Garage Workshop build - advice needed please

[Goesbysteve]

I am currently planning the fit out of an existing double garage as a workshop and I really need some advice. I’d like any thoughts on my situation, plans or anything I’m missing please. I’m sorry for the long post but I hope it gets all the details in. I appreciate anyone who reads it. My post is in two sections. Dealing with flood risk and fit out of garage.

To give you an idea of the location -

The garage is a post 2000 (maybe 2007) dated 3m x 6m single skin concrete block wall with wooden pitched roof and swinging wooden doors. There is a flat concrete slab floor in good condition. I’m unsure if any damp proofing on the slab as it predates our ownership. Should I try to find the builder and get details? Given I live in rural north mid-Wales right next to a river and a stream and the rear of the garage built into earth covering approx 30cm of wall above the slab I am thinking additional protection. Although neighbours have said the house hadn’t flooded in 25+ years of their living here I was told by a previous resident that the property did flood around 1938. From what I’ve been told as we are in the hills water tends to come and go quickly so I am not thinking I need to deal with long term flood. As part of a flood risk mitigation project I probably will look at slot-in flood walls on our front door and the garage. These typically come up to 45cm. However I would want to double up on interior protection for the garage. Maybe a DPM sheet across floor and 30cm up the wall? I wonder if that would deal with any ingress through walls should water get above the concrete slab level. How resistant are concrete blocks and mortar to ingress? Is there any benefit to using a paint on sealer either inside or outside to tank the lower level? Would this actually help? The garage has a black paint on the walls. I’m unsure if this is just regular paint or something like Black Jack bitumen. Anyone know how to tell the difference? It covers the entire wall height.

I realise there will be some comments to not be so mad to build workshop here but it’s an existing building and I don’t have the money to build a workshop from scratch. But help really appreciated on basis of protection. A flood is actually an unlikely occurrence but I’d like to understand my options.

As to the fit out. The roof is built of C24 2x3 rafters and ceiling beams/ties. Doubled up in some locations. Both ends and centre I think. Currently I use the roof space to store suitcases, TV shipping boxes etc. nothing of considerable weight. I would like to insulate between beams and lay sheet material to allow better storage up there. Would -50mm PIR be suitable? I’m limited on thickness given the 3x2s used. I would probably put plasterboard on the underside. Should 2x3s be able to take that weight or am I asking too much of them? What options might I have of reinforcing? Can I build out the walls first then get to the ceiling at a later date?

On the walls I’d like to insulate. I realise there are many options but I was leaning towards a stud wall of 2x3 with -50mm rigid PIR insulation and OSB faced. I will want to add robust shelving to the walls. There is one double glazed window in the back wall. I’ve seen battening direct to walls with -25mm blue treated timber and a DPM roll between wall and batten. I wasn’t sure if that is ok too in my situation. The latter would reduce the space taken up a little. If I go the stud wall route how much of an air-gap should I leave? I’ve seen various ways probably on buildings with far more space than I have here so I don’t want to just go with what had worked for someone else’s building. What are my other options here? If I use PIR that has a film layer both sides do I need an additional breathable layer? I know OSB along with other typical wall panelling is not that water resistant, especially if actually in water. I know there are metal and plastic options but I think they are going to be out of my budget. OSB would he replaceable should the worst happen. How does PIR hold up to water?

On the floor I wasn’t going to insulate specifically. I would add a DPM if necessary and lay interlocking PVC garage tiles. My thoughts here is if I do get any water I want anything on the floor to be essentially waterproof and removable.

The electrics are already in the garage and I have plenty of sockets. RCD and wiring is at the top of the walls and sockets at 1.5m above the floor. I guess I need to release everything from the wall and the replace them back on the surface.

That leaves the wooden doors. They are well fitting and heavy. There is a 25mm rise on the floor level that the doors close against. I’m not sure what to do here but given effort I’d go to to insulate they will be the weak link. Would PIR be an option here again?

I was thinking about having an oil filled radiator set low to come on at 5C. I can then set it higher to get the workshop warm enough to work in. 16C would do me plenty. I don’t mind wearing a fleece lined work shirt! I know little on capacity of rad to deal with freezing temps outside in a garage my size. Would appreciate any advice on this too,

Finally at the end! Thanks for reading. Sorry if I have missed anything or not explained well enough. please do ask for clarification. Again I really do appreciate your advice.


Diolch yn fawr!

Warm regards,

Steve

 

[Goesbysteve]

I have just realised these are “breeze blocks” with no render on the outside. I think they have been painted with something to try to waterproof it. I’ll need to dig around the rear where the ground build up against the wall to see what below. Hmmmm I can see more cost coming in. At least external wouldn’t delay interior build. But i feel tanking or DPM roll on the floor is needed.

may have to find money from switching from PIR to rockwool.

 

[Jameshow]

I think your roof structure is fine for boarding out and plasterboarding.

If your worried about a flood which you say the risk is very low then how about doing the bottom 1/2 ft in cement board which is impervious to water?

Putting a coating of liquid DPM wouldn't go amiss.

Digging out behind the garage and back filling with gravel might be an idea if you feel the back wall to be damp?

If your putting a flood defense system in then I don't think you need to worry too much apart from the worry that your away when it does flood.

 

[MikeJhn]

If flooding is a risk, ensure all the electrics are at a high level.

Personally I would give up woodworking and spend my time fly fishing in that extremely good looking stream.

 

[Goesbysteve]

If flooding is a risk, ensure all the electrics are at a high level.

Personally I would give up woodworking and spend my time fly fishing in that extremely good looking stream.

It is a lovely looking stream but full of lead due to historic mining. However I think fishing is more about the experience than actually catching anything!

 

[Goesbysteve]

I think your roof structure is fine for boarding out and plasterboarding.

thanks, I’d seen so many YouTube videos of 4x2 and 6x2 used for joists that I was worrying that my thin 3x2 wouldn’t take much weight. I’m not about to go dancing a jig up there but I would want to crawl around to get insulation between the joists. Would you feel ok doing that yourself If that were your roof?

If your worried about a flood which you say the risk is very low then how about doing the bottom 1/2 ft in cement board which is impervious to water?

Interesting idea, thanks. I’d not heard if that. Makes complete sense. I will research that.

Putting a coating of liquid DPM wouldn't go amiss.

I guess a DPM is to stop water travelling into the garage through the slab rather than flood water?

Digging out behind the garage and back filling with gravel might be an idea if you feel the back wall to be damp?

that’s an option. However I have my oil heating tank behind the garage. Even though it sits on a substantial concrete slab itself I’d be concerned about back filling with anything that holds back the ground up hill from subsiding down. I think there are two things I need to worry about here. Non-flood Welsh weather where there is a lot of water held in the ground and actual flood. I can add something to assist in draining for the regular cinditions but when it floods that water could have no where to drain. Hence thinking a barrier to hold it back from the wall long enough fir the flood to drain. It comes and goes quite quickly here. Like hours, not days, touch wood. That said assisting drainage might not be a bad idea the other side assuming I can deal with the support issue.

If your putting a flood defense system in then I don't think you need to worry too much apart from the worry that your away when it does flood.

yes indeed. we get a bit of warning and I work from home. We alsoI have lovely neighbours round here and if away for a holiday I’d be putting the system up and locking it as additional theft protection.

 

[Goesbysteve]

Is moisture resistant plasterboard worth it in my situation?

 

[johna.clements]

that’s an option. However I have my oil heating tank behind the garage. Even though it sits on a substantial concrete slab itself I’d be concerned about back filling with anything that holds back the ground up hill from subsiding down. I think there are two things I need to worry about here. Non-flood Welsh weather where there is a lot of water held in the ground and actual flood. I can add something to assist in draining for the regular cinditions but when it floods that water could have no where to drain. Hence thinking a barrier to hold it back from the wall long enough fir the flood to drain. It comes and goes quite quickly here. Like hours, not days, touch wood. That said assisting drainage might not be a bad idea the other side assuming I can deal with the support issue.

The high ground built up against the garage and the flooding from the river are different issues.

The high ground keeps moisture against the wall which will then penetrate making the inside damp. If you do not want to have a wide shallow trench you could dig a narrow one and retain the 300mm high side with blocks to match the garage or paving slabs laid vertacally or a slight angle. Just leave a gap wide enough to clean out leaves etc A photo would help.

The ground being higher at this point is unlikely to make the flooding from the river worse. It may in fact help in a small way, a bit like putting sand bags against the garage.

Unless there is a bridge just downstream that could be blocked by trees in a flood I would doubt that the water would stay there for long. If the high ground has been underwater it will have soaked up some water but most likely not more than weeks of heavy rain.

 

[niall Y]

A beautiful location, but with the river there a hostage to future flooding. As someone whose workshop has flooded twice in the last 14 years, it can be a bit of a PITA. Anything that you can reasonably do to mitigate the effects, is well worth doing at this early stage.
I can't imagine that the guys who put up your garage did not tank the back, where it touches the bank, otherwise you would already have a problem inside the garage. I would assume that this amounts to more than a couple of coats of 'Black Jack', though you never can tell

When we moved into our property, I converted our lean -to garage into an additional bedroom. This was a concrete block construction. Building regs required that I coat the walls with bitumen paint, before, battening it out and fitting 'Cellotex' panels. The choice was then, either to cover this with foiled plasterboard, or to use a thin polythene sheet with ordinary plasterboard on top. Admittedly, yours is a workshop, but something along these lines, is what is required, Though if it's going to get wet some time during its life, some form of cement board might be better.

With reference to the existing doors, I have improved the insulation of this type of door, by first covering with a breathable membrane, infilling with 'Cellotex', and finally covering with board or T&G

 

[johna.clements]

A beautiful location, but with the river there a hostage to future flooding. As someone whose workshop has flooded twice in the last 14 years, it can be a bit of a PITA. Anything that you can reasonably do to mitigate the effects, is well worth doing at this early stage.
I can't imagine that the guys who put up your garage did not tank the back, where it touches the bank, otherwise you would already have a problem inside the garage. I would assume that this amounts to more than a couple of coats of 'Black Jack', though you never can tell

I would assume that the oil tank was built after the garage. The garage looks well constructed. I would assume that the people who put in the base did not care about the levels of the ground "its just a garage".

 

[Goesbysteve]

A beautiful location, but with the river there a hostage to future flooding. As someone whose workshop has flooded twice in the last 14 years, it can be a bit of a PITA. Anything that you can reasonably do to mitigate the effects, is well worth doing at this early stage.
I can't imagine that the guys who put up your garage did not tank the back, where it touches the bank, otherwise you would already have a problem inside the garage. I would assume that this amounts to more than a couple of coats of 'Black Jack', though you never can tell

Thanks for the reply. Yes it is, we are very lucky. But it comes with risks as you say.

oh I can imagine all sorts not done right here. they painted the breeze block with “something” rather than render or glad it. I’m still to work out what the black paint from top to bottom of walls actually is. I’m going to have an exploratory dig around the rear at the weekend. Assuming rain holds off.

When we moved into our property, I converted our lean -to garage into an additional bedroom. This was a concrete block construction. Building regs required that I coat the walls with bitumen paint, before, battening it out and fitting 'Cellotex' panels. The choice was then, either to cover this with foiled plasterboard, or to use a thin polythene sheet with ordinary plasterboard on top. Admittedly, yours is a workshop, but something along these lines, is what is required, Though if it's going to get wet some time during its life, some form of cement board might be better.

If I understood you correctly the walls were painted with black jack on the inside? What was on the outside out of interest?

I definitely want OSB 3 as the inside surface and my budget is only going to reach to rock wool I think. I believe a 50mm airbag between inside of wall and insulation is required if regs applied here. I may only get 25mm and 50mm insulation with 12.5mm OSB over it So I don’t lose too much space. I’m on my getting the rear half as a workshop and the rest is storage and supplies. E.g. two extra fridge freezers.

was the polythene sheet a vapour barrier? I’m not sure I’d be producing enough moisture that could escape through rock wool and get drawn out by airgap. However on the walls I’m not sure how a threw flow is achieved as it would be in my loft.

With reference to the existing doors, I have improved the insulation of this type of door, by first covering with a breathable membrane, infilling with 'Cellotex', and finally covering with board or T&G

I may be able to go with PIR infill on the doors as anything loose is impractical there. thin painted OSB 3 or exterior ply over it? The doors when open may get some rain. Hence wanting more protection there than the interior walls.

 

[Goesbysteve]

I would assume that the oil tank was built after the garage. The garage looks well constructed. I would assume that the people who put in the base did not care about the levels of the ground "its just a garage".

I think at the same time. The owner then was renovating the entire property.

 

[Goesbysteve]

The high ground built up against the garage and the flooding from the river are different issues.

completely agree

The high ground keeps moisture against the wall which will then penetrate making the inside damp. If you do not want to have a wide shallow trench you could dig a narrow one and retain the 300mm high side with blocks to match the garage or paving slabs laid vertacally or a slight angle. Just leave a gap wide enough to clean out leaves etc A photo would help.

I was thinking the same but living in a conservation area may cause issues even when it’s unseen by anyone and the garage is hardly in keeping.

Yes it’s hard to visualise, I will sort some photos

The ground being higher at this point is unlikely to make the flooding from the river worse. It may in fact help in a small way, a bit like putting sand bags against the garage.

Unless there is a bridge just downstream that could be blocked by trees in a flood I would doubt that the water would stay there for long. If the high ground has been underwater it will have soaked up some water but most likely not more than weeks of heavy rain.

There is a fairly decent stone bridge 50m away downstream. It’s field upstream and not woodland though it only takes one tree. I think the flood defences on the door plus suitable waterproofing on outside of walls would be the best I can do. Is Black Jack high up the wall enough for that?

 

[Jones]

As there's no cavity in the wall you will need to do something to stop rain and possibly flood water coming through the block work. You could add cladding to the outside but that won't reduce flood risk. On the inside the choices are a paint on like bitumen or tanking slurry or a plastic membrane, either DPM or a proprietary tanking membrane. Whatever you use you have to be careful to seal it to the slab and seal any fixings for stud work . For a diyer I would use bitumen or tanking slurry as sealing to the floor will be easier. Once you've done that you don't need an air gap behind the insulation. OSB is considered vapour impermeable so you don't need an additional vapour barrier though it's best to leave a 5mm gap between sheets and then mastic the join.

 

[niall Y]

oh I can imagine all sorts not done right here. they painted the breeze block with “something” rather than render or glad it. I’m still to work out what the black paint from top to bottom of walls actually is. I’m going to have an exploratory dig around the rear at the weekend. Assuming rain holds off.

You should be able test the coating by burning an area, The smell will be quite distinctive. Many years ago, I was working on a property in Norfolk and we used Ness Synthaproof to paint the barge boards on the roof. It was also used to paint the Cob outbuildings to keep them waterproof

If I understood you correctly the walls were painted with black jack on the inside? What was on the outside out of interest?

The Black Jack was applied to the inside - two coats to the concrete blocks, The outside is rendered

I definitely want OSB 3 as the inside surface and my budget is only going to reach to rock wool I think. I believe a 50mm airbag between inside of wall and insulation is required if regs applied here. I may only get 25mm and 50mm insulation with 12.5mm OSB over it So I don’t lose too much space. I’m on my getting the rear half as a workshop and the rest is storage and supplies. E.g. two extra fridge freezers.

I have OSB on the walls of my workshop, and it has survived a couple of 'flash floods'. Slight discolouration, but nothing a coat of paint couldn't disguise

was the polythene sheet a vapour barrier? I’m not sure I’d be producing enough moisture that could escape through rock wool and get drawn out by airgap. However on the walls I’m not sure how a threw flow is achieved as it would be in my loft.

The polythene covered all the walls, behind the plasterboard and was taped to the polythene Radon barrier covering the floor.

 

[Goesbysteve]

As there's no cavity in the wall you will need to do something to stop rain and possibly flood water coming through the block work. You could add cladding to the outside but that won't reduce flood risk. On the inside the choices are a paint on like bitumen or tanking slurry or a plastic membrane, either DPM or a proprietary tanking membrane. Whatever you use you have to be careful to seal it to the slab and seal any fixings for stud work . For a diyer I would use bitumen or tanking slurry as sealing to the floor will be easier. Once you've done that you don't need an air gap behind the insulation. OSB is considered vapour impermeable so you don't need an additional vapour barrier though it's best to leave a 5mm gap between sheets and then mastic the join.

Thanks I’ll need to go research tanking options! Anything wrong with bitumen paint up the wall outside?

 

[Goesbysteve]

You should be able test the coating by burning an area, The smell will be quite distinctive.

Thank you, I will try to test this weekend when I do more photos and have a dig,

Ness Synthaproof

love the name!

The Black Jack was applied to the inside - two coats to the concrete blocks, The outside is rendered

I have OSB on the walls of my workshop, and it has survived a couple of 'flash floods'. Slight discolouration, but nothing a coat of paint couldn't disguise

The polythene covered all the walls, behind the plasterboard and was taped to the polythene Radon barrier covering the floor.

Thanks. Very much. Any reason you didn’t go the tanking slurry option suggested elsewhere?

 

[Spectric]

Have you thought of harnessing the power of that river to produce electricity?

 

[Jameshow]

Thanks I’ll need to go research tanking options! Anything wrong with bitumen paint up the wall outside?

If the inside is dry I'd think you can put closed cell foam against the brickwork and it will remain dry?

It's a garage after all.

 

[Goesbysteve]

Have you thought of harnessing the power of that river to produce electricity?

Interesting thought, watch this space! Lol