Advice needed

[Kgthomas]

Hi All,

 

Several years ago, we poured a concrete slab with the intention of building a garage. Unfortunately for several reasons the project came to a halt, mostly due to funding and needing to replace other items in the house, such as heating.

 

Now we are in the position to complete the garage. 

 

The slab is 8x11m and the intention is for the garage to be the full size of this. The slab was poured in 30N concrete and was 12" deep at the edges and 6" deep at the middle, the base was reinforced with steel and had a DPM installed. The slab was dug out and backfilled with gravel also prior to having the slab poured. There is a pathway around the slab which will be below the level of the slab and will be concreted.

 

I have priced the garage in steel however I am wondering if concrete block may be a better option? Either way we intend to have a steel roof on the garage. I was considering a single block wall but was advised by a retired builder, that I would suffer from damp due to having poured a slab rather than using a trench foundation with a floor installed after a couple of courses of blocks. The retired builder said we would be better with a steel garage as there would not be any issues with damp. This does confuse me, I do not understand how the steel would prevent any damp? I know concrete blocks are not watertight but I think sand and cement render is? If the retired builder was suggesting damp could rise up from the slab then I would imagine that would be the same with a steel garage? The retired builder said the only other option would be a cavity wall to prevent damp.

 

I had asked a builder for an estimate to build a cavity wall structure with steel roof and his estimate was approximately £3-4k more than steel. I have priced concrete blocks locally and they do appear to be a cost effective option, I know that is only part of what I would need. 

 

To me, if I opt for the steel garage then there will be no insulation and more importantly, I am limited on where I can put items like plugs etc. The solution around this is that I clad the inside of the shed and put insulation in, this however increases the costs again. If I was to opt for the concrete block and steel roof construction, then if I go cavity wall I have an increased construction cost. If I go single block wall with sand and cement render, that should be waterproof? Then again I could clad the inside of the shed and put insulation in? Don't know if that works out cheaper or not? 

 

We live in a rural area and as such we do get quite high winds at times so do have some concern over a steel structure also, it will look completely different to the house which has a pebble dash render. Whilst the sand and cement will not be pebble dashed it would be painted to match the house. Or I believe it can be coloured white also?

 

Any advice or suggestions are welcome.

 

Thanks

[markc]

I think he means the steel building cladding would extend over the slab and take water down past it, where a block build without a outer leaf rebate (can’t remember the proper name for the step) will allow water to creep under the DPM and into the garage.

 

[Kgthomas]

14 minutes ago, markc said:

I think he means the steel building cladding would extend over the slab and take water down past it, where a block build without a outer leaf rebate (can’t remember the proper name for the step) will allow water to creep under the DPM and into the garage.

 

 

Ah ok, that makes sense. So am I right in thinking if the block work was taken to the edge of the slab and then a sand and cement render applied to the block work, this extends the exterior past the slab? Or is there another way around this?

 

Thanks for the reply

[Mr Punter]

If the DPM under the slab extends up the sides of the slab you can lap it with the DPC for the blockwork, although you may need to some additional paint or stick on membrane if the 2 would not otherwise meet.  Rendered single skin block is fine for a garage.  You will need some internal piers as the walls are quite long.  If you have a metal roof you can get panels sandwiched with foam insulation, which prevents condensation forming and dripping down.

Edited March 1, 2022 by Mr Punter

[Kgthomas]

1 minute ago, Mr Punter said:

If the DPM under the slab extends up the sides of the slab you can lap it with the DPC for the blockwork, although you may need to some additional paint or stick on membrane if the 2 would not otherwise meet.  Rendered single skin block is fine for a garage.  You will need some internal piers as the walls are q

Thanks for the reply. The DPM does not extend beyond the slab, the person who laid the slab did not do that. So if I brick it up, I might have to look at options for that. 
 

I was thinking I might need to put internal piers in the walls given the lengths. I had also considered the idea of supporting the roof on steel framing and using concrete blocks in between. So for the side walls I’d have 5 uprights so roughly 2.75m of wall in each section and for the end there would be 4 uprights so roughly 2.7m of wall in each section. Only part which may be difficult is the end with the garage door. Obviously the corner uprights would double up between sides and end. Though don’t know how well the sand and cement would adhere to the steel? Or if that would bring moisture into the steel and cause it to rot. 
 

Though, maybe I am over complicating it?

[Kgthomas]

Have seen that you can get non drip panels for the roof also. Though was told if you put skylight panels in that’s where you are most likely to get condensation. 

[markc]

Mortar will not adhere to steel.

you could take a membrane down the bottom part of wall and over slab edge, then render over.

single skin roof sheets will form condensation whether solid of skylights.

too many variables here to give meaningful answers, maybe you need to sort out your build method etc. First and then we can assist with details if you cannot find anything by searching past posts

[Carrerahill]

4 hours ago, Kgthomas said:

The retired builder said we would be better with a steel garage as there would not be any issues with damp.

 

This is not strictly correct.

 

I have a block garage on a 10" slab and have had no issues with damp. 

 

I ensured that the ground levels around the slab are lower with no build up, and I took the render down to a bellcast bead just a smidge above the slab, I caught the DPC and pinned it up, just caught it and no more, I used a 2 coat silicone system which you can spray with a hose all day long and you never see damp coming through the mortar joints of the garage (which is always the first to let water migrate through). From ground level up to below the ballcast I used a smooth cement render with waterproof additive which I then painted with water-proofer a year on and after weeks of dry weather. 

 

FGL is about 6" lower than the slab, except at the front door. Garage is so dry that even when using my nailgun to build things the exhaust port air is enough to kick up all the floor dust even after the 2 weeks of solid rain we had up until the weekend. 

 

I also laid my slab on a DPC, which you said you have done too, so where is the damp issue? 

 

 

Edited March 1, 2022 by Carrerahill

[joe90]

I also have a single skin (rendered) garage/workshop on a slab with no damp problems!!!

[Kgthomas]

2 hours ago, markc said:

Mortar will not adhere to steel.

you could take a membrane down the bottom part of wall and over slab edge, then render over.

single skin roof sheets will form condensation whether solid of skylights.

too many variables here to give meaningful answers, maybe you need to sort out your build method etc. First and then we can assist with details if you cannot find anything by searching past posts

Thanks, you are right - first of I need to figure out how I want to build it. If going with the block and steel framing, I will have to see how I can get the mortar to adhere to the steel. 

[Kgthomas]

1 hour ago, joe90 said:

I also have a single skin (rendered) garage/workshop on a slab with no damp problems!!!

Good to know you have done something similar and not had damp problems. 

[Kgthomas]

2 hours ago, Carrerahill said:

 

This is not strictly correct.

 

I have a block garage on a 10" slab and have had no issues with damp. 

 

I ensured that the ground levels around the slab are lower with no build up, and I took the render down to a bellcast bead just a smidge above the slab, I caught the DPC and pinned it up, just caught it and no more, I used a 2 coat silicone system which you can spray with a hose all day long and you never see damp coming through the mortar joints of the garage (which is always the first to let water migrate through). From ground level up to below the ballcast I used a smooth cement render with waterproof additive which I then painted with water-proofer a year on and after weeks of dry weather. 

 

FGL is about 6" lower than the slab, except at the front door. Garage is so dry that even when using my nailgun to build things the exhaust port air is enough to kick up all the floor dust even after the 2 weeks of solid rain we had up until the weekend. 

 

I also laid my slab on a DPC, which you said you have done too, so where is the damp issue? 

 

 

Thank you so much for such a detailed response. It is great to know that you have a similar setup to what I have in mind and not having issues with damp. Makes sense to take the bellcast to just above the slab. 
 

The path around the slab will be below the level of the slab and if I slope the path towards the rear of the garage, as I have a land drain behind the garage. 
 

Using silicone to seal the bellcast and sand / cement with waterproofed below makes sense. Then paint to add another layer of protection. 
 

I do prefer the option of block and if I could include a steel frame for the roof then that should give good strength there too. Just need to look at options for getting mortar to adhere to the steel. I’ve seen an adhesive for attaching block work to steel from CT1. 

[joe90]

My workshop is similar in size and no steels  just standard roof trusses and metal roof.

[Kgthomas]

5 minutes ago, joe90 said:

My workshop is similar in size and no steels  just standard roof trusses and metal roof.

Ah good to know. Did you build piers into the block work too?

[joe90]

Just now, Kgthomas said:

Ah good to know. Did you build piers into the block work too?

Yes, as per building regs. I believe a max of 3 mtrs between piers is lowed In a 100mm wall but piers not required if 150mm blocks are used. (You may need to check this with building control).

[Kgthomas]

8 hours ago, joe90 said:

Yes, as per building regs. I believe a max of 3 mtrs between piers is lowed In a 100mm wall but piers not required if 150mm blocks are used. (You may need to check this with building control).

Thanks, I must check that out with building control. 

[joe90]

45 minutes ago, Kgthomas said:

Thanks, I must check that out with building control. 

There is a website diagram somewhere, could not find it last night.

[Kgthomas]

58 minutes ago, joe90 said:

There is a website diagram somewhere, could not find it last night.

Had a look this morning on building control regs for NI. It does state a maximum floor area of 70m2 so don’t know if that will allow me to use the info I found. I will have to check with them. It says the wall has to be at least 90mm thick with piers every 3m. Doesn’t say anything about if the blocks are thicker. I was considering using 150mm anyway and the piers have to be at least 3x the thickness so that would work out ok. I think. 

[joe90]

Well why not go with that on your plans and see what building control say!

Edited March 2, 2022 by joe90

[PeterW]

13 hours ago, Kgthomas said:

Just need to look at options for getting mortar to adhere to the steel.

For horizontal steels get your steel supplier to tack weld on a 75mm wide strip of diamond mesh along the beam tops prior to dipping or painting and this will bond your mortar to the steels. For vertical joints you want to use welded tabs or frame cramps screwed or bolted to the flanges as the mortar is in sheer and will not grip. 

[Carrerahill]

13 hours ago, Kgthomas said:

Thank you so much for such a detailed response. It is great to know that you have a similar setup to what I have in mind and not having issues with damp. Makes sense to take the bellcast to just above the slab. 
 

The path around the slab will be below the level of the slab and if I slope the path towards the rear of the garage, as I have a land drain behind the garage. 
 

Using silicone to seal the bellcast and sand / cement with waterproofed below makes sense. Then paint to add another layer of protection. 
 

I do prefer the option of block and if I could include a steel frame for the roof then that should give good strength there too. Just need to look at options for getting mortar to adhere to the steel. I’ve seen an adhesive for attaching block work to steel from CT1. 

Just to be clear, the silicone I refer to was not to seal the bead, that was the render product type I used from Enewall. It basically repels water but has breathability. So, from soffit to bellcast bead I covered the whole wall making the block water repellent, however, I would argue waterproof in normal circumstances, although I understand it will not truly be “waterproof”.

 

Then a waterproofed sand/cement smooth render below which was worked in hard against the bellcast to form a continuous protective cover.

 

You could build a steel portal frame and infill with block. I nearly did this for speed of getting a roof over so I could work on the build in the rain, but I was impatient and didn't want to wait for my steel kit to come. Usually the block is just dressed into the web of the steel so that it forms a solid locked in panel, although the mortar does not stick, it locks it in firmly. Also, I have seen the steels with a wall tie system bolted or Hiltigun nailed on. Works well. You need to dress up the plinth detail properly at the based of the steels and protect the steel so that it doesn't rot out in 30 odd years. The other design used frequently is for the steels to be kept within the weatherproofing envelope of the building to keep the steel dry.  

[Kgthomas]

26 minutes ago, joe90 said:

Well why not go with that on your plans and see what building control say!

 Sounds like the best plan ?

[Kgthomas]

15 minutes ago, PeterW said:

For horizontal steels get your steel supplier to tack weld on a 75mm wide strip of diamond mesh along the beam tops prior to dipping or painting and this will bond your mortar to the steels. For vertical joints you want to use welded tabs or frame cramps screwed or bolted to the flanges as the mortar is in sheer and will not grip. 

That’s a good idea, I will speak to them about that thanks ?

[Kgthomas]

2 minutes ago, Carrerahill said:

Just to be clear, the silicone I refer to was not to seal the bead, that was the render product type I used from Enewall. It basically repels water but has breathability. So, from soffit to bellcast bead I covered the whole wall making the block water repellent, however, I would argue waterproof in normal circumstances, although I understand it will not truly be “waterproof”.

 

Then a waterproofed sand/cement smooth render below which was worked in hard against the bellcast to form a continuous protective cover.

 

You could build a steel portal frame and infill with block. I nearly did this for speed of getting a roof over so I could work on the build in the rain, but I was impatient and didn't want to wait for my steel kit to come. Usually the block is just dressed into the web of the steel so that it forms a solid locked in panel, although the mortar does not stick, it locks it in firmly. Also, I have seen the steels with a wall tie system bolted or Hiltigun nailed on. Works well. You need to dress up the plinth detail properly at the based of the steels and protect the steel so that it doesn't rot out in 30 odd years. The other design used frequently is for the steels to be kept within the weatherproofing envelope of the building to keep the steel dry.  

Thanks for that. I am considering going with concrete block with piers rather than steel frame. Will at least remove one issue. Would be planning to use 150mm blocks so that should help. Then that removes any risk of the steel rotting. 
 

Had a look at the silicone system you mentioned and understand what you mean. 
 

Thanks to everyone’s help I think this is becoming clearer in my head. 

[Carrerahill]

1 hour ago, Kgthomas said:

Thanks for that. I am considering going with concrete block with piers rather than steel frame. Will at least remove one issue. Would be planning to use 150mm blocks so that should help. Then that removes any risk of the steel rotting. 
 

Had a look at the silicone system you mentioned and understand what you mean. 
 

Thanks to everyone’s help I think this is becoming clearer in my head. 

Given the price of steel, and the added complexity of building with steel portals, I would still build in block. Block prices have stayed about the same, as has sand and cement so I think. 6" block with piers within the long walls will make for a solid garage.

 

One piece of advice on the piers, plan their location! I didn't and now have some annoying splits in my racking where I need to end the racks, have wasted space, then a pier, then start the racking again. Also try and plan piers to align with things like roof trusses. I have 4 trusses and purlins to have a more open feeling garage with a big open roof area, it does look good and gives loads of height for moving big bits of wood and things about and the ability for me to suspend bikes etc.