Roofing Joist Size

18 Apr 2007
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United Kingdom
I'd like to replace an ageing lean-to built between my house and garage. It's 6.2m x 4.2m, built from 4x2, plastic corrugated roof with a supporting wall about 1m along the 4.2m width (joists run in the direction of the 4.2m width).

I plan to remove the supporting wall section and replace the roof with a flat felt roof on top of plywood sheets. The roof doesn't need to bear much load, although I (or someone) will probably have to get on to parts of it to lay the felt.

How do I work out what sized joists I need?
How far apart should they be spaced?

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So you are removing the supporting wall? or the part of the roof on one side of the supporting wall? What's the longest unsupported span you'll have to cover? What direction do the joists run? away from the side of the house or parallel to?
Yes, I plan to remove the supporting/dividing wall to open up the entire 4.2m width - it's not a particularly substantial wall, 50% glass, rest is 4x2. I won't be touching the walls supporting the end of the joists (house wall one end, brick wall the other).

Longest unsupported span is 4.2m

Joists will run away from the side of the house.

Rough plan below. The lean-to section is built over the driveway between the house, garage and the dividing wall. 4.2m across. Just over 1.2m of this span (nearest the house) is converted into a porchway via the supporting wall that I plan to remove.

The garage is too small to accommodate tools and work on a car. I can't afford to rebuild the garage over this entire area, so for now, I'd like to redo the lean-to with a decent roof and double doors at the front. That way the lean-to provides enough room to work on a car and I can open the garage door to get access to tools.

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| HOUSE | ! |
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I'd go with a min. 7x2's with 400mm centres. 2 runs of bracing (noggins) firring strips away from house, 12mm wbp or 11mm OSB3 on top.
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For a simple lean to as this, screwed down properly it won't go anywhere or fall apart. I recommend only as a min. for this particular type of construction, not as the standard solution for all flat roofs/habitable areas or garages.

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