Queries boarding out Garage

10 May 2012
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United Kingdom
I have searched the Forum topics and cannot find an exact answer to my query so am posting it as a new topic. Am a newbie to this forum so please forgive errors in terminology :oops:

Having recently moved, the house has a detached gable-ended double garage (custom built 1980) which I wish to use as a workshop (as well as sheltering the car). As a first step I would like to board out the pitched roof for additional storage.

Internally the garage measures 6 metres in length and and is 5.58 metres wide. It is single skinned (breeze-block walls 100 mm thick), with external cement rendering. The internal height of the pitch roof (measured from the wall plate to the top of the ridge board), is 1525 mm with the rafters inclined at an angle of about 24°.

The roof (which is tiled) is supported by 14 pairs of 100 mm x 50 mm rafters spaced at 400 mm intervals. The rafters are supported half way along their length by two substantial under purlins of 170 mm x 80 mm (one on each side of the roof), which run at right angles to the rafters with the ends supported by the two gable end walls. The rafters rest (with a birds mouth) on a timber wall plate measuring 95 mm wide by 45 mm thick, on top of the breeze block walls. There are no existing joists save for 4 ties measuring 100 mm x 50 mm spanning the garage and bolted to the rafters at each end with M12 coach bolts. The central tie has some struts (between tie and rafters) for additional support.

The general layout is shown in the attached annotated photos.

Photo 1. Internal view of roof space of garage, lengthwise

Photo 2. Internal view of roof space of garage, breadthwise

My questions are:
1. Is it feasible to install joists across the internal span of 5.58 metres, resting on the existing wall plates?

2. If so, what height and width should the joists be (assuming a spacing of about 400 mm between the centres of each joist)?

3. How should the joists be attached to the existing timber wall plates? I was considering using THESE JOIST HANGERS?

4. Any other options for installing joists?

5. What sort of boarding would be suitable for screwing to the joists (e.g. 1220mm x 320mm x 18 mm lengths of floor grade chipboard)?

Any advice gratefully received.

Tim :)
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8x2 joists @ 400 c/s would be OK . They may deflect a bit but would be OK for bulky but light storage. If you put some strutting between them at mid-span, you will stiffen the platform up.
The hanger you suggested would be OK. If you wanted to avoid the hangers, you could rest the joists directly on the wall plate but you would need to chamfer the tops of them to get under the roofing.
18mm chipboard would be OK.
Very many thanks for that Tony.
If you wanted to avoid the hangers, you could rest the joists directly on the wall plate but you would need to chamfer the tops of them to get under the roofing.
I had considered not using hangers but I think I would need to remove some roofing tiles to get the joists in place which I thought would be more of a hassle than using hangers.

Thanks again


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