bloody good job I saved a copy before hitting submit - that's the 3rd or 4th time nothings come out and I'd have been v annoyed to write this out again - moderator - can you look into this plz? I'm not the first to mention it either.
Ok - this is my department - stand back! I ran a construction plastics shop for 7 years and know pretty much everything there is about it.
Most of the information above is accurate, however Flynns mate who won't cap over wood is only half right. There three things to consider: state of the wood underneath, how well the guttering works and ofc budget.
In an ideal world removing the old wood fascia and fitting replacement boards - usually starting around 16mm serves 2 purposes - allowing you to check and repair any trestle ends and basically ensures you won't have to do anything else after fitting for at least 10-15 years (depending on manufacturer). Some boards will last longer than that but it would be a good idea to ensure it's still solid and secure, especially around corners and joints.
One really important thing I will absolutely stress is you MUST MUST MUST ASK AND GET PROOF IN WRITING THAT THE BOARDS ARE MADE LEAD FREE. Many manufacturers claim lead free when they are not. The lead additive leads to a phenomena known as "pinking" whereby the plastic discolours, usually like nicotine staining, but sometimes an actual pink colour can be seen; it's permanent and will not fade over time - if anything it could get worse and will look awful.
Swish are lead free as that's who I used to get supplied by (known as Summit boards as Leons mentioned above), however a brand called "Everwhite" does contain lead (or did until a 3 years ago when I stopped) and I've heard horror stories of fitters having to go back and redo the job at their cost only a few years later because of it discolouring so quickly.
If you plan to fit it yourself and go the capping over wood route, set a pair of polypins top and bottom at around 500mm on centre - seems overkill but you'd be surprised how many times I've heard of boards being ripped off because of high winds and stingy nailing - you might stretch that a bit - but less than 1 mtr or you'll have just 10 nails in a 5mtr length which is to little IMH(and expert)O. Polypins (or the longer 65mm nails for replacement boards) are an absolute must - they are ringnails and are almost impossible to get back out, which is what you need, so ideally mark the spacing when the board is on the ground, they are more expensive than normal nails but have an integral moulded head which will not come off during normal use thereby eliminating rust, long as you fully drive them home ofc.
If you do cap over as above, then thoroughly check the backer board for soundness and cut out and replace as necessary, and if you have to replace a section between rafters then cut to the next on both sides; half thickness of the rafter - or add some sister timbers and double them up if there's room, these boards have no strength on their own and rely solely on the timber underneath - remember that once you've fitted the cappings you won't be going near them again for 10 years or longer, so giving the wood another treatment with something wouldn't be overkill; the capping WILL NOT make the wood underneath watertight as humidity and the like can still get there, as can driving rain (depending on roof design) so keep that in mind.
If you use replacement boards and the larger nails I recommend predrilling - the inner core of the boards are soft, but the outer skin has been known to get radial cracks around a nail driven hole which may allow water through - but I'm a bit of a "belt and braces" kinda guy. Set the polynails for EACH rafter again a pair - these boards are heavy and will need all the support they can get with the weight of guttering a winds to contend with - if the rafters allow try and fix the guttering clips to the rafters too - you'd be amazed how heavy a 4mtr length of half round filled with water weighs. If it were I, I'd consider adding sister blocks to the rafters for the gutter screws rather than rely on just screwing into the plastic alone, that would pretty much ensure a good solid foundation for both.
When you come to join 2 lengths together either at a corner or straight leave AT LEAST 10mm between the boards, the corners and jointers are plenty wide enough to cover this and this will allow lots of room for expansion - too little and they could meet and bow out, with all the problems that come with it. When you fit them use polypins (the smaller ones) and only nail on one side leaving the other free - depending again on how paranoid you are, you might wish to add a line of silicone to each side of the jointer / corner before fitting.
If you plan to use uPVC soffit as well then you can either use ventilated boards (most expensive but simplest to fit) or plain boards and add a ventilating strip either to run the entire length, or in vertical strips spaced evenly. You don't need too many, but they are imperative to ensure clean airflow through your roofspace to prevent dry rot. However these are only necessary if you don't have another form of roof ventilation like vented ridge tiles or other tile vents. If you have a converted loft then check what was done to ensure ventilation and fit accordingly.
If you are considering doing the guttering at the same time there's one cardinal rule: make sure the water flows DOWNHILL towards the downpipe. Selling this stuff had a side effect - "gutterspotting" an occupational hazard and too many "professional" builders forget this simple thing judgeing by the hundreds I've seen that are poorly set out.
Another important rule is don't skimp - always make sure the guttering lengths are pushed together at the joints as in there's about 2" or so inside the joint / corner / running outlet (the bit at the top of the downpipe) on BOTH sides. The plastic WILL shrink over time - some more than others so if doing that means you have to buy 1 more length of guttering to fill that 6" gap at the end; then do so - because a joint or corner leaking because it's too short will cause problems and will require a fiddly splice to fix costing you a lot more than the £15 or so for the extra length.
If your guttering uses next doors downpipe, you might want to take this opportunity to create your own instead, this takes the maintenance issue (or lack thereof) to your own terms - badly maintained guttering can lead to blocks, leaks or worst case; water running down your outside wall for an extended period which can cause all manner of damage to your home.
If you buy the guttering yourself, do yourself a favor and buy a dozen extra guttering clips, 6 or so downpipe clips and a couple extra joints and corners and store them safe - remove the rubbers from the joints & corners (if you can - some can some can't) and seal them in an airtight bag. Plastic manufacturers DO go out of business or change part dimensions - if I had a pound for every customer trying to buy these few things for decades old guttering I would have a pretty penny saved - especially so if you buy Marley; as far as I've experienced several brands are interchangeable, but Marley ONLY fits Marley.
When you fit the downpipe clips, screw a stainless steel self tapping screw through it into the pipe - that will stop it slipping down - DO NOT UNDER ANY CIRCUMSTANCES solvent weld any of the guttering parts together, especially the downpipes - they WILL get blocked and you'll curse the day if you do - I don't care what a thousand builders tell you - DON'T DO IT!!
If for any reason the downpipe clips don't reach the wall (because of the soffit size and offset) house shape or other, a simple solution is to get a small length of small dia copper pipe (you could use wood but it will eventually rot so be prepared for that) and make some spacer bushings with longer screws - again stainless steel if you have them or some other means to prevent rusting as if / when the pipe gets blocked you'll probably have to remove a few clips to free the pipe to deal with the blockage (hence the spares just in case).
I'm in two minds about the leaf protection systems - some work, some don't - it depends on how many leaves you get on your roof (if any), the ones that don't work end up with the leaves sitting on the surface and preventing the water run off going into the guttering - again water on outside walls, or splashing everywhere.
OK that's enough for now - I went on a bit but those are my first thoughts on what you might wish to consider - any other questions feel free to PM me and I'll try and help.
