Garage/shed roof advice please

[Wightsparks]

Our 1960's bungalow has a separate outbuilding 7.5m x 3.4m It's a fairly lightly built wooden structure, at the level of the house at one end with a (block built) garage underneath at the level of the drive at the other. The roof is a single shallow pitch (about 10-15degs).

Unfortunately the design was a bit amiss in that it starts at head height at the door and goes down from there .

It was originally roofed with lightweight corrugated plastic, but as a stop-gap when it started leaking last year (ish...) I just built a frame round the outside and added some too cheap, lightweight shed felt on top.

This has come to the end of it's life with the recent winds so now I need to do a proper job - it's floor is the ceiling of my workshop so leaks not appreciated

Pic before roof felt added.



100x50 beams running cross ways and lighter 50mm supports on top.

Ideally I want to change the pitch so it slopes the other way so there is standing room in the whole place. At a later date I might commandeer it as another workshop.
Not going to make major changes or spend a fortune, so just raise the other side, run new beams across and roof it. Oh and I need to be able to plan it well so I can do it in a day!

Q's please
- Rough beam size and spacing and cross member spacing (partly dependant on roof choice)?

Roofing options
- Corrugated steel (coated)
- Plastic of some sort
- OSB (12mm?) + ? + proper roof felt
- Something I have not thought of!

 

[addjunkie]

Corrugated steel...condensation is a nightmare...even with the anticondensation felt.

Corrugated steel insulated would be my suggestion, but its expensive.

My shed is osb with onduline over the top.

 

[gaz1]

if you slope it towards the property you will cause much more problems than what you have now

alternative change it to a flat roof slight slope

any chance of an outside photo of bungalow and shed roof structure as maybe able to offer an alternative to use instead

your options

plastic roof again will drip like crazy when working in there same with box profile roof

insulated is the way to go but you lose head height and also light as well

roof felt or grp roofing your choice is expensive if torched on but best way or grp it by roller

 

[Wightsparks]

It is a separate outbuilding so slope makes no odds I would re-gutter of course.
Pic hunting..... (this was just before we bought the place, taken from the roof of the bungalow)

 

[addjunkie]

Easy enough to lift the roof, slope either way just the length of the extensions changes. Think the do it in a day is ambitious.

Think I would have the joists closer together or use 4x2 purlings rather than what look like 2x2...

You could use the waterprof osb that is tounge and groved, the have a proper flet roof put on the next day.

 

[Burdekin]

Here's the roof joist sizes recommended for varied spans: https://www.broxbourne.gov.uk/resident-planning-and-building-building-control/timber-span-guide

I have just put a metal roof on a new extension at my workshop the sheet sizes were 3.5m long and I got 6 lengths (they are 1m wide when joined together), a 3.6m clear sheet for making 3 skylights and it came with all of the screws and flashings and it was £600 delivered. OSB, breathable membrane, battens then metal roof should stop condensation but a ceiling, dpm, and 50mm of insulation would definitely have no condensations issues. Unfortunately it's not a cheap option.

 

[knighty]

I think at a minimum you're going to have to pull down / re-do the lower wall if you want to raise it up

you could use the same cladding/whatever on the outside and just add some extra to the top to extend it if you want ?

you really want a wall beam under the end of each roof joist - wall beams don;t need to be anything fancy, 4x2 will be fine


if you want it done quick, you could always build the new wall ready and then lift it into position with the help of a mate (and then fit the outser skin on after

you could do the same with the roof if you had 3 lads to help you ?

 

[eLuSiVeMiTe]

Easy enough to lift the roof, slope either way just the length of the extensions changes. Think the do it in a day is ambitious.

Think I would have the joists closer together or use 4x2 purlings rather than what look like 2x2...

You could use the waterprof osb that is tounge and groved, the have a proper flet roof put on the next day.

My thoughts too.

Span calc here http://www.home-extension.co.uk/tech2.html

400 or 600 centers

 

[Wightsparks]

Sounds like OSB and a felt top (Cold glue and nails as I will be doing it!) is the way to go.

I will double up the joists, increase the size and use wall beams as suggested. Bear in mind this has been as is for the last 57 years and it's a shed not a house!
What should the purlins be size and spacing? - Can I hang them to be the same level as the joists rather than on top? would seem to make sense for the OSB support.

Unfortunately the wall that will be raised is the one in the back of the pic and access is limited due to forest!

I was hoping to re-use the existing joists but they are not in very good condition and I think it will be easier to re-make

This job is getting bigger by the second! At least a day of prep then a day of build - obviously not the time of year to be roofing so I just need to minimise the open roof time.

 

[addjunkie]

If your joists were bigger and spaced correctly you wouldnt need purlings going with a sheet osb roof and felt

 

[Wightsparks]

Thanks for your help everyone. I'll post a pic when i have finished it on Friday


Well "a" Friday anyway...

 

[gaz1]

could you take 4 photos of each inside wall

i believe youve got the wall above garage door and some of the brick back wall in photo need front wall and the other side wall

 

[Burdekin]

If you don't know use OSB3 boards, they're suitable for humid external use and not much price difference I don't think to OSB2.

 

[gaz1]

idealy that shed is only going to be for light use only

it may have an old concrete garage roof 5" thick from the looks of it single wall built any drilling will cause damage to the original concrete base or garage roof concrete

really it should be double walled and strengthened by steel beams more as a support structure for the garage roof to support more weight based on the old concrete shed roof style

unless you can provide photos inside of garage showing roof details this would be better

pre framing front and sides is a great idea speeds up everything, back wall can be built up quickly in a day

support woods for the beams fastened to original wall so roof beams can be placed in place to reseal the roof

 

[addjunkie]

The two long walls looks to be ply on framing, is the ply good or starting to delaminate...

 

[Wightsparks]

Gaz1 no concrete involved above the garage. Lots of large joists on 400mm centres form the ceiling of the garage/floor of the above shed with T+G floorboards - that bit was done properly! The roof of the shed was corrugated plastic on random woodwork.

Looked at it in more detail today in the light - after I had a brainwave and used a large bit of opaque industrial poly-tunnel plastic to make it watertight (so main panic over!).
Basically it is all junk except the floor. Joists are mounted to the underneath of a capping beam and screwed to the uprights (of various sizes) and supported by shelves (!), purlins on top. Joist spacing ranges from 1.2m to 1.6m , random bits of diagonal bracing.

Basically the plan now is to take most of it down and start again. Thinking at the moment is to build the back wall in two sections inside the existing structure, flat on the floor ready to be lifted vertical (including exterior panelling as access from the other side is a pain) all marked up with joist positions etc. Front wall in two sections pre-built outside but without panels - (I may re use the end panels not sure yet) joists pre cut.

Span is 3.1m, so 47x150 C16 for the joists at 611 centres and thinking 47x100 C16 for the rest ie sole plate (screwed through floor boards into joists below), uprights, and 2 x caps. Then 11mm OSB3 for roof and walls.

.... Lot of work for a leaky roof!

 

[gaz1]

thats good then

as your making the wall higher you can jack the back up and load it for the time being by using 3x2 drilled to the wall to support while wall being built

that back wall for speed build it in blocks as goes for sides and front id build them in panels fr quick fixing and getting on with other things

if i was doing it myself id build the panels first

then rip out the lot as quick as possible if nothing stored in there rip it off day before

put your 3 panels up and fix them solid fastening to back wall and then build the wall up as quick as can be

the last one a new roof fitted 4/5" beams your choice if you just cover for the night again

last but not least fit the felt on the sides once the roof is either covered\ boarded as it becomes more weathered

 

[Burdekin]

Span is 3.1m, so 47x150 C16 for the joists at 611 centres and thinking 47x100 C16 for the rest ie sole plate (screwed through floor boards into joists below), uprights, and 2 x caps. Then 11mm OSB3 for roof and walls.

.... Lot of work for a leaky roof!

That's the sizes I went for as well hat but went for 400mm centres. Some of the progress pics.

 

[Wightsparks]

As felt = 18mm OSB = big joists = complete re-build as the walls are not strong enough - materials came out well north of a grand and that's at trade prices.
Had a re-think and am now going for Coroline which is a corrugated, pressure formed bitumen/fibre with a 15 year guarantee. Also it is light and needs no board backing at 10deg pitch, just purlins at 45cm's - which I can put on half as many joists so I only re-make the one wall to make it higher. Still £750 but I need a roof!
As an added bonus if I want a felt/board roof later I can add the walls, extra joists and boards at a later date.

 

[Phil_D]

If you're using any formed bitumen sheet back it up with boards.

A bit of decent sun followed by rain and it will sag and puddle then drip through - even on the 300mm centres on my cousin's shed roof (which obviously can't be covered by any guarantee for a variety of reasons)...

I’ve just replaced a bitumin roof that did exactly that, was probably less than 10 years old too. Went for GRP in the end, wasn’t the cheapest but wanted something more substantial to keep all my stuff dry!