Air circulation under obs.

[Macavity]

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.

In fairness, it is water-proof chipboard. But I bow to the majority, and retrospection. <G> I still have hopes that flashband, having eradicated any flow of "standing" water, underneath the frame, will render THOSE timbers no more likely to rot than the other parts. I have quite a lot of (incidental!) holes drilled through the floor, and presumably some air-flow(?). I can even monitor the state of (i.e. see) the flags laid in concrete too. I'm less certain I want to allow a regular influx of spores etc. at this stage...

Feel free to depress me, but I suspect the structure will significantly "outlive" me...

[Freff]

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

Digz

Excuse my slowness, I was having a senior moment.

[Freff]

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

As I understand it an airbrick is required per 500²mm floor area. Malcolm's floor area is 10.56.

My original guesstimate was based on the floor area and unprotected ground void.

[malc-c]

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.

Spot on then...

4.8m x 2.2m (15.75' x 7.3' approx void area)

With regards to Alexanders Observatories, and what others have posted, it may be years before any rot sets in, or to the degree where the obsy needs dismantling and the joists or floor replaced. But then again, I'm no expert, and if left untreated, problems could start surfacing withing months of completion ?

I'm sure there are guys here who have a commercial observatory, converted shed or custom self build that is years old, with no inbuilt ventilation, or damp proofing. But for me I didn't want to take the risk and followed Tony's advice. In hindsight it would of been better to of installed the air bricks at the time of building the dwarf wall, but then we all live and learn

[martin_h]

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.

There might be a membrane under the slab....but why put a wooden floor over a nice smooth concrete one??

I would have put a wall plate down...with DPC under and built up from there!

[Macavity]

There might be a membrane under the slab....but why put a wooden floor over a nice smooth concrete one??

I would have put a wall plate down...with DPC under and built up from there!

Both pre-build modders (me!) and "pure" DIY'ers can learn from one another? The challenges are different though. No use complaining that a pre-build doesn't conform precisely to your vision. <G> But, even with the resources of... NASA, CERN etc., mistakes are still made. The solution - multiple meetings, visits to "factories", just ain't feasible from an amateur perspective.

A think a GOOD (obvious?) start is to make the base SLIGHTLY smaller than the walls. <wink> That way, "run-off" continues on downwards. But, precise knowledge of the dimensions are needed. It may be that the "Cardiff guys" will see - And address (any) water pooling. I spent time (more money!) adding bloomin' mini-gutters. Experience suggests these are largely decorative...

[zedds]

I spent time (more money!) adding bloomin' mini-gutters. Experience suggests these are largely decorative...

I was thiking of adding guttering to mine but i realised water will still get to the sides of the slab were the joists meet the base, which is what i want to stop. I will try flashband around the base of the floor. Worth a go.

[martin_h]

Plenty of air space under mine...its built on a slope, the front is 2 feet off the ground!!

[Gina]

Mine's on sloping ground too but not as much as that. So unless I build the ground up mine will also be well ventilated. I plan to leave it like that unless a problem occurs.

[Digz]

Digz

Excuse my slowness, I was having a senior moment.

No worries, it happens to the best of us

As I understand it an airbrick is required per 500²mm floor area. Malcolm's floor area is 10.56.

My original guesstimate was based on the floor area and unprotected ground void.

This is correct providing your air brick has a free area of 500mm². Most air bricks are in excess of this, hence using the figures above, based on area 1 brick is needed, based on length of the longest wall two air bricks are needed (that is based on the timloc air brick I linked to earlier)

I think the key thing to remember is that these rules are usually applied to larger scale projects from houses to office blocks where we are talking of areas ranging from several hundred square meters to several thousand square meters.

Given the general size of observatories I would hazard a guess that if you have a suspended floor that 6 air bricks (3 on each opposing side) would generally be sufficient.

Also, leaving the the sides or end open negates the need for this.

[Freff]

No worries, it happens to the best of us

This is correct providing your air brick has a free area of 500mm². Most air bricks are in excess of this, hence using the figures above, based on area 1 brick is needed, based on length of the longest wall two air bricks are needed (that is based on the timloc air brick I linked to earlier)

I think the key thing to remember is that these rules are usually applied to larger scale projects from houses to office blocks where we are talking of areas ranging from several hundred square meters to several thousand square meters.

Given the general size of observatories I would hazard a guess that if you have a suspended floor that 6 air bricks (3 on each opposing side) would generally be sufficient.

Also, leaving the the sides or end open negates the need for this.

Sir I bow to your profession and conceed to being two bricks out.

As a military engineer (R.E ) and many years in the building trade, I have unwittingly adopted the belt and braces philosophy and occasionally have over-engineered to be safe rather than sorry.

[Digz]

Sir I bow to your profession and conceed to being two bricks out.

As a military engineer (R.E ) and many years in the building trade, I have unwittingly adopted the belt and braces philosophy and occasionally have over-engineered to be safe rather than sorry.

There is certainly nothing wrong with the belt and braces approach, it will be the same approach I will adopt with mine when I get round to building it