Subfloor venting

[r896neo]

Hi all, havent been around for ages for all the usual reasons, yawn.

Anyway my Mother in law has bought a bungalow and wants the paving done. They want a level garden and have ramp and level access at the front door. At the rear a small extenstion will be built with a stepped tray dpc etc to allow for level access through sliding doors.

However she wants to raise the whole level around the rear of the house to remove the gradual 8'' slope and also get the garden falling away from the building as the settlement has caused the original 3x2 path to drop maybe 3'' and water which used to fall onto the lawn pools along the path in many areas. The dpc clearance is fine the only issue is the sub floor vents.

The house is however not timber floor but concrete block and beam/ possibly precast slabs and i assume this is because of poor ground and it was probably piled (piling of houses across the road being built currently)

So my query is because the sub floor needs vented but not as well as a timber subfloor could i create a small 4-6''trench all along the back wall to keep the subfloor vents clear and cover it with a grill?

A bit like a tiny lightwell to a basement. Anyone ever done this approach? Comments of wise up, stupid idea etc welcome.

[seanandruby]

have you read this

[KAMIKAZEE DIY]

Hi guys,

Clicked on Sean's "click here link", and its a page I've already read a few times before.

Apologies if this sounds like a stupid question but when I first read page and still when I read it today I can't understand why splash-back from rain is a hazard any different to normal rain fall?

Am I misreading something? Stupid question? Any explanations?

Curious to understand it better.

Cheers

[r896neo]

Hi Sean, yes I read that page thanks.

My query is more relating to the air flow and vent aspect? How much venting does a block and beam floor need? Will creating a 4" trench provide enough?

Has anyone done this and detailed it neatly?

[seanandruby]

A trench doesn't vent the subfloor, air vents do that. It should already have the telescopic vents installed. It's usually 3 or 4 vents depending on house size. If as you say you have 150 below dpc then i can't see what the problem is ??? If however it is level threshold, then a small edging arrangement surrounding the vent should do the trick.

[lutonlagerlout]

we come across this often with flush patios and ramps
the way we address it is with a 215mm concrete gully surround,around each airbrick
ventilation is important
cheers LLL

[KAMIKAZEE DIY]

Any takers on the DPC and splash back effect from rainfall? I don't doubt main sites references to it, it's just id like to understand it.

Knowing how to do something is great, but sometimes knowing why you do it is just as important. Sourced a lot of info from you guys that I appreciate, so hope I don't sound patronising.

Cheers

[seanandruby]

I s'pose any rain on a wall is not ideal.
Maybe with the splash course your getting double bubble...rain coming down the brickwork and also splashing upwards from the flags etc: it could be a throw back to the early days when hardly any dpc was placed. With "modern" brickwork it is usual to have class f2 bricks below damp ( i think, LLL will correct me if i'm wrong
) :;):
I wouldn't lose any sleep over it Kam,' just go with building regs part c 5.5 and you can't go wrong, as long as you know your in compliance. There seems to be a trend on sites to put in level threshold and a drainage ditch 150ish wide on disabled access side, all along that wall and any softscape areas breach the rule. Air vents get raised up. I don't agree with it myself.

[lutonlagerlout]

heavy rain can bounce up to 150mm that is why the dpc should be 150mm above paved areas
there are workarounds but none easy or cheap
LLL

[Tony McC]

I can't understand why splash-back from rain is a hazard any different to normal rain fall?

The short answer is that it isn't, but, for some reason lost in the mists of time, it was considered a threat at the time the dpc level was set at (as it was back then) 6" above ground level.

The primary reason, as I've always understood it, for the 150mm 'upstand' is to provide bwk an opportunity to lose moisture through natural evaporation before that moisture has a chance to reach the dpc, which is the final 'barrier' to damp rising through the masonry to levels wehere it might harm the building structure.

[KAMIKAZEE DIY]

Thanks for that tony.

It had always puzzled me.

Your a walking, talking encyclopedia.

Cheers

[seanandruby]

Your welcome :;):