leaf (which is nice because it saves some money). All the best, Paul

That is good news - thanks for the heads up!

Hello, Sorry for the radio silence, I've been doing a lot of research and falling through deeper and deeper rabbit holes. My current area of research is the roof space in regards to cold and warm roofs and I'm struggling with some concepts. My studio will be in a timber clad garage, so naturally is a cold roof design. This is how I see the construction (sorry for mangling your image that I found elsewhere Paul!) Does this look right? Any reason why I shouldn't apply my outer layer under the outer rafters rather than creating a 50mm batten from the ceiling? Can I do away with the ridge vent and just have the gap between the fascia board and the outer wall? (with some chicken wire or similar to stop pests getting in) Do I have to place anything on the outside of the outer drywall layers in the roof space to prevent moisture? I'd also love to know the theory of why we place the vapour barrier on the outside of the inner layer of plasterboard rather than anywhere else - just so I understand rather than questioning the position itself. Is it so it keeps moisture in the studio rather than stopping moisture penetrating from outside? Apologies for the abundance of questions, just trying to get my head around this. Cheers!

when you insulate walls you create a temperature gradient across them with the warmest being on the inside and coolest on the outside during the winter months. when you take warm, moist air and cool it you find that the moisture in it condenses, usually on to the surfaces of the container it is in. If you pass this same air through the walls of your building the same condensation process occurs but instead of being inside, it accumulates inside the insulation in your walls, roof or floors.

when you insulate walls you create a temperature gradient across them . . . when you take warm, moist air and cool it you find that the moisture in it condenses

I don't intend to hi-jack this thread Fred & Glenn but I was wondering if effective de-humidification of the air inside the inner leaf can have a mitigating effect on moisture forming between the leaves? Best wishes, John.

it would be beneficial to have a properly controlled level of humidity - instruments, electronics, people etc all benefit.

Fred Pearson wrote:

Hello, Sorry for the radio silence, I've been doing a lot of research and falling through deeper and deeper rabbit holes. My current area of research is the roof space in regards to cold and warm roofs and I'm struggling with some concepts. My studio will be in a timber clad garage, so naturally is a cold roof design. This is how I see the construction (sorry for mangling your image that I found elsewhere Paul!)

Not preserved: Cold Roof Thought 1.png

Does this look right? Any reason why I shouldn't apply my outer layer under the outer rafters rather than creating a 50mm batten from the ceiling? Can I do away with the ridge vent and just have the gap between the fascia board and the outer wall? (with some chicken wire or similar to stop pests getting in) Do I have to place anything on the outside of the outer drywall layers in the roof space to prevent moisture? I'd also love to know the theory of why we place the vapour barrier on the outside of the inner layer of plasterboard rather than anywhere else - just so I understand rather than questioning the position itself. Is it so it keeps moisture in the studio rather than stopping moisture penetrating from outside? Apologies for the abundance of questions, just trying to get my head around this. Cheers!

Hi, You can put the outer leaf mass layers directly on the bottom of the rafters, no problem, it is a lot easier to do that. The only reason for using battens and cutting the mass layers in to fit between the rafters is if you have a height restriction and you want to maximise not only inner ceiling height but also maintain the required cavity depth to reach your isolation goal between the leaves. If you do not have a height restriction, or if you do but are willing to compromise on height, then fixing directly to the bottom of the rafters is the way to go. Vapour barrier needs to go on the warmest side of the insulation (not counting the acoustic treatment inside the envelope) so that any vapour that makes it through the drywall/OSB (which, is very unlikely anyway since they should be sealed air tight) will be retarded by the vapour barrier, which btw, also needs to be completely sealed and continuous - no point having a vapour barrier (or retarder) at all if there are any gaps or holes in it. Make sure you use the right type of barrier here as well. Once you have a fresh air supply and return and air con installed then any moisture inside the building should be managed by the system, if you can install an HRV then all the better as this should also have a dehumidifier built in. In the UK you can do away with a ridge vent IF the total width of the building is no more than 6 meters. You will either need a continuous 10mm gap all the way along the eaves on both sides of the building, or if using the small circular 70mm vents then you'll need one of those spaced every 160mm. You must guarantee that there is a free flow of air on the under side of the roof deck from one side to the other, completely unobstructed between every rafter bay. If there is anything that obstructs the air flow then it will all be void and you'll very likely have problems. Hope this helps, Paul

Paul, thank you so much for your response. That answers all of my questions, makes perfect sense and has given me a much better understanding of the whole cold roof thing. I'll definitely have 10mm gap all around the eaves, but might put a few small round vents in the gable end as close to the ridge line as I can, to ensure airflow.

Ok, slight issue. My rafters in the roof run perpendicular to the pitch, so they won't naturally create an air gap if I attach the outer layer of the MSM to the bottom of these. Can I space out the outer layer using 50mm battens from the bottom of the rafters as suggested in the previous picture. Will this create enough free-flowing air, or will the air get trapped in between the rafters seeing as they're perpendicular to the pitch? Image below:

Fred Pearson wrote:

Ok, slight issue. My rafters in the roof run perpendicular to the pitch, so they won't naturally create an air gap if I attach the outer layer of the MSM to the bottom of these. Can I space out the outer layer using 50mm battens from the bottom of the rafters as suggested in the previous picture. Will this create enough free-flowing air, or will the air get trapped in between the rafters seeing as they're perpendicular to the pitch? Image below:

Not preserved: Rafters.png

Fred, I'm sorry if you've already explained this in full detail but my time is limited and memory poor! What type of roof do you have? Tiles? Metal Sheets? Is there a ply/osb roof deck on top of the rafters, under the roofing? Outside, is there an overhang? How much? Any soffits? Any existing insulation up by the rafters? Any existing ventilation anywhere? Paul

Hi Paul, Thanks for your reply and questions. I don't have the building yet, but can answer all of these with reasonable confidence. It'll be felt tiles (I think laid in segments, so not really individual tiles). On top of the rafters but under the felt will be OSB. Outside there will be an overhang of around 20-30cm with no soffits (you'll be able to see the roof OSB from the outside). There will be no existing insulation. There will be no existing ventilation other than the gap between the rafters and the top of the wall. I hope this helps as I'm still scratching my head a bit at this. Cheers,

Fred Pearson wrote:

Hi Paul, Thanks for your reply and questions. I don't have the building yet, but can answer all of these with reasonable confidence. It'll be felt tiles (I think laid in segments, so not really individual tiles). On top of the rafters but under the felt will be OSB. Outside there will be an overhang of around 20-30cm with no soffits (you'll be able to see the roof OSB from the outside). There will be no existing insulation. There will be no existing ventilation other than the gap between the rafters and the top of the wall. I hope this helps as I'm still scratching my head a bit at this. Cheers,

If it's not built yet, then why not just build it with the rafters going the other way? Paul

Hi Paul, For time purposes, I've steered away from constructing the building myself and am going for a prefab that'll go up in 2 days rather than the months it would take me. I've found the company I want to use and this is how they do their rafters. If I can tweak my design so I can add adequate ventilation with this orientation of rafter that would be perfect, although I'm not entirely opposed to sourcing something else if this simply won't work. Cheers, Fred

presumably the need to provide roofing ventilation as part of build code so, does that mean the venting it something they're doing on top of the rafters? maybe if you can share their detailed build specs/plans for the roofing we could figure this out.

Hey gullfo, Thanks for your response. In terms of ventilation they do have small round vents in the gable ends (which I will opt to not have as they'll be right in the middle of the air gap) and a gap between the eaves and the rafters. Lovely promo picture of the vents in the gable ends below. Could the ventilation work like this? Cheers, Fred

Fred Pearson wrote:

Hey gullfo, Thanks for your response. In terms of ventilation they do have small round vents in the gable ends (which I will opt to not have as they'll be right in the middle of the air gap) and a gap between the eaves and the rafters. Lovely promo picture of the vents in the gable ends below.

Not preserved: Screen Shot 2021-01-19 at 18.14.13.png

Could the ventilation work like this?

Not preserved: Screen Shot 2021-01-19 at 18.27.29.png

Cheers, Fred

The proposed prefab building is 1. not intended to be used as part of a completely air tight, double leaf assembly 2. the normal way of venting, which would apply for a residential or commercial building does not apply because of it's intended purpose is as a garden shed where there are usually no constant sources of moisture for extended periods of time i.e. no humans breathing out hot air/sweating/boiling the kettle, therefore 3. the design of these shed by default are very leaky anyway; cracks between the doors/windows/framing etc. The problem you have with your proposed venting idea (along the gable end fascias) is that you will not have a constant, unobstructed airflow from one end to the other between EVERY pair of rafters. In your proposal you are suggesting bringing air in from the eaves and venting it out of the gable end vents, the problem I see with that is potentially only the lowest rafter bay (the one nearest the eaves) will actually be fully vented, all the others will have progressively less air flow since air will travel the path of least resistance. It might work, but you would need to speak to someone in building control or a cold roof expert. Now, you could vent from one gable end to the other but there's a couple of problems with that.... Now, what is that? You've drawn it as a solid white object but I'm assuming it is supposed to be framing? If it is framing, then yes, technically you could have vents from one gable end to the other BUT that would be longer than 6m I am assuming? In actuality, it would *probably* work fine, but it's an unknown. By deviating from the usual proven way of doing things you run the risk of it not working and therefore having problems later on down the line. if your building is going to be subject to building control then I do not think it would pass. If it is not subject to building control (or you build this on the down low) then you also take the risk upon yourself. You could do either of the following: build it like your proposal, but instead of the vents on the gable ends, just install a ridge vent. This way the air gets drawn in from the bottom at the eaves and then vents out of the top at the ridge. The only unusual thing in this instance is that usually the air flow would flow directly under the roof deck without any resistance from rafters, whereas in your roof the air would have to flow UNDER the rafters first, which means the air will not "contact" every part of the roof deck... it will skim along the bottom of the rafters. Have you considered getting rid of the vents altogether and just installing a warm roof? By doing a warm roof it doesn't matter which way the rafters run since you do not need any venting or air flow below the roof deck at all. Paul