Water proof membranes

[Out int shed]

Hello All, I hope someone out there can help me.

Im about to start putting up my new workshed - a 15m2 single pitched outbuilding, made from timber stud work, exterior face in loglap, interior in ply with a 16mm polycarb roof.

The workshop is located in my back yard which consists of a concrete floor but on top of the concrete there is an 1" thick treated deck. The workshop will sit on this.

So, there is a concrete floor, then an air gap of about 1/3 of a foot, then a timber deck onto which I will fix my shed. The fllor of the workshed will float ontop of the deck with about 2" between the decking boards and the bottom of the floor of the shed.

My question is, do I need a damp proof membrance to sit between the top of the deck and the underside of the new floor?

 

[MikeG.]

I'll look at this at lunchtime........
........if my addled brain remembers!

Mike

 

[MikeG.]

OK.........just struggling a bit to understand this one.........

You have an existing concrete slab, of the right sort of size? Is this at, above or below ground level? Critical information!!! You also talk about a timber deck above this.......what is that? Is it part of your new shed, or something that is already there? If it is already there, is it the same size as your proposed shed?

You also have a new shed building. Does this mean you have bought one, or that you propose constructing it yourself? What are the dimensons of the support timbers for the shed floor?

Answers to the above will help a lot.........but so would a photo or two.

Mike

 

[Out int shed]

Hi Mike,

I have an existing concrete slab of the right size (25m2) above ground level.

On top of the slab there are over engineered 3x4's treated joists bolted to the concrete

There is an existing 1" timber deck floating on top of these joists

The timber deck covers 25m2 - same as the concrete

The missus wants to keep some of the decking (approx 10m2) leaving me 15m2 to play with for my shed.

The shed is going to be built by my fair hands and because I have to keep some of the deck - and therefore the joists it sits on - I am going to build the shed straight on top of the deck.

I intend on fixing 2X3's straight onto the deck. The 2X3's will be used to layout the sole plates and shed floor joists (400mm centres)

On top of the sole plates will go 2X3 stud work. On top of the joists will go some 32mm flooring.

I hope thats made it a wee bit clearer - I will sort out piccies if needed.

So, back to the question, do I need a membrane to sit inbetween the decking and the new flooring?

Thanks in advance for your replies - Stuck and Confused!

 

[MikeG.]

Right, that's much clearer now!

I'm going home now...........I'll have a look at this later.

Mike

 

[head clansman]

Hi

IMHO Place a damp proof course under the bolted timber and between the concrete slab and bolt the timber to the slab but don't forget to seal the head of any screws or bolts that go through the timber to stop any rising damp , and make sure all timber is very well treated as well including the ends , once shed is built it wont get done again.

this will mean decking has to come up but it better that than build an expensive shed only to find a couple of years down the line the timber bolted to the concrete has rotted. IMHO I really don't see the need the for a membrane above the decking cant quite see why you would need it , i was always taught a good air flow will keep things dry enough . hc

Ps don't forget to treat the undersides of any floor board as well .

 

[Out int shed]

thanks h clan

That's good advice regarding the treating of the underside of the floor boards, perhaps that will be enough of a damp proof course?

 

[MikeG.]

OIS,

right...lets have a go at this.

My recommendation would be to keep the decking and joists only where you need them for the remnant decking. Remove the decking & joists under the location of your new shed. You would then be starting from slab level with your shed construction rather than on top of the existing timbers. I would then build a brick plinth 3 courses high, (which would take your sole plate comfortably clear of the decking), and then build your shed on top of that. If you wanted to raise your internal floor level to the level of the surrounding deck you could then do 50mm of Kingspan and a floating floor.

I know that isn't what you are planning to do, and doesn't answer your question directly, but it would be a much better job. You are planning to sit your shed on timbers which are directly on top of concrete, and this is something to avoid wherever you can.

It is good news that your concrete stands above the surrounding ground level. Not only does this help it stay dry it also means that if you put a DPM down on top of the concrete you aren't creating a swimming pool.

As for doing it the way you proposed, I can see no good purpose for a piece of plastic. There is rarely much benefit having plastic between 2 pieces of timber, because the sole purpose of plastic is to keep one side dry, protected from moisture on the other side. If the wet side is made of timber is it going to fail anyway.

I urge you to have a rethink!! You won't regret it......its a better job, and your shed will last years longer. There will be no nasty little void purpose made for rats and mice. I bet it won't cost you much more money, either, because the cost of the few bricks you need will be nearly offset by your savings on timber floor joists.

You are lucky to have the most expensive and hard-work part of the build done for you............the slab!

Hope this helps!

Mike

 

[MikeG.]

It's occurred to me that you could even leave the joists where they are and still have a brick plinth.........cutting out a section to allow the brick plinth to cross their line. Remove the decking from the area within the shed, and infil between the joists with insulation before laying your new flooring on the old joists. The downside of this approach would be that you would be unable to lay a DPM.

You could even lay the flooring directly on top of the decking, instead of removing it. The key, though, to doing a decent timber-framed shed is to get it up on a plinth at least 6 inches above anything wet.

Mike

 

[Out int shed]

Thanks for your advice Mike.

i'd not thought about leaving the existing decking joists where they were and building a brick course around them.

That may be the way forward for me.

I shall certainly post some pictures when its all under way. Just gotta wait for the snow to go!

 

[Digit]

What snow? :roll:

Roy.

 

[head clansman]

hi

Out int shed Posted: Thu Feb 12, 2009 7:09 pm Post subject:

--------------------------------------------------------------------------------

thanks h clan

That's good advice regarding the treating of the underside of the floor boards, perhaps that will be enough of a damp proof course?


No not really , you need to make sure what your going to build off is sound with no rot or damp that can penetrate up through the very joist your intending to build off of . as mike says you need to be minimum of six inch above the concrete level mainly because of rain splash in heavy rain, if you treat all timber and have plenty of air flow you will be ok for many many years to come . hc

 

[Out int shed]

Well, I decided to follow your advice chaps - and after a lot of huffing and puffing, I got the deck up and am preparing the flooring for the workshop.

I had thought the decking rested on 2X3 that simply sat on the existing concrete slab - turns out the decking sat on 2X4's that were anchor bolted to the existing concrete floor!

I should have realised, or rather remembered as it was me that laid the decking some 5 years ago!

My back is now bent over double and I have that weird tingling you get all over after serious physical labour - I even have it in my finger tips which doesn't help with the day job as I spend most of it typing!

I will sort out a photo posting account tonight and start to stick up WIP pics for the workshop - hopefully I can get more sound advice as I go.