Base plate on concrete pad

To make sure the plate is square, use Pythagoras theorem. This is normally refereed to as the 3-4-5 rule in builders term.

So in your instance we have:

The hypotenuse (c) is the longest edge in a right angle triangle. This is worked out as a2 + b2 + c2. The three dots is short for therefore.



Therefore:

 

SIMPLE...

 

I found 3, 4, 5' ing it a little difficult and bought one of these >
http://www.screwfix.com/p/forge-ste...arch-_-SearchRec-_-Area1&_requestid=50584#_=p
But that is probably due to me just being a DIY' er

 

Pythagoras theorem ma botty.

James, chust measure and compare the two diagonals; measure from the outside corner tip of each diagonally opposite corner to corner. Compare the two measurements.

If one is longer than t'other, then tap that corner inwards a bit. Recheck. When both diagonals are the same, your 4 corners will be exactly 90o.

(Provided you have cut each side accurately to length - the opposite sides must be identical in length.)

Also make sure the corner 'joins' don;t move or slip whilst taking these diagonal measurements.

I would first check the concrete slab chust to see if it is 'square'.

Then loosely attach the 4 timber corners together - loose enough to allow a few degrees of turning movement, but not so loose as they can actually slip past eachother (If you have scraps of ply or summat, position the timber butting against eachother as they should be, place the scrap piece on top over both ends and fire a screw down into each end - that should be enough to stop them slipping past eachother as you adjust them, but will also allow a small 'turning' movement.

How are you planning to fix them down? Do you have a DPM under the whole slab (hopefully)? And are you planning to add a DPC under the timber too to be sure to be sure?

And is that baseplate timber treated? If not are you planning to soak is in summat once in place?

If the timbers are bowed, chust get them all 'square' first as mentioned above, mark the tip of each corner on to the concrete (perhaps tap a masonry nail in a few mm) and then run a thin line between these points - that'll give you the exact straight sides. Fix the plates down, and move them to these lines before fixing them mid-ships.

The shed you plan to build on to this - will the shed sides go down past the top of the concrete slab so that all the rainwater runs to ground and doesn't 'pool' on the slab? 'Cos pooling is baaaaad...

 

DA hit the nail on the head, diagonal corner measurements are the best way to square a square, this is how we set out brickwork for square.

 

I think the usual way to get the base plate level is to bed it on a layer of mortar. Other than that, you'll need to be slipping numerous packing pieces all over the place.

To get the plates level, tho', they'll need to sit flat - not warped in that direction. Worth fixing the corners together so's they don't move around, get the whole frame absolutely square and then keep it in that position by nailing a couple of cross-braces to the top, going diagonally from one side to an adjacent side (if too long to go to the actual diagonal corners.)

Then a stiff mix, possibly a DPC on top, lay on the frame and tap down taking levels in all directions (good to have a straight edge that'll go across from one side to t'other for the level to sit on.)

As for concrete pad sticking out? I dunno - angle grinder?

 

I'd put a DPC betwixt the concrete and the base plate - provided no water can land on that DPC and then sit there soaking into t'wood.

I would also saturate spray/brush some timber preserver on to that base plate to be sure to be sure. Just soak it.

 

If it's totally untreated, paint it all in a wood preservative, the bitumen paint will do if you already have it, then sit it on a DPC

 

DA beat me again, must try harder.