How to join a glass flat (low pitch) roof to a pitched slate roof?

[richrips]

Hi All,

Background:
I'm building an oak-framed lean-to on the side of our single garage, to cover a patio area that is currently covered in rain every single day... I have acquired 6 good used double-glazed units each of 2632*668mm from a conservatory roof and plan to use them as the roof of the new structure. The garage is single story, with the lowest slate of the pitched roof finishing 2550mm above patio floor level. The 6 glass panels will be attached using aluminium glazing bars supported by 7 oak rafters of 50*150mm section, with both ends of the rafters resting on oak beams of 150*150mm section. Each oak beam will be supported by oak posts, with the beams and posts adjacent to the garage being attached to the garage by stainless threaded bars set in epoxy. The posts and beam away from the garage will have two diagonal braces running parallel to the garage wall, with the rafters providing stability perpendicular to these. Pictures and very simplified outline below.

Problem:
The most challenging issue currently is maintaining maximum headroom below the lowest oak beam, which will be beneath the end of the rafters away from the garage, on the edge of the patio. For this reason, I would like to fit the glass at a very shallow pitch, and I'm currently working with 4 degrees from horizontal. This seems appropriate as we live on a hill in Cornwall, so snow-loading risk is minimal and there are no trees nearby to drop foliage on to the structure. The easiest method of joining the glass roof to the pitched slate roof, would be to start the glass beneath the existing guttering. However, this makes clearing the gutters in the middle very difficult. I would gain ~100mm in height if I remove the existing guttering from the garage, and position the high end of the glass roof exactly where the pitched roof ends. At a pitch of 4 degrees, the glass would fall 184mm across its span. Glass unit is 24mm thick, rafter (with birds mouth cutout) is ~100mm, beam is 150mm, so that leaves headroom of ~2092mm. That isn't bad headroom in my opinion, and I could probably go lower (1900mm?), either by increasing the pitch of the glass roof, or by starting at a lower point on the garage.

Questions:
1. If my plan above seems reasonable, what is the best method of flashing from the pitched garage roof, on to the low-pitched glass roof? I want this to be a good join as access will be difficult one the glass panels are in place.

2. If my plan seems unreasonable, what could be adjusted/changed to improve things given the space and materials available?

 

[catlad]

You could cut a couple of inch of the bottom of your fascia board to gain some hight and then slide some lead behind, screw through the fascia into the lead to hold it in place . approx 5' lengths by 9" should do for flashing.

 

[robinbanks]

Is clearing the gutters the only concern ? I ask because you're considering a change that will see the joint be asked to carry a fair bit of water, and I wouldn't have said that just gutter clearing is a strong enough reason to do it, when a set of drain rods, a step ladder, and that hose there attached to the rods should see it easy enough to clear the guttering from either end

The recommended minimum fall for a flat roof in 0.72 degrees, by the way

 

[richrips]

Thanks for the replies both, much appreciated. I've been measuring/inspecting a little closer and found a few things:

1. Bottom slates rest on the fascia board. Not having much roofing experience, I thought the fascia board was only there to close the roof from weather/birds/insects at the end of the trusses/rafters. Hadn't realised it supports slates. The fascia looks to be in good condition. There's ~70mm of roofing membrane overhanging the fascia beneath the slates.

2. Regarding the oak beam that will attach to the garage, and the point above about the fascia board being somewhat structural, I think it may be best to site this beam flush against the wall between the top of the door/window openings and the bottom of the fascia board. If I lower it, it will encroach on the openings below, and if I raise it, I either have to partly/totally remove the fascia, or move the beam, and supporting posts, away from the wall by the thickness of the fascia, which is not ideal aesthetically or structurally to my mind.

Is clearing the gutters the only concern ? I ask because you're considering a change that will see the joint be asked to carry a fair bit of water, and I wouldn't have said that just gutter clearing is a strong enough reason to do it, when a set of drain rods, a step ladder, and that hose there attached to the rods should see it easy enough to clear the guttering from either end

The recommended minimum fall for a flat roof in 0.72 degrees, by the way

Regarding gutters, I'm erring towards removing them above the new glass lean-to for a few reasons:
A. Aesthetics: I think a seamless transition from slate to glass with a simple change in angle will look cleaner than having guttering between the two, particularly when viewed from upstairs windows of our house.
B. Space availability: The position of the oak beam adjacent to the garage is potentially restricted as described in point 2 above. The rafters of the lean-to will have to sit on the top of that beam which is essentially level with the bottom of the fascia board. The rafters are going to be 150mm deep in section. Whilst birds-mouth cutouts could reduce the height of the rafter above the beam, but this will still leave ~ 80mm if using the deepest birds-mouth (50% of the 150mm rafter plus a little to account for the pitch of the rafter rising across the beam). On top of that is a rubber gasket, 24mm glass unit, ~10mm glazing bars, and flashing. Currently, at its lowest point, the guttering is 80mm above the bottom of the fascia. Hence leaving the guttering in its current position leaves barely sufficient room to fit in the lean-to roof between guttering and the new beam. If there was another 100mm spare it would seem more appealing to leave the guttering since, as you say, it shelters the joint from slate/pitched to glass/flat roof and reduces rain water flow over that joint. I do think it would still be somewhat hard to clean, particularly the white guttering that we have that shows dirt/algae very badly.

Regarding roof pitch, that's interesting to know 0.72 degrees is a minimum regarding flat roofs. From what I have read about using glass, a slightly steeper pitch seems to be recommended quite often, with mention that this helps the glass self-clean by making water run off faster, particularly at the lower edge where a trim is fitted over the end of the glazing, inevitably producing a small lip that will hold a small amount of water, but more as the pitch is reduced. Hence my current aim of 4 or 5 degrees pitch.

You could cut a couple of inch of the bottom of your fascia board to gain some hight and then slide some lead behind, screw through the fascia into the lead to hold it in place . approx 5' lengths by 9" should do for flashing.

This is a good suggestion and I like the idea of lead as a long-term flashing solution. Having measured I just don't think I have space to have the glass low enough for this setup to fit. My picture in my first post with the oak sketched on is a bit misleading as it doesn't show the rafters, which would be above the oak frame shown. Sorry about that!

Current plan:

- Beam adjacent to garage: Position between door/window openings and existing fascia board.

- Rafters of lean-to: Rest on top of beam. Attach with screws through fascia board from inside garage, and screw down through rafter in to beam.

- Joint between slate/pitched roof and glass/flat roof: There's ~70mm of roofing membrane overhanging outside the fascia. I'm considering sticking this down to the top edge of the glass roof with an appropriate external sealant/adhesive. Then apply 150mm flashband, half on the glass and half on the roofing membrane. Hopefully this should produce a decent seal where the adhesive/sealant acts as a backup to the flashband, whilst being flexible enough to curve over the glazing bars that will be present above each rafter. I could tuck extra roofing membrane or DPC (or lead?) beneath the existing membrane and slightly up wards inside the garage beneath the pitched roof, if this would help?

I'll add a couple of pics below to help visualise the points above.

Let me know your thoughts please and all suggestions are welcome

Thanks again, Rich

 

[robinbanks]

Euww flashband. Consider Leadax if you don't want lead salts staining all down your glass

 

[richrips]

Euww flashband

Having just read a few threads on Flashband and similar, I now realise this is a contentious issue Alternative materials or arrangements of materials are welcome

 

[robinbanks]

Not sure flashband would work there; it doesn't really have any rigidity if its own, and relies on adhering to the substrate for that, so you perhaps wouldnt look to bond it to roofing "felt". Also, if the bituminous substance on the underside got hot in the summer I could see it running the risk of oil based stains on the glass..

Per the above, even if you apply patination oil well, using real lead will eventually leave you with glass stains from lead salts (they can be removed but it's a painful job due to access) so perhaps consider a lead alternative such as leadax. It can be fitted in one long run too

 

[richrips]

Not sure flashband would work there; it doesn't really have any rigidity if its own, and relies on adhering to the substrate for that, so you perhaps wouldnt look to bond it to roofing "felt". Also, if the bituminous substance on the underside got hot in the summer I could see it running the risk of oil based stains on the glass..

Per the above, even if you apply patination oil well, using real lead will eventually leave you with glass stains from lead salts (they can be removed but it's a painful job due to access) so perhaps consider a lead alternative such as leadax. It can be fitted in one long run too

Thanks for the tip, Leadax looks like an interesting product, workable and durable without this staining potential sounds good. Regarding fitting it, do you think it is better to try to tuck it up underneath the existing roofing membrane (compared to my sketch above showing the flashband stuck on top of the existing roofing membrane)? I can see it being fiddly as I'll have to push the bottom row of tiles and roofing membrane upwards in order to open a thin gap in to which I can slide the leadax, between the fascia board and the roofing membrane. However, that would seem like a more reliable seal since each layer will then be tucked up beneath the one above, so water should flow off, even without any sealants/adhesives. Would you be able to suggest the best way to seal the leadax down to the top of the glass? it has to be a pretty good seal as the glass can only have a shallow 4-5 degree pitch. I'm particularly curious about how to get a good seal where the leadax has to bend over the glazing bars between adjacent glass panels.

Glazing bars I'm likely to use are these:

P261 Rafter Glazing Bar System | Birchwood Trading

P261 Glazing Bar System is ideal for lean-to roofs and other projects that utilise rafters as the support structure. Available from Birchwood Trading.

www.birchwoodtrading.co.uk

...although it is very tempting to make my own glazing bars from flat aluminium bar and rubber seals, although I don't see many folk discussing this as an option. I can see that the curved profile should have more rigidity, and thus a better seal, but the cost difference is massive...

 

[robinbanks]

I would probably mechanically fix the leadax to the fascia under the tile overhang and under the projection of the felt, so the felt can shed any water it collects over the fixings and onto the flashing

Leadax is intended to be a lead alternative, it's pliable but fairly heavy and can be shaped like lead can to follow a profile like over glazing bars. It it's slightly more springy than lead and doesn't shape exactly the same but it's close. It's an alu mesh with a squishy plastic coating

As such if you shaped it in so it still had a bit of a bow to it, then fixed it to fascia I dare say it would stay put well due to its own innate springiness, but if you have any concerns Leadax do a proprietary sealant and adhesive that can be used to stick it down. It would likely be enough to pump a bit under the parts where the glazing bars are and then weight it while it sets, that way you wouldn't look up through the glass and see a big bead/smear of black adhesive
At the very top of the glass a bead of sealant could be placed to deter wind driven rain if you weren't sealing anything to the glass

Blue glass, black dot bead of sealant, grey leadax, black spiky screw and washer (or batten) into orange fascia, pink roofing felt, straight black slate


The other thing I like about the leadax sealant is it's relatively easy to clean off stuff. I like silicones that leave oils behind that are a pain on glass this stuff seems to be some kind of rubber polymer; you can get it off your hands just by rubbing them together til it dries, beads/balls up and falls off. Weird stuff! Reminds me of Graft IPT if you ever heard of it