Low pitch roof - UK specific question?

Question

Having built the walls of our extension we are at the stage of putting the roof on.

Trouble is when I calculated the pitch I forgot that the depth of the joist and thickness of the wall plate would have an effect on the roof pitch. We have 1.1m rise in 5m giving a 10.4' pitch ( I was aiming for 12.5 degree but ....) I cannot change the pitch by lowering the front wall (would look really odd on the house with window door and floor heights) The rear wall is constrained by windows so no adjustment there.

I am faced with using a very low pitch tile OR using a board, felt and tile finish.

Two questions

1) what would the group choose , with reasons if poss

2) If its option 2 ( board and tiles) what exactly is the make up of this : what thickness board, battens, counter battens ? what felt or membrane is used and how is it fixed.

In short could somebody walk me through the layers of a board , felt and tile finish roof .... please

any help as ever......

M

Answer 1

go to the builders merchants and get some info and/or speak to your architect, you didnt use one did you

you need to speak to planning and building regs as permission has been given on the appearance specified and structure/energy consumption. change in roof need to be agreed with them before doing the work. info first lets you put alternatives to them

Answer 2

forget the tiled option ..you wont get any tile to go that low ...you might get an interlocking slate but they are very dear ....no its 18mm ply and torch on felt with celotex in between rafters

Answer 3

The tile is not a good option - wind driven rain will get under, ditto slates which can end up chattering.

There are four options, I would say - built up felt, a membrane system, asphalt or metal. The membrane system eg Sarnfil, may be difficult as a diy job but is far superior to felt. Lead or Zinc is another option but it needs to be properly laid. It can last a very long time, but this is reflected in the price. Asphalt is usually best laid on concrete, and is really a job for a specialist.

So felting - try this site: http://www.hometips.com/content/builtup_...

The thing to remember is that the first layer is not fully bonded so there is some freedom of movement - without it the felt will fatigue quite rapidly and let the water in.

If you have had to get planning permission for your extension, you need to check what material you got consent for, btw.

If you have to go down the tile route: When you have rafters you don't really need counterbattens - just drape your felt (Tyvek) over the rafters and then put your tiling laths (must be tanalised) usually 38 x 50 or so - smaller and they tend to split when you nail (you should get advice from the tile people, though) and off you go - nail with non ferrous nails, of course. make sure your felt is dressed into the gutter. Your insulation goes above your ceiling - the tyvek allows it all to breathe.

Answer 4

The way I see it you have two options.

1. Consider and treat your 10.4 degree pitched roof as a flat roof, e.g. 18mm boarding with three layer of 'torch on' built up felt. I would get a professional flat roofer in for this,as it is skilled and dangerous work.Then put the tiles/slates you were going to put on, on 25 x 50 battens, on 25 x 50 counter battens directly on top of the felt. (the final layer does not have to be mineral finish,making that part of it slightly cheaper...and you don't have to dress the edges either.  You have the piece of mind that it won't leak and you can still have the finished look you require, and it will last a lot longer than a normal flat roof as it will be protected from the elements. (your fascias/ barge boards will have to be about 70mm deeper than they would have been.

2.Start by building the roof as high as you can, eg10.4 degrees, then s***w and glue firring pieces built up directly onto your rafters, 2x2 to 0 or 3x2 to 0  (depending on which is best for your span) until you reach the magic 12.5 degrees the building regs require. Around the bottom of the window/s form 'wells' by removing the firrings to suit, and put noggings in between the rafters to create a flat roofed area with the required fall, and cover with code five lead dressed up under the sides of the tiles and dressed over (at least six inches)  onto the pitched roof.  (get hold of a copy of the 'Cookson good lead guide' for handy tips and diagrams from your local Jewsons/builders merchants)

Edit:

Having read what I've written and in veiw of what other have said I felt the need to add more.

Regarding my option one, I ought to clarify that your roof would still be pitched -as high as you can- with a tile finish,(as per your building regs approval or planning permission) but with three layers of built up felt underneath. this would need venting in the tradition manner with soffit or under barge board vents, as any flat roof would. Drape the front edge of the felf into the gutter as usual  and fill the face of the gap on the front caused by the counter batten and battens with strips of black plastic 'over fascia' vents, these allow any water to run through. You do not need any underslaters felt or 'Tyvek' as this area -above the flat roof- does not need venting, only around the rafters.

The reason the manufacturers of the tiles put a minimum pitch on their products is because of the effects of capilary action of water, travelling back under the headlap.You WILL need counter battens to let any water run down the torch on felt.

Batten sizes are usually 19 x 38 or 25 x 50, never 38 x 50

Wind driven rain' would not get into the roof any more than any other pitched roof.

This soultion was first made aware to me by my local building inspector, as so was approved.