Celotex type insulation.

[Dave]

Two questions or thoughts on this topic which I started.

I have a bungalow and wiuld not be too worried about roof climbing and don't need extra long ladders. Would it not be at least a help if I put ventilation in the two gable ends at oppodite sides of the ridge as I will be leaving a couple of feet or so between the ridge board and the insulation board which of course leaves a good coupling space between the eaves. To my mind ventilation only happens with air flow and opposite side of a roof will have different air pressure during any breeze or wind which should drive some air through even without ridge ventilation. I know that warm moist air will rise due to convection and can condense depending on dew point, but I think any breezy day would clear stagnant air. How often do we get a totally calm set of days in the UK?? I sometimes think building regs are a bit over the top and are dreamed up for job creation.

Notwithstanding because my rof timbers are at risk, does anyone know of a semi-round ridge tile replacement with ventilation built in. I'm sure I've seen rooves with the odd ventilator on the ridge and not a continuous strip.

Dave

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[The Natural Philosopher]

YOu are perfdectly correct. What causes ROT is water that persists for long enough to get spores established. That's not a soaking from a shower or a week of winter condensation (dew). Its months of damp that can't escape.

Almost ANY vent will do.

[John Rumm]

Yup if you have coupled all the rafter gaps to a big space at the top, then you can just ventilate the big space and do pretty much the same job.

When I did my loft, we just took off the exiting ridge tiles and then bedded them back on with some cavity wall weep vents installed under them at a downwards slant... investigation with a smoke match showed they worked nicely.

These kind of things:

[John Rumm]

My lawn had loads of wet dew on it the other day... that's quite well ventilated!

[John Rumm]

Yup... tis why you need ventilation for a traditional cold deck flat roof. Even if you have an adequate vapour barrier to block the escape of warm moist air into the roof void, you will still get some condensation in the void space.

Modern warm deck construction (i.e. with the insulation "outside" the roof timbers) does away with the problem generally.

BS 5250 "Control of condensation in buildings" is worth a read for chapter and verse...

[The Natural Philosopher]

But does it still?

[Tim Watts]

Clever - and much more subtle than tile vents.