Loft Boarding a single detached garage (ed.)

[Sam Warburton]

Hi everyone!

We're looking at boarding our 6m x 3m single garage up. We are just going to store empty suitcases, christmas tree etc nothing heavy. Am I right in thinking we can just screw boards straight into the joists? One concern I have is that garage can be a bit drafty and moisture can get in, I dont want the boards sagging after a while obviously, would these boards be OK? (https://www.diy.com/departments/chipboard-loft-panel-l-1-22m-w-0-33m-t-18mm-pack-of-3/1673191_BQ.prd)

Any suggestions/ideas are welcome, I'm a DIY newbie so any help is appreciated.

I've attached a photo of said garage.

 

[JohnD]

I think you mean boarding the roof space, on the trussed rafters.

Almost certainly these trusses will have been designed and purchased for the maximum economy and minimum cost, with sufficient strength to hold up the weight of the roof tiles, and anticipated snow load, and winds, and nothing else. Even the weight of your chipboard may be too much.

If you measure the thickness of the timber they are made of; the distance between them, and the span between supporting walls, somebody will be able to calculate safe load.

It will not be much.

 

[Sam Warburton]

The width of the timber is 35mm and gap between each timber is 5700mm. The width between the two supporting walls is 6m.

 

[ads1]

I'm no expert but I had a similar garage in my old house. My roof truss bottom chords were 72x35mm and spanned 5m. I put a lump of 4x2 between the gable walls on joist hangers under the bottom chords in the middle of the span for extra support. I also used truss clips on each roof truss and the new 4x2 to negate any twist. I then boarded with 18mm loft boards. I had all sorts up there, Garden chairs etc but anything remotely heavy was near the wall plates. I never had an issue in 4 years.

 

[Blaggins]

I think you mean boarding the roof space, on the trussed rafters.

Sorry to post jump, but what's a trussed rafter versus a joist? i thought joists were the horizontals that you would board on, and rafters were the angled ones that the pitched roof is fixed to?

 

[JohnD]

Most modern roofs are made with prefabricated units that have a "W" of interconnected wooden parts forming several triangles. They add rigidity and strength so the units are calculated and engineered for minimum cost using the thinnest possible timber. The "W" obstructs the space in the loft and cannot be cut out or the roof will collapse. The loading calculation for the roof does not include using it as a floor.

https://www.localarchitectsdirect.co.uk/seven-deadly-sins-trussed-rafter-construction-part-1

 

[Blaggins]

Most modern roofs are made with prefabricated units that have a "W" of interconnected wooden parts forming several triangles. They add rigidity and strength so the units are calculated and engineered for minimum cost using the thinnest possible timber. The "W" obstructs the space in the loft and cannot be cut out or the roof will collapse. The loading calculation for the roof does not include using it as a floor.

View attachment 224270

https://www.localarchitectsdirect.co.uk/seven-deadly-sins-trussed-rafter-construction-part-1

That’s perfect, thank you. That’s exactly how my roof is. In that diagram am I right in thinking the joist is the horizontal timber across the bottom?

 

[JohnD]

in my terminology, in a loft, yes, though the truss designer doesn't seem to call it that.

Those trusses are quite weak, just strong enough to take the weight of tiles above and ceiling below, and when people want to convert a loft to a habitable room, they have to add new flooring joists that are much bigger (and reinforce or rebuild the rest of the roof as they need to cut out some of the "W" to make space.

Older houses usually have a "cut roof" made up on site from thicker timbers in a less scientific and economical way.

 

[Deleted member 174758]

In that diagram am I right in thinking the joist is the horizontal timber across the bottom?

The correct term is "bottom chord". Unlike a joist, which is thicker and wider and designed to carry a load, a bottom chord in a truss roof is more there to tie the rafters together and stop them spreading than it is to carry weight. That's why it is permissable to end join two pieces together with nailer plates (bottom chord splices). Not very strong at all, I'm afraid