Greenhouse design

[grahame]

I’m looking for advice to design and build a replacement greenhouse.
I have an existing stone gable + three dwarf walls – the wooden structure was dangerously rotten and I dismantled it a couple of years ago. ( It went on the bonfire – first mistake – I should have done a post-mortem to see how it was made)

It’s approx 4.6 x 3.5m footprint. With the profile of the existing gable the greenhouse side walls will be approx 1400mm to eaves and the ridge is 1250mm above that. Quotes for a bespoke custom-built replacement are way beyond my means.

I’m thinking of a fairly simple structure in Western Red Cedar + 4mm toughened glass – maybe polycarbonate twinwall roof as I’m in NE Scotland and climate change hasn’t arrived here yet.
I’ve got reasonable skills, Triton saw and router tables and a planer-thicknesser but very little idea regarding design, structural strength and best way to construct.
On the web I can find very little when it comes to practical details. I’d appreciate any help. Is there a decent book anyone recommends?
If anyone would like to consider taking this on for a reasonable fee I’ll be happy to continue this conversation by PM. Maybe I should also start a project thread if only to give me a prod to get it started and finished!

 

[RobinBHM]

A greenhouse is quite a challenging joinery project, but a very interesting project and possible with the tools you have.

I guess you dont have a morticer or tenoner, so I would suggest making the framework non rebated and plant on a bead to form a glazing rebate. Theoretically it could be all screwed together but alignment would be difficult without a joint so maybe consider a domino for this (i dont have one but folks on here rave about them).

Aly greenhouses are glazed with clips and would suggest seeing if this could be done. I seem to recall alton use clips at the bottom. I would avoid the traditional sprigs and putty.

Western red cedar is expensive, similar in price to hardwood. If available locally you might consider siberian larch, although it is much harder to machine.

I would see what timber sections are available and then design your framework around that. For example cedar is available in 6 x 2, so you could make the framework 70 x 45 or similar.

As a starting point, I would draw a plan of your walls and gable end. Decide on the position of the framework on the wall, say 15mm back from the outside brickwork then draw in the framework footprint. Mark the 2 70 x 70 corners and mark the door position (say 10mm narrower than the dwarf wall gap for the door). If you are butting up to your gable end wall, I would leave at least 15mm for tolerance and include a 70mm x 70mm post at this position.

The spaces you have between posts and or door gap can then be divided up to suit the window framework. The idea is to make the glass the same width.

Most importantly, take pics for the forum!

 

[grahame]

Thanks for your interest and advice.
WRC appeals because it’s traditional durable easy to work and smells nice.
My first thoughts were to make the wall section framework – three rectangles and a triangle with dowels/glue/screws and rebate these for glazing with butyl strip/glass/silicone-sealed wood beading. Then make the roof in two sections coming together with vertical ridge boards and maybe fairly high ties. I’ve got plenty room to lay out and make these sections indoors through the winter (and maybe even put the whole thing together) But how best to join at the eaves? Do I need another rail joining the bottom ends of the rafters? Or just temporarily space them then join each separately to the uprights?
I envisaged rubber seals and metal glazing bars for the polycarbonate sitting flush on the rafters and overshooting unsupported at the eaves into the gutter. I don’t know enough about the details for opening roof-lights, how to seal round the edges etc. I’ve folded back the original lead flashing on the gable to seal again here and would probably use zinc ridge profile to match the rest of the buildings

Before I can start on exact dimensions I really need to know how strong this needs to be. At ~600mm centres there would be 8/6 uprights in the walls – doubles at the glazed end and uprights and first rafter fixed to the stone gable at the other. With a relatively light roof would 45 x 45 be sufficient? The rafters will be ~2.1m do I need 45 x 75 or even 95 for this.
Should the bottom rail be like a window cill with an overhang and drip groove? Should it sit on waterproof layer?

As you see there's a lot I don't know.

 

[RobinBHM]

Western red cedar is certainly a lovely aromatic timber to work with. However it varies quite a lot in quality and is not very strong.

If you are fitting the glass with timber glazing beads, then butyl tape or silicone will work well. It would have the advantage of making the glass part of the structure so increasing strength and preventing rattle. Butyl tape can be a nightmare for deglazing as it really welds itself to the glass.

The side frames will be best made with a form of headplate, which will allow you a fixing point for the common rafters. If you make this wider than your frame section and overhang the extra width to the outside, then you will be creating an overhang which will help to protect the side frames.

When you cut the rafters, cut the ends with both a plumb and seat cut. The seat cut will allow the rafter to rest on the headplate (flush to the outside) and the seat cut will allow you to fix a fascia board. This fascia board needs to be tall enough for a gutter.

If you have the space to assemble the roof indoors, then it is worthwhile. The headplate can be set on the ground on spacers to level and you can pitch the roof from that (the ridge can be temporally supported with battens and clamps).

45 x 70mm rafters may be ok but be aware that cedar is not very strong, so you may want to get a piece this section and see how much it bows with a bit of weight.

You could make a few rafters much deeper section to coincide with the frame jambs and fit ties at the points (say at 1/3 and 2/3rds). Tie bars will be vital for stability, without them the side frames will bow out.

You could use a roof capping system like capex from exitex, or XR bars from Clear Amber (they are in Scotland). The narrow capping bars are designed for 45mm rafters.

45mm x 45mm for the side frames is a bit light in cedar, although I think Alton use quite a small section.

If you can design the side frames so that the bottom section is not rebated, but finishes to the back of the glass, which is held with plastic clips then you wont need a bottom bead which will rot in time.

You will need an external cill, with a drip that protrudes over the brick wall. I would sit the frames on a layer of DPC (just a cheap one from toolstation will do).