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Building my new garage workshop


steveweblin
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I am building a timber frame 5 x 7.2m workshop.

I have recently taken the shuttering off the base of my new garage workshop.  The slab is cast over a Damp Proof Membrane (DPM).  Unfortunately concrete has got past the DPM at the side. I will be putting a Damp Proof Course (DPC) at the edge of the slab under the floor plate.

What if anything do I need to do the side of the slab?

SlabEdge16-08-2020 image02 800.jpg

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Steve, DPM in houses was not not introduced in the 70/80's (my house slab has no DPM) and garages often have none. So don't worry unless it is swampland.

Better than using dpc is to put a course or 2 of bricks down, but your slab looks to be above ground anyway. 

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The issue at the sides is not the ground water rising up, but the rain water seeping in. If you are building with timber frame then you need the timber to be kept off the ground - i.e. sat on the slab - so that it doesn't rot. You then need the waterproof cladding (even if it's only treated timber like a shed) to overhang the slab so that water running down the walls doesn't end up on top of the slab. The latter was the mistake made on the old pre-fab garage that I knocked down here, and it resulted in a river running across the garage floor whenever it rained!

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1 hour ago, NonMember said:

The issue at the sides is not the ground water rising up, but the rain water seeping in. If you are building with timber frame then you need the timber to be kept off the ground - i.e. sat on the slab - so that it doesn't rot. You then need the waterproof cladding (even if it's only treated timber like a shed) to overhang the slab so that water running down the walls doesn't end up on top of the slab. The latter was the mistake made on the old pre-fab garage that I knocked down here, and it resulted in a river running across the garage floor whenever it rained!

+1 on that, and also Clive's suggestion to put down two, maybe three rows of bricks on their own damp course. You can then attach the timber frame to the bricks so that it overhangs all the way round as Rob says, so that rain will drip off the outside and not be inclined to find a way in, and the timber is also up off the ground so damp will not creep upwards. 

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Don't forget about air flow through the structure as well. So many people spend all their time focused on preventing water getting in without considering the need to promote the movement of air inside. If you're putting a pitched roof on it then consider vented soffits and a couple of roof vents, otherwise you'll end up with mold and subsequently rot. 

As far as the brick courses are concerned, if you use standard porous brick then at least lay some sill gasket membrane between them and the framing. Blue bricks would be the best option.

Ian 

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I have a large pile of brick pavers that I could use to lift the timber off the slab.  I would put a DPC under the timber framing on top of the pavers.

I plan to have a pitched roof covered with plastic coated steel sheets.  Vented soffits yes or possibly no actual wood soffit just netting to keep the wildlife out. Planning to have the roof overhang the eaves at each end.  As somebody said to me "put a big hat on your shed to keep the rain off the walls"

The walls cladding is still to be finalised as it seems hard to get tongue and grooved cladding at present. May end up doing the cladding with steel.  Whatever I use will be fitted to overhang the slab edge but stop short of the earth.

 

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If you are using steel cladding (walls and or roof) be prepared for indoor rain when the temperature changes. The condensation is a complete nightmare.  https://www.ebay.co.uk/itm/10M-Double-Aluminium-Bubble-Foil-Insulation-Loft-Caravan-Shed-Home-Thermal-Foil/353079982780?hash=item523534fabc:g:zlUAAOSwnMhewjgg you will need stuff like this to stop it. No amount of ventilation will stop it.

Tony.

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Anti-condensation roofing is a must; on my last shed I had no windows but two rooflights and the water dripped from these in surprising amounts, but nowhere else, as the rest of the roof was anti-condensation sheeting.

It was metal-framed and bolted straight to the concrete floor, and the bottom metal box section rusted amazingly quickly. Even in the block-built garage next to it, there were days when everything, even the tools, were soaking wet with condensation, and that, as Tony says, was a well-ventilated garage.

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I'm also not a fan of the corrugated steel roof stuff. My garage has nice solid breeze-block walls with foam insulation and a full house-spec tiled roof. For a lower cost option, I'd opt for wood and asphalt, like a shed. Wood is a lot less prone to condensation forming than steel.

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My last garage had plain coated steel roof and did have condensation.

This time I may be using insulated sheets.  Or fit a membrane under the sheets + board under the roof timbers with ply.

The walls will also include wrap under the cladding and ply on the inside.  Wall outer face was going to be timber cladding but that seems very hard to source at the moment hence may be using steel sheets.

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I grew about metal sheeting roofs, my first garage concrete sectional with galvanised sheet had condensation dripping from the roof. I had to put a liner of plastic under the roof to protect my Spitfire.

May be better to sheet it with wood and felt, or even slate tiles. 2nd hand ones my may be cheap and more permanent than felt.

Graham

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My sheds were reroofed ten years ago with OSB under bitumen roofing felt. The OSB absorbed the damp and rotted from the inside up. Now got galv steel roofing sheets and... yes.. condensation in the winter, just measured the metal temperature during the nice weather, 49 degrees C.

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2 hours ago, NonMember said:

I have gigabit Ethernet to the garage and a WiFi hotspot in there. I have the same to the shed with the model railway in the garden. But then I'm a software engineer by day...

Now your talking, any pic's of railway Rob?

Tony.

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not a garage  but i built my  6 x 3 mtr workshop from odds and sods  , the walls are  timber  25mm   T &G shelving removed from the factory clad with 18mm shiplap sits on a 3x2 frames on a raised cement bed to stop any under run of rain  water sits  over old slabs  no dpc  initial had a corrugated plastic roofing  this made condensation like the titanic , replaced with triple box poly carbonate and this solved the drips completely   as with any clear roof  it does need shading in the summer , but no drips or Oops its gone rusty tooling ,  dry and now  20 years old 

pete

 

,

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1 hour ago, Pete Lewis said:

not a garage  but i built my  6 x 3 mtr workshop from odds and sods  , the walls are  timber  25mm   T &G shelving removed from the factory clad with 18mm shiplap sits on a 3x2 frames on a raised cement bed to stop any under run of rain  water sits  over old slabs  no dpc  initial had a corrugated plastic roofing  this made condensation like the titanic , replaced with triple box poly carbonate and this solved the drips completely   as with any clear roof  it does need shading in the summer , but no drips or Oops its gone rusty tooling ,  dry and now  20 years old 

pete

 

,

Triple box polycarbonate great idea.  I was thinking of how to add top lighting.

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On 17/08/2020 at 13:24, SixasStandard said:

Don't forget about air flow through the structure as well. So many people spend all their time focused on preventing water getting in without considering the need to promote the movement of air inside. If you're putting a pitched roof on it then consider vented soffits and a couple of roof vents, otherwise you'll end up with mold and subsequently rot. 

As far as the brick courses are concerned, if you use standard porous brick then at least lay some sill gasket membrane between them and the framing. Blue bricks would be the best option.

Ian 

Double plus for the above!

So many people moan about condensation in the garage, then spend much on dehumidifiers, Carcoons etc.      I live next to Cumbria, wettest part of the country, in Lancashire, next wettest.    My garage is brick, with an unlined slate roof, with one skylight that drips slightly in heavy rain.    Condensation is NEVER a problem, as none of the doors fit!     It's draughty, and cold in winter, but because there is a frequent change of air, any moisture is blown out, it doesn't drop out.

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1 hour ago, JohnD said:

Double plus for the above!

So many people moan about condensation in the garage, then spend much on dehumidifiers, Carcoons etc.      I live next to Cumbria, wettest part of the country, in Lancashire, next wettest.    My garage is brick, with an unlined slate roof, with one skylight that drips slightly in heavy rain.    Condensation is NEVER a problem, as none of the doors fit!     It's draughty, and cold in winter, but because there is a frequent change of air, any moisture is blown out, it doesn't drop out.

I learnt the hard way , damp was never a problem in my brick built garage , the top window was always left on the latch . I closed it in November last year as we were going away came back and garage walls and ceiling were mouldy ,

Paul 

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I have a steel framed and clad garage. 50mm twin skin insulated roof, and I dry lined the single skin walls. 

Floor has a DPM in it, not had the slightest hint of any moisture in it, even on any of the large lumps of cast iron that used to be wringing wet in the old garages.

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1 hour ago, thescrapman said:

I have a steel framed and clad garage. 50mm twin skin insulated roof, and I dry lined the single skin walls. 

Floor has a DPM in it, not had the slightest hint of any moisture in it, even on any of the large lumps of cast iron that used to be wringing wet in the old garages.

Colin What is twin skin insulated. 

I was thinking of using insulted steel roof panels.  They have the normal plastic coated exterior surface and rather the anticondensation flock layer have a composite base layer. The inner face is white bit like polyripple covering the insulation.  Choice of insulation depth I was thinking of 40mm. I will try to find a photo or diagram to illustrate.

As I said earlier I may go for steel wall cladding as getting any wood cladding here on the IOW is proving hard.  Only seems to be short lengths like used in fence panels available just now.  My garage/workshop is 7.2 x 5 meters so want/need long lengths if using wood cladding.

InsulatedRoofSheetsCapture.JPG

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