Doing something about depression in flat garage roof.

In article , Etaoin Shrdlu writes

The fact you have a sag suggests that the beams are inadequate or that the roof wasn't constructed correctly (flat roofs aren't meant to lie flat but have something like a 1 in 40 fall.

Do the beams run the full length of the garage or are they across the width, and what size/length are they.

There may be mileage in correcting the sag by adding some mid-way support (underneath at right angles) or if the roof was laid with insufficient fall you could jack up one end to give it more fall.

Jacking could be done with spreading timbers lifted by acro props with the raised timbers having a packer beam put underneath or some other form of fixing devised.

Shut the bedroom curtains.

Bill

Why not build a car port over it to keep the rain off?

Bill

Neighbours say the previous owner made the roof himself. It seems hard to believe, but they were there before mum and dad were, so I suppose they ought to know. Anyway, he had a bit of a reputation as a bodger.

They go across the width. From memory, I'd guess 4 x 2".

The next door neighbour (other side of the garage) have extended their house onto the garage wall. I'd have to have a good look at the arrangement before getting the jack out :-)

Flowerpots and thirsty plants?

Ok, assuming this is single garage width then 4x2 sounds adequate for a light roof. More likely a case of inadequate fall in the design rather than sag.

For ref, 3x2 was the going rate for bouncy workmens' site huts in the past and is a bit light but 4x2 should be workable.

Look at which side you could jack to gain a fall for drainage and look at where the water could drain to. Even if you don't jack, this is something that you will need to consider as the water has to go somewhere.

Making it drain to the rear would be a huge change. If the intended fall is to the neighbour's wall then you may have to create a valley gutter on that side, including possibly jacking the while roof to make room for it.

Have fun :-)

Doesn't warrant that much work, I think. On the other hand, if she keeps going on about it...

That's a good idea. I'd probably do that if it was mine. I'll suggest it.

Possibly - it actually covers a garage and a kitchen extension.

It is supposed to run into a drain pipe at the back. But the fall of the roof isn't sufficient to get it all to that end.

I'll try :-)

It's what mothers do, if you fix that then she'll find something else to complain about :-)

If you try to move the structure, you may well cause a leak.

You can buy cans of bitumous gloop intended to make roof repairs, you could progressively add layers of this stuff to build up the top surface.

Turn it into an inverted roof?

Dad actually has some of this, and is looking for something to do with it. Gravity should keep it flat, I think.

Indeed - the proper solution usually being to lay the joists level, and then place "firings" (i.e. a thin tapered joist) on top of them to create the fall in the deck.

In article , Etaoin Shrdlu writes

Draining the long way probably wasn't the best way but it is what it is.

At this point I'd suggest getting on the roof and doing a survey with a long (say 3m) piece of straight wood an a level to see what the fall is and to see it there are any localised dips. If you find a generally adequate fall with an isolated dip then you can perhaps carry out some localised remedial work (not a pond of tar :-!).

If you find it was designed to fail then maybe think about shedding the water some other way, such as to the side.

Yes, I hadn't appreciated that until recently when I was replacing the deck on a 'flat' roof and encountered the tapered additions. In that case, a taper was cut on the top of the joists for the last 25% of the span and the remained catered for by a tapered addition. It achieved the correct fall without too much reduction in strength on the main timbers.

Despite the 'correct' fall, that roof had always been a bugger for leaks and so I fibreglassed it when I replaced the deck with no problems since.

For a conventional rooms it makes sense to keep the underside truly flat but maybe not for a garage where the slope wouldn't upset the eye.

bitumen has lots of uses!

NT

Would it be easier to build up layers using roof felt and then covering with a few layers of Bitumen gloop to seal the layers.

I'm glad you mentioned this. I believe that the 'firings' are there, but in this low area they have rotted, and new boards have been put down without replacing them. I think the solution now is to hammer wedges in to replace them in the rotten areas.