sub floor ventilation and exterior sub floor cover up

[Paul Mac]

I have a timber home with colour bond corrugated iron cladding.
I need to find some vents about 230mm x 75mm for sub floor ventilation that will look as good as possible on corrugated iron.
I have also had my house restumped and the subfloor and perimeter stumps are exposed to the elements.(about 300mm-400mm)
I need to cover the exposed sides with something thin and waterproof knowing that fill is going to but up to them and eventually pave the exterior up to the house floor level.
I have been told hardie plank is a possible solution but is there anything else that may be better.

[journeyman Mick]

Paul,
in a word DON'T! If you bring the outside ground level up to your floor level you will:
a) cause extreme drainage problems as the ground under your house will be lower than that around it.
b) send an open invitation to every termite for a mile around to come and eat your house. If the ground level is the same as your floor level it will be virtually impossible to maintain a physical termite barrier meaning you'll be getting it sprayed with toxic chemicals on a regular basis.
Normal practice for low set timber construction is either leave it as is, or if you don't like the look of it some form of lattice or palings. Don't worry about your stumps getting weather, they should be treated hardwood if they're milled timber, or a category 1 durable timber if in the round. If you really want/have to bring the soil level up you will need to lay a brick or block perimeter wall under your house, waterproof it and provide plenty of vents. You will need to keep ground level 100mm below the top of the wall and it will need to have a dampcourse above ground level. You will also need to provide a termite barrier. There's also the question of how far above ground level your vents need to be. (I have absolutely no idea there as I never deal with subfloor vents)
Forget about hardiplank for building a retaining wall ('cause that's what you're doing, building a retaining wall under your house) it will bend and buckle under the weight, also the supporting stud wall will be sitting on the ground, great for termites and rot.

Mick

[Paul Mac]

Thanks very much Mick for your reply... really appreciate your advice.
As the surrounding landscape is just below house floor level and above sub floor level I feel I will have to go for option B paving to just below house floor level (allowing for vents) so I can have sufficient fall away from the house.
So I guess I will have to build a block perimeter.
I was thinking of maybe laying it along side the stumps/wall rather than under it having a small gap between the retainig wall and the stumps allowing air to flow between them.
The row of pavers alongside the house could then be laid on something like a small slither of hardi plank (on top of the retaining wall) to stop paving sand falling through the gap.
I have found a good venting system (galvo. flexible. that will flush up aginst corrugations in iron and sit just below house floor level) and along with my research found a formula that calculates air flow requirements etc. so I will have plenty of air movement under the floor.
Is this whole idea at all feasable?
Would the dampcourse you mentioned be heavy plastic layed along the house side of the retaing wall and under the retaing wall?
Thanks again Mick, hope to hear from you soon.
Cheers

[journeyman Mick]

Paul,
just trying to picture your situation. Is the finished ground level up around floor height all around the house? Is the ground level original or has it been brought up by your (or your neighbour's) landscaping? If water gets under the house is there anywhere it can run to? If the ground level under your house is lower than the surrounds can you run drains to a lower point? (ideally out to the street). What are the house stumps made out of? The more information you can give the better that I and others can try to help with a solution.

Mick

[Paul Mac]

G'day Mick
The surrounding finished ground level (lawns/landscaping etc) is below house floor level and just above the ground level under the house. I am fairly sure the ground level under the house is just below street level. These levels all came with the house as it is over 100 years old. In fact I think it was the first house in the area.
The stumps are made of Jarrah and have been pre-soaked in creosote (I think thats how you spell it) and have all been fitted with ant caps.
I am situated in South Australia where rainfall is fairly low and as the house is well sheltered you would almost never get flowing water under the house to warrant drains. The house is also located near the coast and not very far down is sand hence very good drainage.
So I hope this extra info helps... and thanks again for your speedy reply
Cheers
Avagoodweekend

[journeyman Mick]

Paul,
I think in your situation the best bet is going to be a retaining wall that's spaced from your perimeter stumps by at least 50 - 100 mm. If you build it hard against your stumps it may cause problems with termites and fungal decay. If it is spaced off your stumps you won't need a dampcourse and if, as you say you have very little rainfall you probably don't need to waterproof the wall either. However as a cheap form of insurance it may pay to give it 2 coats of silasec (on the soil side) before you back fill. Silasec is mixed with water and cement to form a slurry which you paint on with a broom. I'm still concerned with your drainage but I guess that's because I live in a very high rainfall area. You should leave some way to access under the house for possible plumbing problems and to access for termite inspections. To make termite activity easier to spot you might want to paint a 50mm white band around every stump. That way you can shine a torch under there and instantly spot any termite galleries that have been built. If you keep any garden beds clear of the house it should prevent access from ground level into the house. Everything (garden beds, plants, tools, odds and ends should be kept a minimum of 1 metre away from the house as anything bridging from the ground to the house can become a termite entry point. (I've seen them cause thou$and$ worth of damage because the homeowner had left some tomato stakes leaning against the house). If you want to get the gap between the garden and the house smaller I guess you could cap the retaining wall with large pavers but you will need to keep a bit of a gap and you will need to be vigilant in your termite inspections. To prevent the underfloor area becoming a swamp you need to ensure your gutters are kept clear (to prevent overflow) and that your downpipes discharge well away from the house. I still can't picture having the ground level under a floor lower than its surrounds and my first reaction when I read your last post was that you should lift your house and restump it. However as you've just restumped it's not likely to be an option. Best of luck

Mick

[bitingmidge]

Paul,

I agree entirely with all Mick has said.

I have one further concern however: If you do as described around the entire perimeter of the house, you will have no access in future to enable you to remove dead cats, or rectify any other problem which may arise. Try to leave at least a crawl access for the future.

I know you live in the dryest state in the dryest continent on earth, but I still don't like the thought of an undrained subfloor.

cheers,

P

[Paul Mac]

Thanks so much for your time and information, it has greatly increased my confidence in going ahead with the job and complete it knowing all was thought of before the job and not after when it is too late.
It was amazing to find that after enquiring with several builders here in SA that only one came up with a tangeable solution along the same lines as you Mick.
Just to add... all stormwater runoffs are being set by a laser to give maximum fall and they all will be feed to the street (which is legal in SA). There are manholes to the subfloor from inside to help with inspections and as there is no plumbing under the area of interest, the only way water can get under the house is if there is a huge flood in the area.
(being on sand will make for good drainage aswell.)

So here I go - Head down ass up

Thanks again for you time I really appreciate it.

Cheers