Garage/workshop flat roof options - Joist hangers or wall plate?

13 Dec 2014
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West Midlands
United Kingdom
Hi All

I am planning ahead in regards to my flat roof for the garage. It is completely isolated in the garden and there are no walls on either side to use the bolted wall plate. The builder is in process of laying the block work sometimes this week. I have watched EPDM videos on youtube and it looks like a reliable and easy way to go about.

Bricky has told to me that he may not have time to do the flat roof and it is something a competent DIYer can do. So, I am more than willing to take up the challenge if I don't find someone else willing to do it in reasonable time and cost.

Width of the garage is around 3m and therefore I am thinking to use 6" x 2" timber for roof rafters with 400mm spacing.

These are the questions I have.

Option 1 is to do a wall plate on the top course and then use furrings to create the slope? (seems to be a lot common) However this technique will need a wall straps if I am right and inside the garage, since I am not plastering, these wall straps will be clearly visible (not the end of the world). Second issue this option may cause is the overall height which may get over 2.5m. The door is roughly 2.1m + lintel + timber for wall plate + rafters (6 x 2) + OSB3 board.

Option 2 is to use joist hangers, not sure if this is even an option tbh. Can the Masonary joist hangers be attached on the top of the last block with masonry screws? This will avoid having wall straps but strength wise, is this okay or there will be too much load on top layer course of the blocks?

Option 3 Attach a timber (8 x 2) across the larger sides (using rawbolts) and use timber (bolted wall plate). Joist hangers will be attached to this of which the roof rafters will be running across.

If you can kindly suggest which method is better, please do. Thanks.
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Straps are to stop the wind lifting the roof off (hence they are long and anchored low down the wall, so the weight of the higher up masonry restrains them. You will need them regardless of how you fix the roof joists at the top of the wall. Unless you are desperate for a flat "ceiling" just have one side of the garage wall higher than the other to provide the fall. The method of fixing the roof normally has no influence on the height (fascias will cover joists and wall plate, and are needed to fix the EPDM trims and the gutters to).

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