Air circulation under obs.

[zedds]

Hello all.

I may be answering my own question here but i just want to see it in writing.

I have a ROR observatory created from a modified bespoke 10ftx7ft apex shed. HERE is the build thread.

Have i created a issue with life of the floor of the obs by blocking the open end of the floor with 2x2?

1st pic shows how the floor came delivered.

2nd pic shows how it is now with the 2x2 in place.

I suspect that it will cause a problem long term by keeping any moisture under the obs.

I am thinking of removing the 2x2 tomorrow but any advice would be appreciated.

Thanks

[malc-c]

You've seen Tony's comments regarding the lack of air bricks in my build

To be honest I don't know, as your "shed" is sitting straight onto a concrete base, so damp could come straight up as there isn't any DPC barrier between the wood and the base. For me I have around 3 and a half cubic meters of air between the ground and floor, which at the moment can't be vented, so it will get damp and lead to rot. You only have one end capped off so it may not be as acute

[Freff]

Yes I would. Remove the 2" x 2" and either leave open or put some 1/4" sq rigid mesh. This will stop rodents nesting and moggie getting trapped.

[laser_jock99]

I think it would be better with some free air circulating. Don't want to trap any moisture under there or the timbers might decay.

[Freff]

I just went back over your build. As Malcolm noted there is no DPC between the concrete and timbers. Unless the timber has been been specially treated, damp wil become a problem. Tanalised alone wont do it.

[Helen]

I'm having to rebuild my Obs because of a problem with a rotting floor, so this is a very timely thread!! I think in my eagnerness to stop rodents getting in (I did have a small problem) I blocked off the air circulation and also created a way for water to get into the side of the floor.

I've got a patio slab. So should I put damp proof membrane down on the slab to protect the floor supports? or if I just leave the ends open (with mesh) and build on the floor supports will that be better?

Thanks

Helen

[zedds]

The base of the obs had 2 coats of oil based timber preservative before i laid down the floor.

Looking back i should have put some sort of barrier between the concrete and the wood.

[malc-c]

Years ago we lived in a mobile home, and whilst these didn't sit directly on a concrete base, the floor had a dpc between the floor material (chipboard ! - and joists).

On my build I laid a DPC on the top of the 2nd course of bricks, but didn't put anything between the ground and joist's or flooring. Whilst I too covered the joists in preservative, and used exterior grade ply flooring, having damp air between them and the ground isn't good.

Wayne used a DPC membrane on his build http://stargazerslounge.com/diy-observatories/103639-wetherview-diy-obsy-build-begins-2.html#post1449682 but placed it under the floor on top of the joists, (as per our mobile home) rather than under the framework, only time will tell if that proves ideal.

[Macavity]

I sealed around all the edges with "Flashband" - To stop the pooled rainwater on the oversize (FLAT!) paved base, traveling underneath the shed timbers. I do propose to put in a few ventilation holes above the "water line" at some stage though.

[Freff]

I have never liked the idea of laying timber on concrete slab only separated by DPC. There is a tendency for water to pool.

Best if you can raise up on a row of at least one bricks high, in effect a dwarf wall. Water can't pool on top of a brick or similar.

If I remember Wayne's build the membrane used would act as a moisture barrier only.

[Gina]

I'll have to do something about the gap between the floor and the ground in my build. At present it's open so I need to do something to stop rodents, cats and other critters getting underneath. I already have air bricks, so the ventilation can easily be catered for. I'd forgotten that when I said I'd finished concreting! Looks like there's some more to do.

[George]

Mine ROR shed sits on 4 5" steel h section steel which I rescued from a skip, you should make sure there is air circulation under wooden structures otherwise stagnent damp will rot wood over time.

[focaldepth]

I've been pondering this one since mine went up. Mine is like the first picture. Shed on paving slabs just above soil line. This is exactly how the shed manufacture recommends.

I considered DPC but decided it would only make matters worse. We have a lot of mice etc. Funny I never thought of mesh. I suppose the paving could have been laid on DPC but that is not a normal way of doing it.

I've decided it will be some mesh for me. Oh goody bleeding fingers to look forward to from cutting mesh. Wonder if it comes in narrow rolls.

[focaldepth]

Good build by the way, don't forget to pass it on to Helen to add to the list(s).

[Freff]

Wayne used a DPC membrane on his build http://stargazerslounge.com/diy-observatories/103639-wetherview-diy-obsy-build-begins-2.html#post1449682 but placed it under the floor on top of the joists, (as per our mobile home) rather than under the framework, only time will tell if that proves ideal.

I have just had a look at Wayne's build. And although he has done a fantastic job, I'd have put a DPC membrane on top of the sand to stop raising damp. The sand and concrete blocks are forever going to be sucking up moisture from the ground.

The first timbers look well treated and probably have DPC beneath as well as on top. This is good but again these are vulnerable to standing water, especially if the concrete blocks sag slightly in the centre. They wont be immune from rot.

[Freff]

This is what I mean. This is minimum protection I'd use.

[zedds]

Thanks everyone. All useful comments.I`ll be replacing the 2x2 with mesh. Good idea Tony.

[focaldepth]

Found this.

EXPAMET EXMET GALVINISED STEEL MESH 115MM X 20 METRES | eBay UK

Probably cheaper at a builders yard but Buy It Now is so easy.

[Vic&Al]

I had never even given this topic any thought! As the shed firm set up the shed I assumed there would be no problems. Mine is straight onto paving slabs with no ventilation!

Looks like I will have to drill a few holes! also I think I should re-treat the timber after drilling as non treated wood would then be exposed.

Thanks for the post:icon_salut: could end up being an expensive fix if not addressed

Al

[Digz]

A suspended floor of any construction should have the void ventilated as mentioned above to remove moisture.

There are various ways of achieving this and it will be important to stop vermin etc entering the the void.

If you can sit any base on a DPC then this will prolong the longevity.

[zedds]

I sealed around all the edges with "Flashband" - To stop the pooled rainwater on the oversize (FLAT!) paved base, traveling underneath the shed timbers.

I`m keen on the idea of keeping water from entering under the timbers but i don`t think i have enough base to attach the flashband to.

I`m now thinking of using some sort of sealant paint around the three sides. Like chlorinated rubber or something similar.

[malc-c]

I've been contemplating how I will replace half a dozen bricks with air bricks, given my very newly found bricklaying skills... Its not so much the removal that is the problem, it's cementing the airbricks back in neatly, and without weakening the structure, and keeping the DPC in place. I'm contemplating simply drilling lots of holes in a brick and then fitting a vent over the top

Something like the above. These have the added bonus of an integral screen that will keep out wasp, etc that may like to nest there.

[Gina]

Good point about wasps - plastic air bricks will not keep wasps out.

BTW. Are you feeling better now Malcolm?

[Freff]

I've been contemplating how I will replace half a dozen bricks with air bricks, given my very newly found bricklaying skills... Its not so much the removal that is the problem, it's cementing the airbricks back in neatly, and without weakening the structure, and keeping the DPC in place. I'm contemplating simply drilling lots of holes in a brick and then fitting a vent over the top

Something like the above. These have the added bonus of an integral screen that will keep out wasp, etc that may like to nest there.

Malcolm if I was you I'd stick to your plan. You wont weaken your structure in the slightest.

Remove a brick about 3 courses down from the wall plate DPC. Rebed the hole with mortar (top of brick) then try and get some mortar on the sides of the airbrick. Don't worry if it falls off you can point it in.

Remember the mortar is only to hold it in place so it only needs about a 1" all round the front. It might prove a bit awkward but it is worth it. You have done a great job on your brickwork, :icon_salut:don't spoil it with plastic vents.

Don't worry about wasps, if you do you'll end up filling every tiny crevice around your home. If they start sniffing around the vents spray the vent cavity with a proprietary pest controller.

[malc-c]

OK Tony you convinced me - airbricks it is then

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