sloping roof


Can anyone help please. My son is building a small brick extension with a sloping roof, which will have concrete interlocking tiles (sandtoft standard). Unfortunately he cant get a carpenter and I have agreed to build the roof (woodwork side) for him. I still have the plans etc from when my extension was built and have taken lots of photos in my loft to get the construction right. I have just checked the distance from wall plate to opposite wall and it is 2.9 metres at one end and 2.95 at the other. How do I get the rafters to run in line for the tiles with this 2 inch difference?. Would I need to cut the birds mouth deeper at one end,?I cant get my head round it. Are there any websites which deal with roofs? Many thanks Neil
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Not entirely with you on this Neil. A slopping tiled roof can't be anywhere near flat or the tiles will only allow water to run back under the tiles, especially in a driving wind.
From the wall plate to opposite wall is a span of 2.9 mtrs? Or are these measurements you give the difference in height of the extensions new wall compared to the height of the existing house wall?
Can the new roof section follow the line of the existing roof on the house? If it can, then remove the tiles and boards on the existing roof and extend the beams to cover the new extension. Then you can replace the boards and tiles to match everything up again.
More data please.
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Thanks for reply. It is single storey extension. Unfortunately not square. the length is 4 metres exact but the sides are 2.9 and 2.95. It is like a lean to shed design, built in to a "L" section of original house. 2 walls original house 2 walls new. The roof will slope from wall plate to below a bedroom window on original house. The pitch is 18 degrees. Hope this makes it a bit clearer. Cheers Neil

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the L shape is 4 mtrs exactly? I think I get this bit. But the 2.9 and 2.95 measurements I'm still not with. Are these the heights of the two walls that make up the whole box? If they are the heights, then a small amount of 5 mm isn't going to put the roof plates out by very much, and could probably be ignored in actual fact.
Where on the old wall has the wall plate been fixed? Does it face the longest 4 mtr long wall? You have said the angle is to be set at 18 degrees off horizontal, so doesn't measuring this give you the exact lengths you need for the beams, and show where they must be positioned to fit properly?
A roof that is to be set with a corner to form two faces can also be simply set out when you have the main rest beam (queen beam) set from the inner corner of the L shape between the house walls and the outer corner of the walls forming the new extension. The rafters are then placed against the queen beam at the angle set by the beam itself, and rested to the outer skin of the extension, allowing for eaves if needed.
For a straight sloping angle formed along the longest length of the room. Then you first need to fix your wall plate height from the angle off the outer skin walls. You can get this near perfect with a string or straight timber and a protractor. Set the angle from the outer skin to the wall of the house. Once you fix the string at the angle needed, you can take measurements from it to find the lengths you need for the rafters. If you want to mark all the points at both ends of the extension to make sure you find any differences in rafter length, then it works just the same way.
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It's 5 cm, or 2 inches.

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Yep. 2.9 *metres* and 2.95 *metres.
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Neil,
I presume that this is a MONO pitch roof, with a wall plate bolted onto the side of the house - but the same method can be applied to a dual pitch with the 'cut' tiles under the ridge.
Remember that the tiles do NOT actually rest on the rafters themselves so there is no great insurmountable problem.
Cut the rafters as normal (allowing for the fact that they will all be different lengths), but adjust the plumb cut for the wallplate to allow for this difference in lengths (the seat cut should stay the same) and then fix them in position.
Once this is done, felt and batten the roof as normal so that the tiles will 'show straight' at the gutter end and when you come to the wall of the house, cut the last row of tiles to reflect the 2" difference. It won't matter if you cut them slightly short as the ends will covered by the lead flashing.
BTW, remember that there is a 'minimum pitch' to a tiled roof to prevent water running back under the tiles - this can differ according to tile type.
Hope this is of some help?
Brian G
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the eave for the bottom course of tiles. equal the gauge for the rest at the longest rafter, at the min head lap of 100mm or 4" for 18 degs, then adjust the gauge for the shortest rafter by increasing the head lap. There aren't many roof tiles that will go down to 18 degs, be careful.
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Thanks to everyone for advice given. Neil

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