Garden Room Timber Cladding - No Roof Overhang. How to Vent?

[Davieboy84]

Hello folks, 

 

I'm waiting on the delivery of my SIPS garden room kit (4.8m x 3.6m), with my foundation and sub-frame now complete.

 

I'm looking into cladding details and we are keen for a contemporary style with no roof overhang around the building, and the cladding running to the top of the walls on all 4 sides. I've added some photos to give an idea of what we're after.

 

 

We will be wrapping the sips panels in tyvek breather membrane and adding vertical and then horizontal battens to form an air gap before fixing the vertical timber cladding.

 

My question is, what is the best way to vent the air gap at the top, considering we will have no overhang?

 

1. Is the EPDM roof covering that I'm using breathable, and will therefore let moisture escape? If so do I just lap it over the cladding at the top and fix my facia board? 

 

2. Is my only option to run a horizontal wooden trim across the top of the cladding at the top to form a pathway for the moist air, as per photo 2?

 

3. Do I need to lap the EPDM over the cladding and then drill vent holes in the cladding near the top (not preferred)

 

4. Anything else I've not considered?

 

I've been searching the web for a definitive answer to this but I'm struggling to find anything. All comments or suggestions (or criticism!) welcome in my search for something I can build that works.

 

Many thanks.

[Dave Jones]

these cold roof abortions rely on not much condensation being created internally in order not to rot out.

[Davieboy84]

So you reckon it would be fine to just wrap the EPDM over the cladding as long as the room was well ventilated when being used? It will have a 2.4m wide bifold that will be opened at all times when in use and another window that will also be kept open for ventilation. We won't be using the room during bad weather.

 

I did consider putting in a small overhang of say 75mm all round the building to allow the air gap behind the cladding to vent. Might go with this to be safe. 

[Alan Ambrose]

This is not what you want to hear, so feel free to ignore

 

>>> no roof overhang around the building

 

Looks cool, but buildings have had overhangs for centuries for a reason - to keep the weather off the walls, so they're not regularly saturated. Similarly with the bottom of the wall, where upsplashes often rot the cladding eventually.

 

>>> Is the EPDM roof covering that I'm using breathable.

 

No, not at all.

 

>>> 3. Do I need to lap the EPDM over the cladding and then drill vent holes in the cladding near the top (not preferred)

 

I would definitely lap the EPDM over, preferably with some kind of overhang and/or kick to keep the water off the cladding.

 

It also looks dead cool without gutters, but...

[Temp]

Is it a SIP roof? If so I don't think it needs any ventilation and no void to ventilate.

 

If not a SIP is it being done under permitted development (roof less than 2.5m) or under planning permission which might allow a bit more - possibly enough for a warm roof construction.

 

If all else fails build this version..

 

 

Run rafters front to back and put vents behind front fasia and a small rear overhang. Only needs about 25mm slot for vents. 

[andyscotland]

Just to be clear, you're asking about ventilation behind the vertical cladding, not about roof ventilation (which I think a couple of the answers have assumed)?

 

The cladding ventilation gap is not really related to the indoor humidity of the space (the SIPS themselves will provide a fairly effective moisture barrier between inside & out, though I also lined mine with a vapour barrier behind the internal lining for belt & braces). It is solely to provide airflow across the back of the cladding boards so they can dry from both sides when they get wet.

 

You don't need a huge amount of airflow - the manufacturer's drawings may well show just having the cavity open at the bottom. I wasn't happy with that, so I just drilled some holes in the top of the cladding just below where the EPDM laps over but above the line of the bottom of the fascia. Then covered with insect mesh, some thin packers, and the fascia. The top ventilation is completely invisible now it's all finished, and even a thin gap at the top should be enough to let natural convection draw the air gently out past the fascia.

[Davieboy84]

On 15/08/2023 at 10:10, Temp said:

Is it a SIP roof? If so I don't think it needs any ventilation and no void to ventilate.

 

If not a SIP is it being done under permitted development (roof less than 2.5m) or under planning permission which might allow a bit more - possibly enough for a warm roof construction.

 

If all else fails build this version..

 

 

Run rafters front to back and put vents behind front fasia and a small rear overhang. Only needs about 25mm slot for vents. 

 

Yes it's a full sip construction; floor, walls and roof. But it's the vertical wall cladding that I'm looking to ventilate.

[Davieboy84]

On 15/08/2023 at 11:38, andyscotland said:

Just to be clear, you're asking about ventilation behind the vertical cladding, not about roof ventilation (which I think a couple of the answers have assumed)?

 

The cladding ventilation gap is not really related to the indoor humidity of the space (the SIPS themselves will provide a fairly effective moisture barrier between inside & out, though I also lined mine with a vapour barrier behind the internal lining for belt & braces). It is solely to provide airflow across the back of the cladding boards so they can dry from both sides when they get wet.

 

You don't need a huge amount of airflow - the manufacturer's drawings may well show just having the cavity open at the bottom. I wasn't happy with that, so I just drilled some holes in the top of the cladding just below where the EPDM laps over but above the line of the bottom of the fascia. Then covered with insect mesh, some thin packers, and the fascia. The top ventilation is completely invisible now it's all finished, and even a thin gap at the top should be enough to let natural convection draw the air gently out past the fascia.

 

Yes that's it, it's purely the vertical cladding that I'm not sure about.

 

Your solution sounds exactly what I should be doing. Do you have any photos you could share so that I can get an idea of what it should look like?

 

Thanks for taking the time to reply 👍 

[andyscotland]

Had a look and unfortunately I don't have any photos, might be able to do a quick sketch tomorrow