Building Workshop

8 Feb 2011
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United Kingdom
Hi there
Basically I have got a single garage in my garden at the moment that leaks and is damp and made from asbestos so getting that taken care of and then wanting to build something a little smaller for a workshop. There are two trees (birch) nearby that are about 15m high so wondering how deep and wide i should dig the foundations. Going to be building it as a cavity wall with brick outer and block inner with insulation and a flat (warm roof) with 100mm celetex wedged between ply. For those thinking it is overkill for a workshop I need it to be warm as I will be also using it to smoke in as not allowed to in the house anymore!
What i need to know is then how deep/wide should the foundations be. What concrete mix to use for the foundations and would i need any root barriers?
Also wondering what the pros cons of using thermalite blocks for the inner wall or whether i should just use 3.5 or 7n concrete blocks.
Also what would be best to use for the cavity insulation?

(additional the trees are about 3m away)
thanks dudes
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Soil type?

What are the trees like? any signs of hollowed out sections, major dead branches or fungal 'ears' coming off the trunk?

You won't really know what's going on until you start digging, if the garage is on an old concrete base that's been there for many years, you may find that the bigger tree roots have given that area a 'miss' In any case, you shouldn't go chopping through any roots larger than about an inch, without having a negative effect on the trees.

Reinforced concrete raft foundation probably your best bet, unless you want to get into pad n beam foundations, with root barriers and expansion boards.

You don't need an inner block wall necessarily, you could just fix insulation boards and then plasterboard or plywood the inside to keep the weight down.

You could also try giving up smoking
why the raft approach and not trench? the soil is heavy clay.
I am quitting smoking but I do like to smoke my churchwarden every now and then.
There is a crack in the existing base making me think that the birch roots have gone underneath the base, lifting it at one side and causing it to crack.
Might just ask my neighbour if we can cut the tree down then wait a year and rebuild.
I was going to just do a 1m deep trench foundation with a linear root barrier. would have thought this would have been overkill for the size of the project. (only going to be 4.8m x 2.4m)
with a single wall i would need some kind of barrier I am guessing as I dont want any moisture getting in or out. (going to be controlled with a hydrometer and a dehumidifier so single wall might start up a capilary action)
The tree in question from the trunk up looks fine and although tall hasnt thickened out yet. There might be a problem with it though as during last autumn there were a lot of mushrooms in the soil around the base leading me to think the roots might be infected.
The key things are the distance of the tree away and the soil volume change potential. Birch is a low water demand tree so that helps. Assuming high soil change potential, if it's 2m away you'll need a 1.6m deep foundation. If it's 3m away you'll need 1.4m deep. The size of the building is irrelevant. A smaller building will just lift and crack more easily than a larger one. You should also fit claymaster to the inside walls of the foundation and a suspended floor - not ground bearing.
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see i am trying to work within the 2.4m allowed height without needing planning permission so adding a suspended floor wouldnt leave me much headroom at all. Are there nay alternatives?
im 1.89m tall so no chance of squeezing in hehe
A ground bearing reinforced floor with an expansion product beneath might do the trick, but not without some risk. I assume you wouldn't need building regs so it would be your choice.
is the risk just that the floor would crack... if so would putting in a reinforced concrete floor of about 200mm do the trick? also for the heigh of building regs does it go from the ground or ground level of the foundations?
And as per my frist question what is good to use for insulation in the cavity?
also if i take up the original concrete base and have a dig around and find no roots would it be acceptable just to put a 2.5m linear barrier at the edge of the old foundations and as the new building is smaller just start the foundations another meter away from that?
If you mean eaves height for permitted development purposes, it's taken from the natural ground level adjacent to the building.

If you're heating the room fully I'd use a 75mm cavity full filled with mineral cavity batts. With turbo blocks on the inside that will comply with current U-value.
have had a dig around (dug a trench about 2m away from the tree 1m deep) found no signs of any thick roots at all. Just a lot of thin fibrous roots. Would i have to dig deeper to find anything or should i have found the big roots?
have had three builder over now all of whome are FMB registered with all of the bells and whistles all of whome have said that wouldnt even need reinforced concrete base so who is right :S
when i mentioned about the floor cracking they said that it would be very unlikely to crack as they would put a base down before the floor slab was laid...
As long as you're happy that they're suitably qualified to provide structural advice and have PI insurance and you employ them on an appropriate contract then what could go wrong? You might also just check with the FMB to make sure they do back their members to provide structural advice and will cover you if things go wrong.

Good luck.
all i want is a damn workshop!!! everything is made so chuffing complicated... the old garage floor was only 100mm thick and only has one crack in it after 40 years. Ive just figured builders just want too much money for what they are doing... priced up all the materials for this build and worked out to £2429.59 that is me buying them myself without any trade discounts etc etc and for poured concrete base (that would be 1.4m deep trench foundations on a 300mm reinforced raft)
so how are builders comming up to me and saying it will cost £5000-£9000 so if I took a middle price of £7000 they would be making a profit of £4570 for three weeks work. Baring in mind this price would be with me clearning the original garage myself and breaking up the original concrete base. Firstly them laying the foundations which would be laying them and a couple of days to dry (I will be digging them out myself) then building up to the DPC etc another half day job and then waiting for it to go off then a few days of laying bricks 4 max for the size of it. then waiting for that to go off before the trusses go in and the insulation for the warm roof (flat)
basically I worked out if they were there for 7 hours each day for the three weeks,(105 hours)(but there would be 2 days not working when the foundation has been laid and another day after the brick has been build up to dpc and another 2 days before the roof trusses are put on so 70 hours in total) roughly they are earning a whopping £65.29 an HOUR Have been told by all builders that there will be 2 people on site which still means they are earning a whopping £32.64/hour so if we take of running cost of vehicles etc that would still leave them earning £30 an hour... this is madness and I know that they have trainer for a long time and have knowledge but some of the ones I have met dont even know their arse from their elbow and are still charging looney money. I have a lot of friends who have trained for years in their specific fields of work and dont earn anywhere near that much. Think I am going to do this one on my own as when I said I could get the materials for that much (ive taken an average price not the cheapest) all of them told me it was impossible until i sent them the quotations from the suppliers then one didnt reply and the others said they could get it cheaper than that which just added insult to injury. I wouldnt mind paying for it if i knew it was going to be an excellent job but one of the companies (remember these are all FMB registered) was using stock photos on their website claiming it was their work. Another showed me some of their work which just by blind luck I knew the house he took me to and the people in it and I know they had the work done by a builder friend that came up from london, when i knocked on the door and they said hi to me he just tried to divert the conversation away from the work... last person couldnt give me any pictures or places where he had worked and then gave me some bull about data protection act prevents him from showing me his previous clients addresses.
sigh rant over... when I start to get this thing going i'll document it all on here with pictures in the hope that others wont have to go through the mess I have had to... Unless someone knows of a decent builder in the stoke on trent area?
You don't need trench and raft, one or t'other. With a slab you just thicken the edges to support the walls, and you won't need 300mm thick for the main floor section.

As Jeds mentioned initially, you can trench fill and use a suspended floor. This needn't necessarily mean raising the floor, but you will need to excavate about a foot beneath for vent and expansion.

Pad and beam is a good solution here, you dig a load of holes, fill them with concrete and join them up with reinforced concrete beams. Can be diy'ed with the right advice.

FMB stickers can be bought on eBay.
how thick would be enough then for the main floor section? If i was to put in a suspended floor how far above the DPC would the floor have to be if the DPC has to be 150mm away from the ground in the first place adding suspended floor after would lose me even more height or is there another way? Pad and beam seems tricky..
to be fair the only part im slightly stumped about is the foundation... what kind of price should I be looking at for having a 1.2m foundation laid. Spoke to building regs and planning building regs told me that for the kind of tree 1.2m was sufficient with claymaster boards around the side and not to put in a root barrier as that could cause the tree to die (not sure how with the barrier only being on one side of the tree) and then planning told me the height of the building could go up to 2.5m but im sure it is 2.4 any ideas on that?

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