Looking for some rooflight advice

[siggy_7]

Fingers crossed, I'm finally going to get around to building my workshop this year. I'm aiming for a building close to 6x5m, single skin Celcon block with internal insulation. Due to space constraints, I am intending to build adjacent to the boundary and I'll be restricted to 2.5m height. I'm therefore intending to build a very shallow pitch roof to maximise internal height.

I can't make my mind up on roofing details. I know that I want a windowless workshop to maximise wall space with some rooflights. I've been very impressed with my Dad's insulated roof panels (trapezoidal Kingspan type) which include some rooflight sections but I think pretty much all of these systems have a minimum 4° pitch (for reasons I can't find out) and I want to go shallower than that. I see that Kingspan offer a shallow-pitch panel suitable for a 1.5° pitch - not sure what the difference in construction detail is? I'm leaning towards an EPDM flat roof deck with upstands for some flat roof skylights. I've looked at some different types of rooflight; for a flat roof I like the look of stepped glazing units where the top pane extends beyond the upstand by ~30mm as the most robust system but the cost of the stepped units seems high at several hundred pounds per light. Ideally I'd put in four lights but maybe two would suffice. I wondered whether a polycarbonate structure might be cheaper than glass but I was surprised to find out that products such as roof domes are a similar cost to glazed units.

Any suggestions for decent sized rooflights in a flat roof for a workshop at a sensible cost gratefully received. I'm sure that there must be quite a few people with this sort of experience but I can't seem to find any!

 

[MikeG.]

Have you looked at polycarbonate dome-type rooflights? They're pretty standard for "flat" rooves and are a lot cheaper than the flat-pane glazed type.

I would be amazed if the Kingspan panels are even slightly competitive cost-wise for such a small project. They're normally used on large steel-framed jobs, and by the time you've paid for transport and possibly some sort of handling equipment they'll be pretty expensive. EPDM would certainly be my number one choice in your circumstance. The reason for the 1.5 degrees thing is because the sagging in the substrate will have water pooling, but 1 in 80 is much shallower than 1.5 degrees, and 1 in 80 is pretty safe with EPDM.

As an aside, I would be designing such a building around a 4.8m span for the timber of the roof, simply because it can be hard to find longer lengths than this, but also because longer lengths will need to be larger sections at closer spacings and thus not just more expensive but also even harder to source. I've no idea why anyone would build a workshop with blockwork walls, thus wasting over 2 square metres of potential floorspace, but that of course is your choice.

 

[siggy_7]

I did look at that type of rooflight yes - as I said I was surprised to find these don't seem to be substantially cheaper than flat glazing. E.g. a double skin Thermadome polycarbonate dome 600x1800 light is £344.63 from that site you recommended, whereas I can get a 600x1500 triple glazed stepped unit from https://toughenedglasssystems.co.uk for £452 (a triple skin dome is about the same price as the triple glazed unit). Maybe I'm cheap, but I was expecting more of a saving for going with what is basically a few bits of clear plastic.

Thanks for the suggestion of limiting the span. I was intending to put a steel down the centre of the building to limit lengths of timber to ~2.5m and buy me another 100mm of headroom with shallower section depths - maybe I'm a bit nuts with this idea though.

I was going with block construction on the basis of it being within 1m of the boundary and over 15sq metres, which requires the construction to be of "substantially non-combustible materials". I presume you were implying I should consider a stud construction - if the whole frame of the building is timber (combustible as far as I know!) then how is this compliant with regulations? I'm very happy to be educated as buildings aren't my line of work.

 

[MikeG.]

........I was going with block construction on the basis of it being within 1m of the boundary and over 15sq metres, which requires the construction to be of "substantially non-combustible materials". I presume you were implying I should consider a stud construction - if the whole frame of the building is timber (combustible as far as I know!) then how is this compliant with regulations? I'm very happy to be educated as buildings aren't my line of work.

It is generally the case that councils treat this regulation as applying only to those parts of the building which are within 1 metre of the boundary, and it is also generally the case that if the outer face is non-combustible then the rest of the wall doesn't need to be. With timber frame close to a boundary you can make it non-combustible (for the sake of this part of the Guidance) by rendering it, or by cladding with cementitious boards (there are lots which mimic timber feather edge boards), or by using crinkly tin of some description. However, because different authorities interpret this in different ways it might be best to check with them prior to starting work........and you want Building Control, not the Planners, for that conversation. Timber frame construction makes the best use of the footprint essentially because it has the insulation within its depth rather than added to the inside as in the construction you were proposing.

 

[Fitzroy]

I recently installed a stepped glazing unit on a flat roof dormer extension. I built the upstand myself from 12mm ply and an 50mm cavity packed with kingspan. The triple glazed stepped unit was custom sized and sourced locally from a firm my builder knows. It transpires stepped unit are expensive as they require a lot of manual intervention in the manufacturing process. Mine ended up slightly cheaper than the ones from the firm you mentioned.

A 600x900 unit was about £340 of which the glass was only about £125, this matches online where a simple triple glazed unit of that size is about a hundred pounds.

I thought long and hard about how to use a regular DG or TG unit to reduce the cost but couldn’t figure out an easy answer.

Fitz.

 

[siggy_7]

Thanks both. I've been having the same thoughts as you Fitz - why are these units so expensive compared to ordinary DG units? Your info helps to understand why. I've made some progress - I've found a company that will sell only the polycarbonate lid for the domes at a much more reasonable price than the full units: https://www.roofglaze.co.uk/products/re ... ooflights/

I can get a 0.6x1.2m dome lid from there for a much more palatable £131, then use that as a top cover for a unit with a conventional double glazing unit sitting below - all in should come to just over £200 per rooflight, which is a fair bit cheaper than I have found elsewhere. Although interestingly, I've also found a site which advertises much cheaper prices for stepped units - according to their online specifier they will supply a stepped unit of that size for less than £100: https://www.sealedunitsonline.co.uk/reg ... fault.aspx

Thanks too for the information on building control interpretation Mike - I'll get in contact with my local BCO who has been helpful in the past. The workshop will be on the boundary of a neighbour who I would charitably describe as "difficult" so I don't want anything to be a potential sticking point further down the line.