Air circulation under obs.

[Gina]

Gina, yes thank you, still got the odd sniffle and sore throat, but otherwise back on my feet

Glad you're feeling a bit better. Hope it all clears up soon

[malc-c]

Well after an hour or so with the club hammer and a bucket full of muck I've fitted 7 air bricks around the base of the observatory.

Granted it isn't pretty (at least not whilst the mortar is still wet), and the terracotta colour is somewhat bright but hopefully with two bricks at either end, two down one side and one down the other, there will be enough of a flow for the air to prevent or at least reduce the risk of rot through dampness.

Tony, thanks for pointing this out. One thing I did notice was that slight musky smell that greeted me when I pushed the bricks through and opened up the void. I'm hoping that I caught it in time, and any moisture that has been trapped will now evaporate and my obsy will remain damp free for years to come

[Gina]

Yes, I think it'll look a lot better when the mortar has dried out. And also I think the bright red plastic will lighten with time as the light works on it, making it a better match with the bricks.

[Freff]

Great job Malcolm, it looks the dogs kahunas.

With no circulating air in the cavity the joists, no matter how well treated would have sucked up the groung moisture in no time.

That smell was the start of rotting grass, bugs etc..

[laser_jock99]

I'm expecting a lot of rain/wind/hail/snow in my location so I'm leaving the bottom completly open. Any rodents are welcome to the shelter- the cats will sort them out if they emerge. My timber frame is kept directly off the concrete by metal fence post brackets or blocks of plastic. Concrete acts like a sponge so avoid direct with timbers. Apply plenty of creosote during construction as you may not be able to treat these areas again once built. The tip my dad gave me was to stand the upright posts in a bucket of creosote for half an hour before fitting. That way they soak up a load of preservative before fitting.

Note the timbers are held above the concrete pad by the black metal fence post bracket.

[malc-c]

Interestingly, I found a thread where a society purchased and had installed a large observatory from Alexanders. The frame they used for the flooring is closed (ie a rectangular frame infilled with joists) and was laid straight onto the concrete slab. They also used chipboard as the flooring which I would assume would be one of the worse choices given the damp conditions it would encouter (both in the construction and use - looking at the rest of the build sequesnce)

This practice seems to negate the information given in this thread regarding air circulation under the base.

[yesyes]

Isn't it different when you have a solid concrete slab? Not too much moisture can come from there...

In your build you have bare soil underneath the floor framing as far as I remember.

[zedds]

Well i have removed the 2x2. I found a nice steel off cut in our workshops at work. So a little work with a pair of tin snips and a couple of coats of hammerite and alls done.

I don`t think there`s much more i can do as the obs is built now. Still looking at keeping water from getting under the side timbers. Maybe try flashband and see how it goes.

Interesting how Alexanders build theirs direct to concrete and no open end.

[Gina]

Maybe damp doesn't come up through concrete as it does through soil but I still think it wants ventilation. AND I think it wants DPC.

I was going to leave my obsy open underneath but OTOH don't want rats taking up residence. Expanded metal mesh would be ideal IMO but I don't have any and not found any at a sensible price. I have some plastic air bricks I can use though.

[Digz]

Like with most things there is a degree of risk involved and that needs to be weighed up against various factors, in this case, such as cost, time, quality, lifespan etc.

In my job as an Architect I am constantly weighing these up when designing and detailing a building. There are a range of standard details which have been tried and tested over the years, such as using DPCs, cavity trays, DPMs etc. I guess it is up to the individual to weigh up their design to see if its needed or not.

As for me, given my experience and knowledge I know what I would do, sure it may seem overkill to some but Ill be safe in the knowledge that my detailling is sufficient for a commercial building therefore my obs will last

[malc-c]

Isn't it different when you have a solid concrete slab? Not too much moisture can come from there...

In your build you have bare soil underneath the floor framing as far as I remember.

True, my build didn't have an infilled concrete slab, so yes there would be a higher risk from the damp air / moisture than a concrete slab. But most commercial sheds make their floor sections such that air can flow from one end to another. Most of the sheds I've had tend to have T&G flooring as well.

Andy, - that meshing looks cool.

I've got 7 airbricks in place at the moment, which hopefully will be enough to ventilate the void (maybe Digz could varify that given his profession?). No doubt once all the grass etc has died off there will be less chance of the floor rotting in years to come.

[Digz]

Im not sure whether it is any different with a concrete slab. A suspensed floor will have a void underneath that isnt air tight. Air will permeate in through cracks and holes anywhere along the structure and might even get through the flooring itself. This can then, obviously carry mositure so you will need an effective way to get it out.

On a recent job I had a suspended floor and void below which was sealed with oversite concrete to prevent anything from growing up into the void below, it still needed ventilating.

Malc, the key is to have them on opposite sides to promote a cross flow of air. The air bricks should provided either 1500mm² per meter run of wall or 500mm² per square meter of floor air (according to Part C of the building regulations approved documents: Planning Portal - Approved Documents - Downloads), whichever gives the greater result. You should be able to calculate your floor area and multiple by 500, the air bricks will then have a rate associated with them and as long as you are over this figure your will be fine. I would hazard a guess and say that 7 air bricks would be sufficient.

[Freff]

A concrete slap for building on should have a membrane. Normally (depending on the building) the hardcore is laid in first, then a finer ballast to fill in large gaps with finally a thin bed of sand. The membrane is then laid over the sand (the sand prevents sharp stones puncturing holes in the membrane) then the concrete poured on top of the membrane.

The membrane will prevent any rising damp through the concrete.

If Alexanders turned up at my site to lay a obsy in the manner shown above, I'd be very concerned. They may have laid a membrane in the image above, there's no way to tell, but the timbers will still be vulnerable to standing water.

The chipboard is a complete no-no.

[noakesy]

get a wood drill and put about 6 holes each side more than enough

[Freff]

Im not sure whether it is any different with a concrete slab. A suspensed floor will have a void underneath that isnt air tight. Air will permeate in through cracks and holes anywhere along the structure and might even get through the flooring itself. This can then, obviously carry mositure so you will need an effective way to get it out.

On a recent job I had a suspended floor and void below which was sealed with oversite concrete to prevent anything from growing up into the void below, it still needed ventilating.

Malc, the key is to have them on opposite sides to promote a cross flow of air. The air bricks should provided either 1500mm² per meter run of wall or 500mm² per square meter of floor air (according to Part C of the building regulations approved documents: Planning Portal - Approved Documents - Downloads). You should be able to calculate your floor area and multiple by 500, the air bricks will then have a rate associated with them and as long as you are over this figure your will be fine. I would hazard a guess and say that 7 air bricks would be sufficient.

Good solid info.

[rwg]

Maybe damp doesn't come up through concrete as it does through soil but I still think it wants ventilation. AND I think it wants DPC.

I was going to leave my obsy open underneath but OTOH don't want rats taking up residence. Expanded metal mesh would be ideal IMO but I don't have any and not found any at a sensible price. I have some plastic air bricks I can use though.

My understanding was that if there is plenty of space (enough for a cat to get in and out) under a shed etc, then the rats will usually stay away - they're not dumb and they'll try to find somewhere safer.

Robin

[Gina]

You should be able to calculate your floor area and multiple by 500, the air bricks will then have a rate associated with them and as long as you are over this figure your will be fine. I would hazard a guess and say that 7 air bricks would be sufficient.

Something odd here... I've just calculated my floor area as just over 10 sq metre, multiplying by 500 gives 5,000. The plastic air bricks are 225mm x 75mm giving an area of 16875 sq mm. That would mean that one air brick should be sufficient (even if the hole area is only 33% of the external size). Say one at each end. I don't think Malcolm's floor is much bigger.

[Digz]

Gina check out this website:

Air brick

Your calculation isnt quite true and it is always better to rely on manufacturers information. In the example linked above their timloc air brick has a free area of 6170mm² per unit therefore you should get away with one brick at each end - remember two opposed external walls should have vents in. I should have mentioned above that you need to calculate both methods and use the one which gives the greater area - Ill edit my post.

Tonym your welcome (and no pun intended eh? :))

[Gina]

Gina check out this website:

Air brick

Your calculation isnt quite true and it is always better to rely on manufacturers information. In the example linked above their timloc air brick has a free area of 6170mm² per unit therefore you should get away with one brick at each end - remember two opposed external walls should have vents in.

Yes, to give a through flow of air.

I should have mentioned above that you need to calculate both methods and use the one which gives the greater area - Ill edit my post

Thank you. Yes, taking the length of wall, it works out at many more air bricks.

[Gina]

My understanding was that if there is plenty of space (enough for a cat to get in and out) under a shed etc, then the rats will usually stay away - they're not dumb and they'll try to find somewhere safer.

Robin

Thanks for that Robin Maybe I'll leave the underfloor open. Save a lot of bother

[Freff]

Tonym your welcome (and no pun intended eh? )

eh!!

[malc-c]

Interesting thread this has turned out to be...

The floor area of my observatory is 10.56 sq m, so I guess I could of got away with a brick at either end - 7 is a bit of an overkill

Checking the build thread for the Cardiff AS observatory there was no sign of a membrane placed before or after the hardcore when constructing the base.

[Freff]

Interesting thread this has turned out to be...

The floor area of my observatory is 10.56 sq m, so I guess I could of got away with a brick at either end - 7 is a bit of an overkill

Checking the build thread for the Cardiff AS observatory there was no sign of a membrane placed before or after the hardcore when constructing the base.

Hi Malcolm

Sorry but I thought your obsy was 16' x 8' when I calculated for the air bricks which would have been 8x bricks.

At 10.56 sq m / 1.5m = 7.04 bricks.

[Digz]

eh!!

Guess not. Solid info - solid bases vs suspended floors? Been a slow day I'll get me coat lol

[Digz]

Hi Malcolm

Sorry but I thought your obsy was 16' x 8' when I calculated for the air bricks which would have been 8x bricks.

At 10.56 sq m / 1.5m = 7.04 bricks.

Talk me through the math there so I can understand where the figures come from.