Workshop Design

[kingcod]

I'm in the early stages of building my workshop. Its going to be 7 x 3.5 m on a concrete raft using lots of inspiration from this site, not least the blockwork and timber design here.



(the coloured blocks are the odd sized ones I need to cut!)

I have a few of questions before I go much further:


1. The concrete base
Q: Is reinforcing with steel bar worth doing or will my 100mm concrete on compacted subbase be enough?

Laying the electric cable into the building I would like it to be set in through the concrete base.
Q: Do I need to cut through the damp proof membrance to allow this to happen? won't this compromise the DPM?
Q: How far in should the conduit for the cable appear? I am guessing the width of my block wall plus cladding plus a bit more?

2. For the roof of the timber frame I have got so far and got stuck in finishing the design of the gable end (red denotes the pieces I am referring to):



Uploaded with ImageShack.us

I have a nice support for the ridge board, but cant see how to get a rafters up each side to the end of the top plate. I am aiming for an overhanging rake but the vertical support just seems to get in the way of acheiving this. Do I butt rafters to the vertical pieces to end up flush with the gable end? It doesn't look such a good idea. I am almost wondering whether to do a complete redesign around ply gussets like this but I like the idea of a ridge board that rafters adhere to. am not sure how a ridge would fit to the ply gussets build.

Any help and advice gratefully received!

 

[OPJ]

Instead of having that support piece at the end, you could try temporarily bracing two rafters together (at the correct angle; allowing for the thickness of the ridge board at the top) and then lift the assembly in to position before fixing to the wall plates. Do this at either end and you can then drop the ridge board in between the two (you may also want to fit a temporary support to prevent it from dropping). Depending on the length, you may need to repeat the same again with another frame near the centre of the ridge, to prevent it from sagging(?).

From there, you could be able to set out the centres for each intermediate rafter, do you seat and plumb cuts and nail them in to place, one at a time. It's actually easier than how I've probably made it sound!

 

[monkeybiter]

With regard to the base, my workshop is of a similar size, but fully blocked double skin walls up to a tiled roof, and the base is approx. 4"-5" thick with 6"-8" around the edges, all on top of sledgehammered rubble.
No rebar, no cracks, after more than 10 years. Maybe it's luck ?

 

[Dibs-h]

I'm in the early stages of building my workshop. Its going to be 7 x 3.5 m on a concrete raft using lots of inspiration from this site, not least the blockwork and timber design here.



(the coloured blocks are the odd sized ones I need to cut!)

I have a few of questions before I go much further:


1. The concrete base
Q: Is reinforcing with steel bar worth doing or will my 100mm concrete on compacted subbase be enough?

Laying the electric cable into the building I would like it to be set in through the concrete base.
Q: Do I need to cut through the damp proof membrance to allow this to happen? won't this compromise the DPM?
Q: How far in should the conduit for the cable appear? I am guessing the width of my block wall plus cladding plus a bit more?

2. For the roof of the timber frame I have got so far and got stuck in finishing the design of the gable end (red denotes the pieces I am referring to):



Uploaded with ImageShack.us

I have a nice support for the ridge board, but cant see how to get a rafters up each side to the end of the top plate. I am aiming for an overhanging rake but the vertical support just seems to get in the way of acheiving this. Do I butt rafters to the vertical pieces to end up flush with the gable end? It doesn't look such a good idea. I am almost wondering whether to do a complete redesign around ply gussets like this but I like the idea of a ridge board that rafters adhere to. am not sure how a ridge would fit to the ply gussets build.

Any help and advice gratefully received!

Cable access - yes you would have to puncture the DPM, but why can't you wrap extra DPM around the pipe and join it to the main sheet of DPM, with some of the Butyl-like joining compound (that appears sepcially made to do that)? As for where - depends on where you want your distribution board and whether you'd want the cable to effectively come up inside the timber walls or not in the walls?

Gable ladder - that's what they call it, the overhang. To achieve this - the ridge board needs to overhang the outer end wall (which you have), as do the wall plates (which you don't), and then you treat it as just another set of rafters (slight simplification - but that's it in essence). Have a look at the following webpage and the 1st picture - a picture speaks a 1000 words and all that,

http://www.askthebuilder.com/578_Roof_F ... sics.shtml

Slab - personally I would reinforce it - marginal extra cost, but it allows you to increase the loading on it and should there ever be any settlement or voids, it'll span those with no issues.

HIH

Dibs

 

[Deejay]

Hello

Laying the electric cable into the building I would like it to be set in through the concrete base.
Q: Do I need to cut through the damp proof membrance to allow this to happen? won't this compromise the DPM?

If it was my building, I would prefer not to perforate the damp proof membrane. I assume that you will feed the power through a SWA cable, which is not easy to bend (minimum bending radius six to eight times the diameter of the cable).

I would suggest that you clip the cable to the outside of a wall , terminate the SWA in a box and fit the consumer unit right behind it on the inside. If you leave long tails on the wires in the SWA they can be fed through a horizontal conduit from the back of the box straight into the back of the CU.

EDIT

If you want the CU somewhere else, connect twin and earth to the SWA in the external box , run that through the wall and take it to the CU.


This has caused a few comments later in the thread. Paul wrote ...

As I was asked to back up my comments earlier with regard to the electrical installation then IF you want a BS7671 Reg. No. as to why you should not fit an external joint box to go from SWA to T&E in your sub main, try 132.7, 133.3, 134.1, 510.1, 510.2, 512.2, 513.1, 521.1, 522 (group), 526.1, 543 (some of this group),.
You would also need to refer to reg 422.3 when undertaking your internal install.

Paul is a registered NICEIC contractor, so you'd be wiser to take his advice, rather than mine.

At the end of the day you (KC) are using a spark, who will issue the Part P certificate, so let him decide.



Cheers

Dave

 

[Rob Platt]

Extend your ridge beyond the gable and support, fix rafters then cut off excess. What`s your roof covering. You have no collars I think you may well get spread at that length. I would think a dog leg raft would be better. Electric cable should be ducted. Put in duct and seal around.
All the best
Rob

 

[Mcluma]

Hello

Laying the electric cable into the building I would like it to be set in through the concrete base.
Q: Do I need to cut through the damp proof membrance to allow this to happen? won't this compromise the DPM?

If it was my building, I would prefer not to perforate the damp proof membrane. I assume that you will feed the power through a SWA cable, which is not easy to bend (minimum bending radius six to eight times the diameter of the cable).

I would suggest that you clip the cable to the outside of a wall , terminate the SWA in a box and fit the consumer unit right behind it on the inside. If you leave long tails on the wires in the SWA they can be fed through a horizontal conduit from the back of the box straight into the back of the CU.

If you want the CU somewhere else, connect twin and earth to the SWA in the external box , run that through the wall and take it to the CU.

Cheers

Dave

That is how I have done it on my cabin build

 

[kingcod]

Thanks for the advice so far. I will stay clear of compromising the DPM by bringing electrics up to ground height outside the buidling and then fitting as you suggest Dave.

Olly I have gone with your suggestion to dispense with the permanent end support. It is looking neater.

Dibs I have shown a ladder gable of sorts

Some measurements to show the size of the thing.



Am I overdoing it with rafters spaced at 400 centres?

Roofing material probably something like OSB and Onduline or felt tiles. I don't think I can afford cedar shakes. Which takes me to the weight of it and potential spread ...

Rob I am not sure the most economical way to prevent spread ... so a couple of options shown - one for a collar (in yellow) set in the top third of the roof, secured with ply, and the other (in red) uses lengthier timbers resting on the top plate - I guess these would be called ceiling joists though they are to do the same job. Would one be better than the other? I know one would cost more!

 

[monkeybiter]

I specifically didn't want crosspieces when I built mine, so I bought a length [~20'] of steel 'I' beam [9" web] which is supported at either end at ridge height, the rafters are supported on the walls and on the ridge, tied together across the ridge. There are no cross pieces to get in the way overhead, and no tendency to spread as the direction of the force on the wall top from the rafter is vertical.

 

[Deejay]

Morning KC

You might find this thread useful ...

post460872.html?hilit=shed#p460872

The author, Mike Garnham, no longer contributes to this forum, but does so on The Wood Haven.

Have a look at ...

http://thewoodhaven.co.uk/phpBB3/viewto ... hilit=shed

When you get round to cutting the roof, have a look at ...

http://www.blocklayer.com/

Cheers

Dave

 

[Rob Platt]

Red is better than yellow because you can spike it directly into the stud heads. But yellow maybe enough if? as follows I cant see any sizes other than 400c/c so whether its overkill or not? i cant see your location. Wind and snow loadings and worse case scenarios should be considered in the roof design and construction. If you put a chunk of steel into the ridge you will have to factor that into the calculations especially as its over a doorway.
All the best
Rob

 

[NetBlindPaul]

I would NOT put a "joint box" on the exterior of the building guys!
By all means take the SWA up the exterior of the building, I would take it up to soffit level and into the building intact then run the SWA to the point where you are going to locate the consumer unit.
How you joint up from there depends on your exact configuration.
Please also remember that the wiring comes under building regulations part p unless you get someone registered with a part p competent persons scheme to undertake the installation.
There are also a few other things to consider when installing electrical systems exterior to the building in which the supply originates, please remember electricity kills!

 

[Deejay]

Evening Paul

'I would NOT put a "joint box" on the exterior of the building guys!'

Is there any reason why this should not be done?

Would it contravene any of the reg's, assuming that a box with the appropriate IP rating was used?

Mcluma and I are probably not the only people who have done this, and I would like to know if it is 'legal'.

Cheers

Dave

 

[NetBlindPaul]

Why add an additional joint into the cable?
IF you use a screwed joint, such as a "chocolate block" this has to be accessible for inspection. (Regs)
You are addiing additional impedance into your supply cable which reduces its "performance", it will increase volt drop and earth fault loop impedance. (Physics)
If the joint fails, which it can you have a fire risk, and the possibility of the failed joint inducing a possibly lethal fault into the premises.
Without the full detail of how the actual install is to be/was done I can'f fully comment.
However, it is not good practice, which is actually against building & electrical regs, as there are better and safer ways of doing it.
There is a section in Part P & other building regs docs that requires "good materials & workmanship", which is basically mirrored in the wiring regs, I don't have the docs to hand at the mo, I am only still here as I am eradicating a virus from my daughters laptop which she desperately needs for her GCSE course work!!!
We can keep discussing this and I will try to find specific references if necessary.

 

[Deejay]

Morning Paul

I think you misread the post.

The use of an adaptable box on the outside removes the need to bend the SWA. The long tails don't add another joint if they terminate directly in the CU.

I'd still like to know if it contravenes the reg's.

Cheers

Dave

 

[Dibs-h]

IF you use a screwed joint, such as a "chocolate block" this has to be accessible for inspection. (Regs)
You are addiing additional impedance into your supply cable which reduces its "performance", it will increase volt drop and earth fault loop impedance. (Physics)

I am sorry, but such rather fantastical claims, one hears a lot with absolutely zero calculations or real theory to back them. A block as has been suggested - how much extra impedance would it add? Do you know? What voltage drop would it add? The words practically NILL spring to mind, for a block of the correct rating. As has been suggested - whilst I might not necessarily do it that way - why wouldn't it be accessible for inspection?

If the joint fails, which it can you have a fire risk, and the possibility of the failed joint inducing a possibly lethal fault into the premises.

A joint made using the correctly rated connector - what fire risk? What possibly lethal fault? Assuming the wiring is to the correct spec\design.

Without the full detail of how the actual install is to be/was done I can'f fully comment.
However, it is not good practice, which is actually against building & electrical regs, as there are better and safer ways of doing it.

Not good practice - how so. Got any sections\clauses\sub-clauses to quote?

I'm sorry, if my post comes across as having a dig at you Paul - it isn't. When claims of x. y & z are made I like to see the substantiation up front. The fact that the word Engineer is over used in the UK and has no protection unlike in other parts of Europe, but that's just my pet hate! (that's probably just my Monday morning grumpyness coming out)

Dibs

 

[kingcod]

Dave - thanks for links - already in my set of bookmarks

Rob - I agree Red ceiling joists look better - and less fiddly to fit. I might try to reduce the total number (1 every 2 rafters?) to avoid overkill. Snow loading - I am on Tyneside so 0.6 - 0.8 Kn/m2 (I think). Gawd knows what that means for my design!

Mike - I am trying to stick to wood so I hope 200 x 500 x c.7000 for my ridge. The I-beam sounds a bit scary to get up there.

Electrics - would something like this suit to terminate the SWA up at soffit level? Thinking on this I intend to get wired up for an alarm so wonder if the electrics should be tamper proof on the outside ....
Whatever happens I will get a qualified sparks to do it!

 

[Dibs-h]

Thinking on this I intend to get wired up for an alarm so wonder if the electrics should be tamper proof on the outside ....
Whatever happens I will get a qualified sparks to do it!

Tamper proof - that will mean bringing the supply in underground and bringing it up thru the slab. Anything else is not tamperproof.

HIH

Dibs

 

[NetBlindPaul]

IF you use a screwed joint, such as a "chocolate block" this has to be accessible for inspection. (Regs)
You are addiing additional impedance into your supply cable which reduces its "performance", it will increase volt drop and earth fault loop impedance. (Physics)

I am sorry, but such rather fantastical claims, one hears a lot with absolutely zero calculations or real theory to back them. A block as has been suggested - how much extra impedance would it add? Do you know? What voltage drop would it add? The words practically NILL spring to mind, for a block of the correct rating. As has been suggested - whilst I might not necessarily do it that way - why wouldn't it be accessible for inspection?

If the joint fails, which it can you have a fire risk, and the possibility of the failed joint inducing a possibly lethal fault into the premises.

A joint made using the correctly rated connector - what fire risk? What possibly lethal fault? Assuming the wiring is to the correct spec\design.

Without the full detail of how the actual install is to be/was done I can'f fully comment.
However, it is not good practice, which is actually against building & electrical regs, as there are better and safer ways of doing it.

Not good practice - how so. Got any sections\clauses\sub-clauses to quote?

I'm sorry, if my post comes across as having a dig at you Paul - it isn't. When claims of x. y & z are made I like to see the substantiation up front. The fact that the word Engineer is over used in the UK and has no protection unlike in other parts of Europe, but that's just my pet hate! (that's probably just my Monday morning grumpyness coming out)

Dibs

Dibs,
I am qualified to practice in countries where the term engineer is protected in law I can assure you as this is one of my pet hates also! I won't list my qualifications, but I did an apprenticeship and then carried on to get letters. I was made redundant from a design/applications engineering post when several of our large customers closed down in the Midlands and the global recession was just starting. From then I went back on the tools, now I am flying completely solo.
I undertake craft type work and I also undertake engineering design & H&S consultancy.

I can quote you reg numbers but unless you have a copy of 7671 & know your way around it, this would be a bit of an overkill.

I will once I get the chance to pull up some references as I promised.

The fire risk is often seen when screwed connections are not correctly made, often by DIYers.
The potentially lethal fault can come from several failure modes, to do a full FMEA I would need the full details of the install, the upstream cct protection and the characteristics of the supply at the origin.

Please also remember that when an electrician comes to work at your home, this becomes his pace of work he thus has to comply with all work place legislation.
If he is to fit this for the constructor he has to also comply with CDM & work at height legislation, thus the joint must be in a position where by it can be inspected without the necessity of working at height as it is possible that live working could be needed at that location.
Thus remove the joint and you remove the requirement for access for inspection & testing at height.

Paul

 

[NetBlindPaul]

Dave - thanks for links - already in my set of bookmarks

Rob - I agree Red ceiling joists look better - and less fiddly to fit. I might try to reduce the total number (1 every 2 rafters?) to avoid overkill. Snow loading - I am on Tyneside so 0.6 - 0.8 Kn/m2 (I think). Gawd knows what that means for my design!

Mike - I am trying to stick to wood so I hope 200 x 500 x c.7000 for my ridge. The I-beam sounds a bit scary to get up there.

Electrics - would something like this suit to terminate the SWA up at soffit level? Thinking on this I intend to get wired up for an alarm so wonder if the electrics should be tamper proof on the outside ....
Whatever happens I will get a qualified sparks to do it!

kingcod,
If you utilise that box then you must be careful as you can experience plastic deformation of the body of the box, allowing the cable glands to loosen, this can allow a corresponding increase in earth fault loop impedance and a reduction in the sealing properties, i.e. IP rating of the enclosure. There is more to this than meets the eye.
Sorry.