Through Ventilation Cross Ventilation Explained

Link to other videos using the test rig above

Flat Roofs - Through Ventilation / Cross Ventilation questions I'm always asked
Through ventilation in a cold flat roof is essential. Without through ventilation, the air in a vented space will not move at the far end. Through ventilation means a vented space with vents on both ends of the channel, and the vented space has to be clear for the air to move though it. You cannot have restrictions. We like to use cross ventilation if possible; basically cross ventilation is through ventilation with a matrix of timbers over the top, to form a venting system that vents left and right as well as front to back. If you have eaves vents, soffit vents or even mushroom vents over a cross vented roof, you will have very good cross ventilation and any way the wind blows you will get movement of air in the vented cavity of your flat roof. Unvented flat roofs have all sorts of problems with moisture buildup; not only do you have to have a good Air Vapour Control Layer (AVCL) at ceiling level, but you also have to have good through and cross ventilation. The size of the vents you use in a flat roof really depends on the size of the roof you need to vent. Generally the more venting you can place on a roof, the better. However, you must make sure the holes to the vents are around 4mm in size to stop birds and small insects from entering the vented cavity. Adding through ventilation to a cold flat roof when you have a skylight is extremely difficult; this is when we like to use cross ventilation on a vented cold roof. The fact that it's crossed means that you will not get any spaces between the joists that have no ventilation.
When doing a loft conversion and a vented cold flat roof to the dormer, and you have a ridge with tiles, today we can fit a dry ridge roof system to vent under the tiles; this enables ventilation not only from the vented cold roof to the vented ridge, but also to vent the pitched roof at the front of the building. You must be careful not to put the vents of the cold roof too low, as water can be blown back into the vented area. Often we see the wrong vented ridges being fitted to the ridge; this means that the venting to the cold flat roof is not through venting, and then you may have problems.

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Steven Dickinson
London flat roofing
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[karlbadham]

Thanks for all these very informative videos.
Steve kindly helped set my mind at rest by answering a few technical questions that arose from awareness of potential problems based on researching this excellent information sharing. Outstanding work Steve.

[steveturnbull]

great information, I don't even work in the building trade but I find these videos fascinating and it almost makes me wish I was a roofer :) Steve helped me with an issue with a cold roof a builder did for me where it was condensing badly and I had to tell the builder myself what needed to be done after I'd watched some of these videos. Fast forward almost a year and my cold roof is now completely condensation free. Originally I had no ventilation at all, then I only had ventilation at the front, which still didn't fix the issue. Now I have through ventilation just using round soffit vents at each end section and its made a massive difference. All the timber is now dry on the deck above the insulation where previously it was sweating and mould forming. Keep up the great work with these fantastic videos.

[proactiveflatroofing]

I have to say Steve, I have been watching your videos for years and love your enthusiasm and practical fact based explanation of the "right" way to put a flat roof together, I would love to have a coffee with your sometime.

[benchippy]

No wonder so many get this detail wrong, there’s not much information about. Thanks Steve, never really contemplated just how ineffective ventilation is without through flow

[wotnoturbo]

Thanks for taking the time & trouble to explain. 👍🏽

[MrAlexshellard]

Just totally changed how I planned on insulating my garage roof, thanks dude!

[HarryASMR]

Using smoke to demonstrate your point? You’re a mad lad 😜 but also dedicated! Thanks for yet another super clear, informative video Steve!

[northeastcorals]

Great video, I had seen cross battening before (probably one of your vids) but pretty much forgotten about it. Glad you reminded me of it's benefits as I need to build a little flat at some point roof this year.

[justinstevenson]

Very good stuff. I always use a steel/aluminum head flashing from the sloped roof to the flat roof with perforations in the metal to ac lol posh two things.

First is your using the head flashing to provide waterproofing to the sloped roof, also then having the perforations (facing downward on a crated lip to avoid leakage) to have airflow coming in.

[paullovesey]

Genuine question. If you can’t put a hole in the vapour barrier, how would you fix it?

[wac]

Hi. Have you done a video where the cross ventilation on the flat roof (garage conversion) is running into a pitched roof of the house. Or a video where there is no space for soffit due to boundary line - vents would have to go under in the facia boards... beneath the gutter level, if at all possible.

[joefanningartist]

What happens when you screw the drywall it's going to make a whole in the vapour barrier?

[miker]

What are those two white (fascia) panels called? The one you're holding at 1:10 with the small lip and the one with holes for ventilation.
I just can't seem to find these panels available anywhere.

[BilbosBrother]

How does the cross battening work with furrings on a flat roof with a sky light?

[andrewthomson]

Hi Steve, I have cut through my membrane and installation to fit spotlights. Now I have sweating on the underside of my roof. As an option, I thought I could install surface mounted ceiling lights and fill the downlight voids in with installation, Sliver tape, and plaster board.
Do you think this would solve the sweating issues?
Kind Regards, Andrew

[andreicotorobai]

thanks for video, how to do if joists are fixed on wall hangers inside the building? is a cold roof, bcz of building hight ..thanks

[coreydwood]

if I cant get that thick insulation board u have above plaster, , can i use styrofoam.. for my 10 by 14 ft extention

[muso]

Any thoughts on plasterboard for the ceiling? I was planning on using 12.5mm tapered edge but is insulated plasterboard useful in a ceiling (flat warm roof) ?

[angelkj]

Thanks for your videos, your real world examples are very interesting.
From what I know, air moves only due to three reasons: stack effect (i.e height difference), wind, mechanical (fan). On flat roofs we don’t have any stack effect, we don’t install fans, and wind is unpredictable and maybe inexistent in some circumstances, so almost no air will flow through.
The experiment is explaining the theory nicely but I think it’s not indicative of reality. I’ve seen in some countries building regs venting being only suggested above a certain pitch. Curious to hear your opinion on this. Cheers

[adamkingdon]

Skylight on a roof with all the insulation installed properly with adequate vapour barriers and a vent on all four sides does this pose a security risk?