Venting a flat roof

[jpadie]

Hello and greetings to all for the festive season

 

On a garden room project I've got a flat cold roof (75mm slope across 3.5m).  For soffits there is an overhang on the high side of 500mm and on the low side about 100mm.   The joinery is 2x7s stuffed with 140mm of PIR.  Providing quite a bit of air space under the osb which sits atop the furring.

 

What's the normal way of providing ventilation to the roof cavity?  Soffit vents on the low side and fascia vents on the fascia of the high side?  Or are soffit vents either side ok?  Or can I reverse so that I have soffit vents on the high side and fascia vents on the low? 

 

Aesthetically venting on the fascia of the high side would be least good.   

 

Is it normal to have mesh on the inside of the vents to protect against critters?  If so what's the advised density for the mesh?  Those I've seen would definitely let through wasps who seem to like to nest everywhere.   And the soffit vents that I see on typical pvc boards look big enough to accept hornets. 

 

Thanks in advance!

 

Justin

 

 

Edited December 26, 2023 by jpadie
Misspelling

[ToughButterCup]

2 hours ago, jpadie said:

...What's the normal way of providing ventilation to the roof cavity?  Soffit vents on the low side and fascia vents on the fascia of the high side?  Or are soffit vents either side ok?  Or can I reverse so that I have soffit vents on the high side and fascia vents on the low? 

...

Is it normal to have mesh on the inside of the vents to protect against critters?  If so what's the advised density for the mesh?

 

Q1 Yes: google it, loads on the market

Q 2 and 3. Loads of good example images on Google and YT

Q4 and 5. Yes.. Google insect screen or associated search terms.

 

 

[gaz_moose]

1mm insect mesh stuck to the back. Some vented soffit looks terrible so choose wisely.

[ETC]

Try Glidevale continuous soffit vents.

[Gus Potter]

On 26/12/2023 at 09:57, jpadie said:

Hello and greetings to all for the festive season

 

On a garden room project I've got a flat cold roof (75mm slope across 3.5m).  For soffits there is an overhang on the high side of 500mm and on the low side about 100mm.   The joinery is 2x7s stuffed with 140mm of PIR.  Providing quite a bit of air space under the osb which sits atop the furring.

 

What's the normal way of providing ventilation to the roof cavity?  Soffit vents on the low side and fascia vents on the fascia of the high side?  Or are soffit vents either side ok?  Or can I reverse so that I have soffit vents on the high side and fascia vents on the low? 

 

Aesthetically venting on the fascia of the high side would be least good.   

 

Is it normal to have mesh on the inside of the vents to protect against critters?  If so what's the advised density for the mesh?  Those I've seen would definitely let through wasps who seem to like to nest everywhere.   And the soffit vents that I see on typical pvc boards look big enough to accept hornets. 

 

Thanks in advance!

 

Justin

Hiya.

 

@gaz_moose and @ETC solutions can work well. Just depends on the type of detailing you want to do to finish off the roof.

 

Cold flat roofs used to be common I (many worked fine) but now we stuff loads of insulation into and under them and BC etc.. and myself are not that keen on them for houses not least.. but..

 

Being pragmatic. It's not a kitchen, utility room or a bedroom where folk are sleeping in, thus the level of water gas generated should be relatively modest most of the time. Many of these older types of cold flat roofs relied on cross flow ventilation, usually via soffit vents. Most of these older types of flat roof are attached, say an extension which can inhibit the ventilation of a cold flat roof.

 

A quick bit of rough theory, I have left out a lot but hope I give enough to let you understand the principles.

 

To achieve cross flow ventilation on a flat roof you need to generate either / and or :

 

1/ A difference in air pressure between the windward and leeward side of the roof and / or have a sufficient slope in the roof to induce air convection currents. You get the best air pressure differential when the walls on the leeward and windward side (your roof joist / air gap span direction) are low and wide compared with the eaves height. This leads the wind to flow up and over the roof rather than round the sides of a structure. By promoting this type of wind flow you'll create the most favourable conditions to vent a flat roof.

 

You have an advantage here as this is a stand alone building so avoid the problems typically associated with a cold flat roof extension.

 

2/ And / or induce air convection currents within the cold vented space we need heat.. that comes from the internal space in the winter. But that is lost heat which we are now keen to avoid. This goes some way towards explaining why cold flat roof were ok in the past but not now.

 

I think this will work if you are careful, when partying.. leave a window open. Also avoid adding log stores and other paraphernalia / planting of trees etc that inhibits air flow round the building.

 

Yes,  you want to stop beasties getting in to the vent void as they will very quickly fill it up to make a lovely home for themselves. Mitigate by providing other habitats; bug hotels, bee habitats, bird boxes etc that you'll be able to see and enjoy. Don't know if you have frogs but you could do a bog garden from the rainwater run off and top that up with the garden hose if need be?

 

 

 

 

 

 

 

 

[gaz_moose]

there is some guy on youtube who builds loads of these garden rooms and he never seems to build any gaps in the roof. he just fills them full of kingspan then seals them up. id be inclined to put vented soffit at each end.

[jpadie]

On 28/12/2023 at 11:52, gaz_moose said:

there is some guy on youtube who builds loads of these garden rooms and he never seems to build any gaps in the roof. he just fills them full of kingspan then seals them up. id be inclined to put vented soffit at each end.

thanks - I've seen so many different approaches (so called hybrid?) that I thought it best to seek the wisdom of the masses! 

thanks - soffit vents it is. 

[jpadie]

On 27/12/2023 at 22:11, Gus Potter said:

Hiya.

 

@gaz_moose and @ETC solutions can work well. Just depends on the type of detailing you want to do to finish off the roof.

 

Cold flat roofs used to be common I (many worked fine) but now we stuff loads of insulation into and under them and BC etc.. and myself are not that keen on them for houses not least.. but..

 

Being pragmatic. It's not a kitchen, utility room or a bedroom where folk are sleeping in, thus the level of water gas generated should be relatively modest most of the time. Many of these older types of cold flat roofs relied on cross flow ventilation, usually via soffit vents. Most of these older types of flat roof are attached, say an extension which can inhibit the ventilation of a cold flat roof.

 

A quick bit of rough theory, I have left out a lot but hope I give enough to let you understand the principles.

 

To achieve cross flow ventilation on a flat roof you need to generate either / and or :

 

1/ A difference in air pressure between the windward and leeward side of the roof and / or have a sufficient slope in the roof to induce air convection currents. You get the best air pressure differential when the walls on the leeward and windward side (your roof joist / air gap span direction) are low and wide compared with the eaves height. This leads the wind to flow up and over the roof rather than round the sides of a structure. By promoting this type of wind flow you'll create the most favourable conditions to vent a flat roof.

 

You have an advantage here as this is a stand alone building so avoid the problems typically associated with a cold flat roof extension.

 

2/ And / or induce air convection currents within the cold vented space we need heat.. that comes from the internal space in the winter. But that is lost heat which we are now keen to avoid. This goes some way towards explaining why cold flat roof were ok in the past but not now.

 

I think this will work if you are careful, when partying.. leave a window open. Also avoid adding log stores and other paraphernalia / planting of trees etc that inhibits air flow round the building.

 

Yes,  you want to stop beasties getting in to the vent void as they will very quickly fill it up to make a lovely home for themselves. Mitigate by providing other habitats; bug hotels, bee habitats, bird boxes etc that you'll be able to see and enjoy. Don't know if you have frogs but you could do a bog garden from the rainwater run off and top that up with the garden hose if need be?

 

 

 

 

 

 

 

 

great response!  many thanks. 

there's no obvious wind/leeward side as the surrounding foliage stop any extreme wind.  if anything the roof joists run perpendicular to the prevailing wind so if I'm lucky I might get some low pressure areas under the soffits.