Sructural question, flat roofed garage.

[Donwill]

Garage is being converted to workshop. Internal size is 2.7 x 5.3 M. Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which are too small for that span but it's been there for 45yrs with very little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the waterproofing I want to externally insulate the felt covered roof by using "Jablite inverted roof" insulation:

to hold the insulation down you need to cover it in 50mm of gravel which is heavy. The roof area is approx 15 Sq M, the volume of gravel would be approx

0.75 Cu M which I think, when wet would weigh in the region of 1500Kgms (distributed load) or approx 100Kgms /Sq M. Obviously I would need to beef up the joists and I thought to double up on the joists bolting through a steel flitch plate in between the joists at say 600mm intervals. Has Anyone tried to do this in situ?

The steel plates , say 100mm x 6mm x 2.7M, and second joists would not rest on the supporting walls and would be approx 100mm shorter than existing joists so lateral support would be needed at both ends and probably in the middle. Would this be strong enough?

Can anyone help with structural information? and or practical experience which they can pass on please.

Regards Don

[Jim K]

can you not just insulate inside and save yourself all the agro and expense??

Cheers

Jim K

[Donwill]

Fair point, I happen to like a warm roof insulation system to avoid condensation problems and rusting tools, I also want exposed beams in the workshop for storage etc. Thanks Don

[Jim K]

er OK but shurely you will always get condensation of air borne moisture onto the coldest things in your garage/workshop.

how would a "warm roof" approach be better than other alternatives in avoiding this?

Cheers Jim K

[Phil L]

as Jim has already pointed out, you are going to a ridiculous expense for nothing. far better would be to insulate from the inside, I can't see your point WRT cold spots/condenstaion - the only 'cold spot' would be the joists themselves and moisture doesn't condense on wood.

I would insulate from undrneath, then cross batten with say, 3 X 2 every metre or so and use these for your storage.

[Donwill]

Yes you are right, but by having the roof insulated you need less energy to raise the air temperature above outside temp.By putting the insulation above the roof the inside of the roof will be warm so no interstitial condensation.

What I'm trying to achieve is a relatively constant temp inside the workshop, one wall of which is in thermal contact with the house, the other three walls are very well insulated and sealed The roof has the largest heat loss. Inside the workshop there is a dehumidifier to lower the dewpoint of the air.

Thanks Don

[Donwill]

Moisture condenses on any thing that is below the dewpoint of the air, wood absorbs the condensation so is not readily obvious.

I also have a problem with headroom which I forgot to mention.

Thanks Don

[Phil L]

OK going back to your OP, you say you're thinking of using steel, and also doubling up on joists, but these extra supports, nor the steel would not reach the walls? - in this case, they are merely adding to the extra weight. Can't you have a 'post' in the centre of the workshop, considering it's never going to be used to house vehicles again? Then you can use a small steel beam, say 50X50 box section, two pieces about

2.8m each, both resting on the wall at each side and touching the underside of each joist in the middle of the workshop, and propped up by an acrow or similar in the centre.

[Tabby]

Surely its far easier to put your polystyrene on the outside then your chosen regular roof covering over it rather than 2" of gravel. Sounds like youre making your life hard.

The roof joists arent too small, as proven by your experience.

As someone said, 45 yrs from a flat roof is unusually long, if you're considering reroofing you might look at it and see if its rotting, usually they are by 45 yrs.

NT

[Donwill]

I think the felting & probably the 20mm ply has been replaced but I think the 4x2s are the original beams according to the neighbours who have lived there since the houses were built, we arrived only 6yrs ago. No rot visible. Regards Don

[John Rumm]

Given its holding up a roof and not a floor for the room above, that is not too surprising!

Which is basically the force used in calculations for standard floor loading...

Not tried it, but it would be relatively easy to do. You would need a face mounting shoe at each end (which could be added after the plate is in and the beams are bolted through) that supports the end of the new joist, flitch plate, and allows the existing joist to pass through the back of it.

So you have and extra 1500kg supported on say 9 joists, plus you ought to allow another 75kg/m^2 for snow and access etc, which give you an all up loading of about 291kg/joist. At 2.7m each that is 108kg/m or 1080N/m linear UDL

with 2 of 100x50 and a full height 6mm flitch plate, that gives a total deflection of just under 5mm which looks ok. The only unknown in that calculation is the weight of the existing roof materials - but at a guess you would appear to have reasonable margins to play with.

[robgraham]

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John - going off the original topic slightly, where does your figure of 75kg/m^2 for wet snow come from ?

I've been doing a roof loading to calculate supports and found a figure of 485 kg /m^3 being quoted for compacted wet snow. On the basis of a melt down ratio of 10:1, 500mm of dry snow would compact to

50mm, which would give a loading of around 25kg/m^2. - and even in the worst winters we don't get 500mm of snow in one fall. The figure you are using would be from 1.5m of dry snow.

Rob

[John Rumm]

My figure was for "snow *and access etc.*" So includes not only the weight of snow, but also someone romping about on the roof for maintenance etc.

Taken from:

41, "Typical unit floor and roof loadings". It uses 0.75KN/m^2 for a timber flat roof.

Indeed, but as mentioned, includes more than just snow.

[John Rumm]

Ah, so that's not really what most would think of as a 45 year old flat roof! Assuming the boarding and waterproofing are maintained and replaced when required, you can expect the support timbers to last indefinitely.

If the felt etc is looking worn, then stripping it, and laying insulation, followed by a thin board covering before refelting would be a fairly easy and low cost process.

[Donwill]

Thank you John for the reassuring information. Does the "Superbeam" give any guidance on the spacing and positioning of the bolts in the flitch beam? Can you explain the purpose/ function of the "shoe" and how does it play a part in the strength of the roof?

Most grateful to you, it was exactly the type of information I was seeking. Cheers Don

[robgraham]

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John - many thanks for coming back with that. My apologies in continuing on the theme, but would I be right in understanding that the 0.75KN/m^2 is the loading on the support timbers/structure and therefore includes the roof covering as well as the wet snow, maintainer weight, etc.?

I'm working on a gently angled roof which is just under 14 m^2 and has a gross weight currently (including snow!) of 800kg - if I take the

0.75KN.m^2 that goes up to 1050kg which is still no problem. If on the otherhand, the loading allowance is all for wet snow then effectively 1 ton of wet snow does worry me somewhat, even if the structure is still well capable of taking it.

Rob

[John Rumm]

It includes the snow and maintainer, but not the covering and roof fabric itself.

Hang on a mo let me find the bit....

Right, for a warm deck timber[1] flat roof, it lists:

Snow and access 0.75 asphalt waterproofing 0.45 Timber joists and insulation 0.2 Ceiling and services 0.15

So that is an all up weight 1.55kN/m^2

The allowance for asphalt seems fairly generous - but probably allows for a tarmac type finish and not just felt. I would have thought three layers of felt would be nearer 10kg/m^2 or 0.1kN/m^2

[1] A tiles pitched roof works out to a similar total - the snow and access is a bit less, but the tiles are a bit more.

[John Rumm]

Yup, it specifies the "normal" every 600mm with each bolt 50mm set in from alternate sides (which given this is a 100mm beam amounts to a row down the middle!). So five bolts total - one each end, one in the middle, and two to fill the gaps.

Well I am assuming the existing joist is set into a cutout in the masonry as was the way things were done then. These days its more usual to fix a joist hanger to the masonry, and let that hold the joist (it saves the risk of the wall wicking moisture to the wood, and also saves the wall needing to be pierced allowing another place that would reduce airtightness of the building.

Since you are retrofitting half the beam width, you could either chop out more wall and set the new beam and plate in the same way as the existing. Or, you could use slightly shorter joists and plates that just bolt to the visible bit, and support each end with a suitable hanger.

Here is a wall mounting hanger (aka shoe) ready to take the end of the large flitch beam you see in the forground:

Most grateful to you, it was exactly the type of information I was seeking. Keep in mind that I am working from a mental picture that may not be spot on, so if in doubt, check!

[robgraham]

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Many thanks, John - it would be interesting to know how these figures are arrived at, as they do seem excessive. I'm looking at a DIY conservatory with DG and the weight of the roof panels at 0.3kN/m^2 is away under the allowance for asphalt.

However even working with those figures which effectively double mine, the support system has a more than 20:1 margin.

Rob